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	Version: linux-6.0.1 spark-3.0.0 hadoop-3.2.0-src hadoop-3.3.0-src spark-2.4.7 linux-4.15.13 hadoop-2.7.4-src Revert   Change

 // SPDX-License-Identifier: GPL-2.0
 /*
  * Generic ring buffer
  *
  * Copyright (C) 2008 Steven Rostedt <srostedt@redhat.com>
  */
 #include <linux/trace_recursion.h>
 #include <linux/trace_events.h>
 #include <linux/ring_buffer.h>
 #include <linux/trace_clock.h>
 #include <linux/sched/clock.h>
 #include <linux/trace_seq.h>
 #include <linux/spinlock.h>
 #include <linux/irq_work.h>
 #include <linux/security.h>
 #include <linux/uaccess.h>
 #include <linux/hardirq.h>
 #include <linux/kthread.h>  /* for self test */
 #include <linux/module.h>
 #include <linux/percpu.h>
 #include <linux/mutex.h>
 #include <linux/delay.h>
 #include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/hash.h>
 #include <linux/list.h>
 #include <linux/cpu.h>
 #include <linux/oom.h>
 
 #include <asm/local.h>
 
 /*
  * The "absolute" timestamp in the buffer is only 59 bits.
  * If a clock has the 5 MSBs set, it needs to be saved and
  * reinserted.
  */
 #define TS_MSB      (0xf8ULL << 56)
 #define ABS_TS_MASK (~TS_MSB)
 
 static void update_pages_handler(struct work_struct *work);
 
 /*
  * The ring buffer header is special. We must manually up keep it.
  */
 int ring_buffer_print_entry_header(struct trace_seq *s)
 {
     trace_seq_puts(s, "# compressed entry header\n");
     trace_seq_puts(s, "\ttype_len    :    5 bits\n");
     trace_seq_puts(s, "\ttime_delta  :   27 bits\n");
     trace_seq_puts(s, "\tarray       :   32 bits\n");
     trace_seq_putc(s, '\n');
     trace_seq_printf(s, "\tpadding     : type == %d\n",
              RINGBUF_TYPE_PADDING);
     trace_seq_printf(s, "\ttime_extend : type == %d\n",
              RINGBUF_TYPE_TIME_EXTEND);
     trace_seq_printf(s, "\ttime_stamp : type == %d\n",
              RINGBUF_TYPE_TIME_STAMP);
     trace_seq_printf(s, "\tdata max type_len  == %d\n",
              RINGBUF_TYPE_DATA_TYPE_LEN_MAX);
 
     return !trace_seq_has_overflowed(s);
 }
 
 /*
  * The ring buffer is made up of a list of pages. A separate list of pages is
  * allocated for each CPU. A writer may only write to a buffer that is
  * associated with the CPU it is currently executing on.  A reader may read
  * from any per cpu buffer.
  *
  * The reader is special. For each per cpu buffer, the reader has its own
  * reader page. When a reader has read the entire reader page, this reader
  * page is swapped with another page in the ring buffer.
  *
  * Now, as long as the writer is off the reader page, the reader can do what
  * ever it wants with that page. The writer will never write to that page
  * again (as long as it is out of the ring buffer).
  *
  * Here's some silly ASCII art.
  *
  *   +------+
  *   |reader|          RING BUFFER
  *   |page  |
  *   +------+        +---+   +---+   +---+
  *                   |   |-->|   |-->|   |
  *                   +---+   +---+   +---+
  *                     ^               |
  *                     |               |
  *                     +---------------+
  *
  *
  *   +------+
  *   |reader|          RING BUFFER
  *   |page  |------------------v
  *   +------+        +---+   +---+   +---+
  *                   |   |-->|   |-->|   |
  *                   +---+   +---+   +---+
  *                     ^               |
  *                     |               |
  *                     +---------------+
  *
  *
  *   +------+
  *   |reader|          RING BUFFER
  *   |page  |------------------v
  *   +------+        +---+   +---+   +---+
  *      ^            |   |-->|   |-->|   |
  *      |            +---+   +---+   +---+
  *      |                              |
  *      |                              |
  *      +------------------------------+
  *
  *
  *   +------+
  *   |buffer|          RING BUFFER
  *   |page  |------------------v
  *   +------+        +---+   +---+   +---+
  *      ^            |   |   |   |-->|   |
  *      |   New      +---+   +---+   +---+
  *      |  Reader------^               |
  *      |   page                       |
  *      +------------------------------+
  *
  *
  * After we make this swap, the reader can hand this page off to the splice
  * code and be done with it. It can even allocate a new page if it needs to
  * and swap that into the ring buffer.
  *
  * We will be using cmpxchg soon to make all this lockless.
  *
  */
 
 /* Used for individual buffers (after the counter) */
 #define RB_BUFFER_OFF       (1 << 20)
 
 #define BUF_PAGE_HDR_SIZE offsetof(struct buffer_data_page, data)
 
 #define RB_EVNT_HDR_SIZE (offsetof(struct ring_buffer_event, array))
 #define RB_ALIGNMENT        4U
 #define RB_MAX_SMALL_DATA   (RB_ALIGNMENT * RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
 #define RB_EVNT_MIN_SIZE    8U  /* two 32bit words */
 
 #ifndef CONFIG_HAVE_64BIT_ALIGNED_ACCESS
 # define RB_FORCE_8BYTE_ALIGNMENT   0
 # define RB_ARCH_ALIGNMENT      RB_ALIGNMENT
 #else
 # define RB_FORCE_8BYTE_ALIGNMENT   1
 # define RB_ARCH_ALIGNMENT      8U
 #endif
 
 #define RB_ALIGN_DATA       __aligned(RB_ARCH_ALIGNMENT)
 
 /* define RINGBUF_TYPE_DATA for 'case RINGBUF_TYPE_DATA:' */
 #define RINGBUF_TYPE_DATA 0 ... RINGBUF_TYPE_DATA_TYPE_LEN_MAX
 
 enum {
     RB_LEN_TIME_EXTEND = 8,
     RB_LEN_TIME_STAMP =  8,
 };
 
 #define skip_time_extend(event) \
     ((struct ring_buffer_event *)((char *)event + RB_LEN_TIME_EXTEND))
 
 #define extended_time(event) \
     (event->type_len >= RINGBUF_TYPE_TIME_EXTEND)
 
 static inline int rb_null_event(struct ring_buffer_event *event)
 {
     return event->type_len == RINGBUF_TYPE_PADDING && !event->time_delta;
 }
 
 static void rb_event_set_padding(struct ring_buffer_event *event)
 {
     /* padding has a NULL time_delta */
     event->type_len = RINGBUF_TYPE_PADDING;
     event->time_delta = 0;
 }
 
 static unsigned
 rb_event_data_length(struct ring_buffer_event *event)
 {
     unsigned length;
 
     if (event->type_len)
         length = event->type_len * RB_ALIGNMENT;
     else
         length = event->array[0];
     return length + RB_EVNT_HDR_SIZE;
 }
 
 /*
  * Return the length of the given event. Will return
  * the length of the time extend if the event is a
  * time extend.
  */
 static inline unsigned
 rb_event_length(struct ring_buffer_event *event)
 {
     switch (event->type_len) {
     case RINGBUF_TYPE_PADDING:
         if (rb_null_event(event))
             /* undefined */
             return -1;
         return  event->array[0] + RB_EVNT_HDR_SIZE;
 
     case RINGBUF_TYPE_TIME_EXTEND:
         return RB_LEN_TIME_EXTEND;
 
     case RINGBUF_TYPE_TIME_STAMP:
         return RB_LEN_TIME_STAMP;
 
     case RINGBUF_TYPE_DATA:
         return rb_event_data_length(event);
     default:
         WARN_ON_ONCE(1);
     }
     /* not hit */
     return 0;
 }
 
 /*
  * Return total length of time extend and data,
  *   or just the event length for all other events.
  */
 static inline unsigned
 rb_event_ts_length(struct ring_buffer_event *event)
 {
     unsigned len = 0;
 
     if (extended_time(event)) {
         /* time extends include the data event after it */
         len = RB_LEN_TIME_EXTEND;
         event = skip_time_extend(event);
     }
     return len + rb_event_length(event);
 }
 
 /**
  * ring_buffer_event_length - return the length of the event
  * @event: the event to get the length of
  *
  * Returns the size of the data load of a data event.
  * If the event is something other than a data event, it
  * returns the size of the event itself. With the exception
  * of a TIME EXTEND, where it still returns the size of the
  * data load of the data event after it.
  */
 unsigned ring_buffer_event_length(struct ring_buffer_event *event)
 {
     unsigned length;
 
     if (extended_time(event))
         event = skip_time_extend(event);
 
     length = rb_event_length(event);
     if (event->type_len > RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
         return length;
     length -= RB_EVNT_HDR_SIZE;
     if (length > RB_MAX_SMALL_DATA + sizeof(event->array[0]))
                 length -= sizeof(event->array[0]);
     return length;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_event_length);
 
 /* inline for ring buffer fast paths */
 static __always_inline void *
 rb_event_data(struct ring_buffer_event *event)
 {
     if (extended_time(event))
         event = skip_time_extend(event);
     WARN_ON_ONCE(event->type_len > RINGBUF_TYPE_DATA_TYPE_LEN_MAX);
     /* If length is in len field, then array[0] has the data */
     if (event->type_len)
         return (void *)&event->array[0];
     /* Otherwise length is in array[0] and array[1] has the data */
     return (void *)&event->array[1];
 }
 
 /**
  * ring_buffer_event_data - return the data of the event
  * @event: the event to get the data from
  */
 void *ring_buffer_event_data(struct ring_buffer_event *event)
 {
     return rb_event_data(event);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_event_data);
 
 #define for_each_buffer_cpu(buffer, cpu)        \
     for_each_cpu(cpu, buffer->cpumask)
 
 #define for_each_online_buffer_cpu(buffer, cpu)     \
     for_each_cpu_and(cpu, buffer->cpumask, cpu_online_mask)
 
 #define TS_SHIFT    27
 #define TS_MASK     ((1ULL << TS_SHIFT) - 1)
 #define TS_DELTA_TEST   (~TS_MASK)
 
 static u64 rb_event_time_stamp(struct ring_buffer_event *event)
 {
     u64 ts;
 
     ts = event->array[0];
     ts <<= TS_SHIFT;
     ts += event->time_delta;
 
     return ts;
 }
 
 /* Flag when events were overwritten */
 #define RB_MISSED_EVENTS    (1 << 31)
 /* Missed count stored at end */
 #define RB_MISSED_STORED    (1 << 30)
 
 struct buffer_data_page {
     u64      time_stamp;    /* page time stamp */
     local_t      commit;    /* write committed index */
     unsigned char    data[] RB_ALIGN_DATA;  /* data of buffer page */
 };
 
 /*
  * Note, the buffer_page list must be first. The buffer pages
  * are allocated in cache lines, which means that each buffer
  * page will be at the beginning of a cache line, and thus
  * the least significant bits will be zero. We use this to
  * add flags in the list struct pointers, to make the ring buffer
  * lockless.
  */
 struct buffer_page {
     struct list_head list;      /* list of buffer pages */
     local_t      write;     /* index for next write */
     unsigned     read;      /* index for next read */
     local_t      entries;   /* entries on this page */
     unsigned long    real_end;  /* real end of data */
     struct buffer_data_page *page;  /* Actual data page */
 };
 
 /*
  * The buffer page counters, write and entries, must be reset
  * atomically when crossing page boundaries. To synchronize this
  * update, two counters are inserted into the number. One is
  * the actual counter for the write position or count on the page.
  *
  * The other is a counter of updaters. Before an update happens
  * the update partition of the counter is incremented. This will
  * allow the updater to update the counter atomically.
  *
  * The counter is 20 bits, and the state data is 12.
  */
 #define RB_WRITE_MASK       0xfffff
 #define RB_WRITE_INTCNT     (1 << 20)
 
 static void rb_init_page(struct buffer_data_page *bpage)
 {
     local_set(&bpage->commit, 0);
 }
 
 /*
  * Also stolen from mm/slob.c. Thanks to Mathieu Desnoyers for pointing
  * this issue out.
  */
 static void free_buffer_page(struct buffer_page *bpage)
 {
     free_page((unsigned long)bpage->page);
     kfree(bpage);
 }
 
 /*
  * We need to fit the time_stamp delta into 27 bits.
  */
 static inline int test_time_stamp(u64 delta)
 {
     if (delta & TS_DELTA_TEST)
         return 1;
     return 0;
 }
 
 #define BUF_PAGE_SIZE (PAGE_SIZE - BUF_PAGE_HDR_SIZE)
 
 /* Max payload is BUF_PAGE_SIZE - header (8bytes) */
 #define BUF_MAX_DATA_SIZE (BUF_PAGE_SIZE - (sizeof(u32) * 2))
 
 int ring_buffer_print_page_header(struct trace_seq *s)
 {
     struct buffer_data_page field;
 
     trace_seq_printf(s, "\tfield: u64 timestamp;\t"
              "offset:0;\tsize:%u;\tsigned:%u;\n",
              (unsigned int)sizeof(field.time_stamp),
              (unsigned int)is_signed_type(u64));
 
     trace_seq_printf(s, "\tfield: local_t commit;\t"
              "offset:%u;\tsize:%u;\tsigned:%u;\n",
              (unsigned int)offsetof(typeof(field), commit),
              (unsigned int)sizeof(field.commit),
              (unsigned int)is_signed_type(long));
 
     trace_seq_printf(s, "\tfield: int overwrite;\t"
              "offset:%u;\tsize:%u;\tsigned:%u;\n",
              (unsigned int)offsetof(typeof(field), commit),
              1,
              (unsigned int)is_signed_type(long));
 
     trace_seq_printf(s, "\tfield: char data;\t"
              "offset:%u;\tsize:%u;\tsigned:%u;\n",
              (unsigned int)offsetof(typeof(field), data),
              (unsigned int)BUF_PAGE_SIZE,
              (unsigned int)is_signed_type(char));
 
     return !trace_seq_has_overflowed(s);
 }
 
 struct rb_irq_work {
     struct irq_work         work;
     wait_queue_head_t       waiters;
     wait_queue_head_t       full_waiters;
     bool                waiters_pending;
     bool                full_waiters_pending;
     bool                wakeup_full;
 };
 
 /*
  * Structure to hold event state and handle nested events.
  */
 struct rb_event_info {
     u64         ts;
     u64         delta;
     u64         before;
     u64         after;
     unsigned long       length;
     struct buffer_page  *tail_page;
     int         add_timestamp;
 };
 
 /*
  * Used for the add_timestamp
  *  NONE
  *  EXTEND - wants a time extend
  *  ABSOLUTE - the buffer requests all events to have absolute time stamps
  *  FORCE - force a full time stamp.
  */
 enum {
     RB_ADD_STAMP_NONE       = 0,
     RB_ADD_STAMP_EXTEND     = BIT(1),
     RB_ADD_STAMP_ABSOLUTE       = BIT(2),
     RB_ADD_STAMP_FORCE      = BIT(3)
 };
 /*
  * Used for which event context the event is in.
  *  TRANSITION = 0
  *  NMI     = 1
  *  IRQ     = 2
  *  SOFTIRQ = 3
  *  NORMAL  = 4
  *
  * See trace_recursive_lock() comment below for more details.
  */
 enum {
     RB_CTX_TRANSITION,
     RB_CTX_NMI,
     RB_CTX_IRQ,
     RB_CTX_SOFTIRQ,
     RB_CTX_NORMAL,
     RB_CTX_MAX
 };
 
 #if BITS_PER_LONG == 32
 #define RB_TIME_32
 #endif
 
 /* To test on 64 bit machines */
 //#define RB_TIME_32
 
 #ifdef RB_TIME_32
 
 struct rb_time_struct {
     local_t     cnt;
     local_t     top;
     local_t     bottom;
     local_t     msb;
 };
 #else
 #include <asm/local64.h>
 struct rb_time_struct {
     local64_t   time;
 };
 #endif
 typedef struct rb_time_struct rb_time_t;
 
 #define MAX_NEST    5
 
 /*
  * head_page == tail_page && head == tail then buffer is empty.
  */
 struct ring_buffer_per_cpu {
     int             cpu;
     atomic_t            record_disabled;
     atomic_t            resize_disabled;
     struct trace_buffer *buffer;
     raw_spinlock_t          reader_lock;    /* serialize readers */
     arch_spinlock_t         lock;
     struct lock_class_key       lock_key;
     struct buffer_data_page     *free_page;
     unsigned long           nr_pages;
     unsigned int            current_context;
     struct list_head        *pages;
     struct buffer_page      *head_page; /* read from head */
     struct buffer_page      *tail_page; /* write to tail */
     struct buffer_page      *commit_page;   /* committed pages */
     struct buffer_page      *reader_page;
     unsigned long           lost_events;
     unsigned long           last_overrun;
     unsigned long           nest;
     local_t             entries_bytes;
     local_t             entries;
     local_t             overrun;
     local_t             commit_overrun;
     local_t             dropped_events;
     local_t             committing;
     local_t             commits;
     local_t             pages_touched;
     local_t             pages_read;
     long                last_pages_touch;
     size_t              shortest_full;
     unsigned long           read;
     unsigned long           read_bytes;
     rb_time_t           write_stamp;
     rb_time_t           before_stamp;
     u64             event_stamp[MAX_NEST];
     u64             read_stamp;
     /* ring buffer pages to update, > 0 to add, < 0 to remove */
     long                nr_pages_to_update;
     struct list_head        new_pages; /* new pages to add */
     struct work_struct      update_pages_work;
     struct completion       update_done;
 
     struct rb_irq_work      irq_work;
 };
 
 struct trace_buffer {
     unsigned            flags;
     int             cpus;
     atomic_t            record_disabled;
     cpumask_var_t           cpumask;
 
     struct lock_class_key       *reader_lock_key;
 
     struct mutex            mutex;
 
     struct ring_buffer_per_cpu  **buffers;
 
     struct hlist_node       node;
     u64             (*clock)(void);
 
     struct rb_irq_work      irq_work;
     bool                time_stamp_abs;
 };
 
 struct ring_buffer_iter {
     struct ring_buffer_per_cpu  *cpu_buffer;
     unsigned long           head;
     unsigned long           next_event;
     struct buffer_page      *head_page;
     struct buffer_page      *cache_reader_page;
     unsigned long           cache_read;
     u64             read_stamp;
     u64             page_stamp;
     struct ring_buffer_event    *event;
     int             missed_events;
 };
 
 #ifdef RB_TIME_32
 
 /*
  * On 32 bit machines, local64_t is very expensive. As the ring
  * buffer doesn't need all the features of a true 64 bit atomic,
  * on 32 bit, it uses these functions (64 still uses local64_t).
  *
  * For the ring buffer, 64 bit required operations for the time is
  * the following:
  *
  *  - Reads may fail if it interrupted a modification of the time stamp.
  *      It will succeed if it did not interrupt another write even if
  *      the read itself is interrupted by a write.
  *      It returns whether it was successful or not.
  *
  *  - Writes always succeed and will overwrite other writes and writes
  *      that were done by events interrupting the current write.
  *
  *  - A write followed by a read of the same time stamp will always succeed,
  *      but may not contain the same value.
  *
  *  - A cmpxchg will fail if it interrupted another write or cmpxchg.
  *      Other than that, it acts like a normal cmpxchg.
  *
  * The 60 bit time stamp is broken up by 30 bits in a top and bottom half
  *  (bottom being the least significant 30 bits of the 60 bit time stamp).
  *
  * The two most significant bits of each half holds a 2 bit counter (0-3).
  * Each update will increment this counter by one.
  * When reading the top and bottom, if the two counter bits match then the
  *  top and bottom together make a valid 60 bit number.
  */
 #define RB_TIME_SHIFT   30
 #define RB_TIME_VAL_MASK ((1 << RB_TIME_SHIFT) - 1)
 #define RB_TIME_MSB_SHIFT    60
 
 static inline int rb_time_cnt(unsigned long val)
 {
     return (val >> RB_TIME_SHIFT) & 3;
 }
 
 static inline u64 rb_time_val(unsigned long top, unsigned long bottom)
 {
     u64 val;
 
     val = top & RB_TIME_VAL_MASK;
     val <<= RB_TIME_SHIFT;
     val |= bottom & RB_TIME_VAL_MASK;
 
     return val;
 }
 
 static inline bool __rb_time_read(rb_time_t *t, u64 *ret, unsigned long *cnt)
 {
     unsigned long top, bottom, msb;
     unsigned long c;
 
     /*
      * If the read is interrupted by a write, then the cnt will
      * be different. Loop until both top and bottom have been read
      * without interruption.
      */
     do {
         c = local_read(&t->cnt);
         top = local_read(&t->top);
         bottom = local_read(&t->bottom);
         msb = local_read(&t->msb);
     } while (c != local_read(&t->cnt));
 
     *cnt = rb_time_cnt(top);
 
     /* If top and bottom counts don't match, this interrupted a write */
     if (*cnt != rb_time_cnt(bottom))
         return false;
 
     /* The shift to msb will lose its cnt bits */
     *ret = rb_time_val(top, bottom) | ((u64)msb << RB_TIME_MSB_SHIFT);
     return true;
 }
 
 static bool rb_time_read(rb_time_t *t, u64 *ret)
 {
     unsigned long cnt;
 
     return __rb_time_read(t, ret, &cnt);
 }
 
 static inline unsigned long rb_time_val_cnt(unsigned long val, unsigned long cnt)
 {
     return (val & RB_TIME_VAL_MASK) | ((cnt & 3) << RB_TIME_SHIFT);
 }
 
 static inline void rb_time_split(u64 val, unsigned long *top, unsigned long *bottom,
                  unsigned long *msb)
 {
     *top = (unsigned long)((val >> RB_TIME_SHIFT) & RB_TIME_VAL_MASK);
     *bottom = (unsigned long)(val & RB_TIME_VAL_MASK);
     *msb = (unsigned long)(val >> RB_TIME_MSB_SHIFT);
 }
 
 static inline void rb_time_val_set(local_t *t, unsigned long val, unsigned long cnt)
 {
     val = rb_time_val_cnt(val, cnt);
     local_set(t, val);
 }
 
 static void rb_time_set(rb_time_t *t, u64 val)
 {
     unsigned long cnt, top, bottom, msb;
 
     rb_time_split(val, &top, &bottom, &msb);
 
     /* Writes always succeed with a valid number even if it gets interrupted. */
     do {
         cnt = local_inc_return(&t->cnt);
         rb_time_val_set(&t->top, top, cnt);
         rb_time_val_set(&t->bottom, bottom, cnt);
         rb_time_val_set(&t->msb, val >> RB_TIME_MSB_SHIFT, cnt);
     } while (cnt != local_read(&t->cnt));
 }
 
 static inline bool
 rb_time_read_cmpxchg(local_t *l, unsigned long expect, unsigned long set)
 {
     unsigned long ret;
 
     ret = local_cmpxchg(l, expect, set);
     return ret == expect;
 }
 
 static int rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
 {
     unsigned long cnt, top, bottom, msb;
     unsigned long cnt2, top2, bottom2, msb2;
     u64 val;
 
     /* The cmpxchg always fails if it interrupted an update */
      if (!__rb_time_read(t, &val, &cnt2))
          return false;
 
      if (val != expect)
          return false;
 
      cnt = local_read(&t->cnt);
      if ((cnt & 3) != cnt2)
          return false;
 
      cnt2 = cnt + 1;
 
      rb_time_split(val, &top, &bottom, &msb);
      top = rb_time_val_cnt(top, cnt);
      bottom = rb_time_val_cnt(bottom, cnt);
 
      rb_time_split(set, &top2, &bottom2, &msb2);
      top2 = rb_time_val_cnt(top2, cnt2);
      bottom2 = rb_time_val_cnt(bottom2, cnt2);
 
     if (!rb_time_read_cmpxchg(&t->cnt, cnt, cnt2))
         return false;
     if (!rb_time_read_cmpxchg(&t->msb, msb, msb2))
         return false;
     if (!rb_time_read_cmpxchg(&t->top, top, top2))
         return false;
     if (!rb_time_read_cmpxchg(&t->bottom, bottom, bottom2))
         return false;
     return true;
 }
 
 #else /* 64 bits */
 
 /* local64_t always succeeds */
 
 static inline bool rb_time_read(rb_time_t *t, u64 *ret)
 {
     *ret = local64_read(&t->time);
     return true;
 }
 static void rb_time_set(rb_time_t *t, u64 val)
 {
     local64_set(&t->time, val);
 }
 
 static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
 {
     u64 val;
     val = local64_cmpxchg(&t->time, expect, set);
     return val == expect;
 }
 #endif
 
 /*
  * Enable this to make sure that the event passed to
  * ring_buffer_event_time_stamp() is not committed and also
  * is on the buffer that it passed in.
  */
 //#define RB_VERIFY_EVENT
 #ifdef RB_VERIFY_EVENT
 static struct list_head *rb_list_head(struct list_head *list);
 static void verify_event(struct ring_buffer_per_cpu *cpu_buffer,
              void *event)
 {
     struct buffer_page *page = cpu_buffer->commit_page;
     struct buffer_page *tail_page = READ_ONCE(cpu_buffer->tail_page);
     struct list_head *next;
     long commit, write;
     unsigned long addr = (unsigned long)event;
     bool done = false;
     int stop = 0;
 
     /* Make sure the event exists and is not committed yet */
     do {
         if (page == tail_page || WARN_ON_ONCE(stop++ > 100))
             done = true;
         commit = local_read(&page->page->commit);
         write = local_read(&page->write);
         if (addr >= (unsigned long)&page->page->data[commit] &&
             addr < (unsigned long)&page->page->data[write])
             return;
 
         next = rb_list_head(page->list.next);
         page = list_entry(next, struct buffer_page, list);
     } while (!done);
     WARN_ON_ONCE(1);
 }
 #else
 static inline void verify_event(struct ring_buffer_per_cpu *cpu_buffer,
              void *event)
 {
 }
 #endif
 
 /*
  * The absolute time stamp drops the 5 MSBs and some clocks may
  * require them. The rb_fix_abs_ts() will take a previous full
  * time stamp, and add the 5 MSB of that time stamp on to the
  * saved absolute time stamp. Then they are compared in case of
  * the unlikely event that the latest time stamp incremented
  * the 5 MSB.
  */
 static inline u64 rb_fix_abs_ts(u64 abs, u64 save_ts)
 {
     if (save_ts & TS_MSB) {
         abs |= save_ts & TS_MSB;
         /* Check for overflow */
         if (unlikely(abs < save_ts))
             abs += 1ULL << 59;
     }
     return abs;
 }
 
 static inline u64 rb_time_stamp(struct trace_buffer *buffer);
 
 /**
  * ring_buffer_event_time_stamp - return the event's current time stamp
  * @buffer: The buffer that the event is on
  * @event: the event to get the time stamp of
  *
  * Note, this must be called after @event is reserved, and before it is
  * committed to the ring buffer. And must be called from the same
  * context where the event was reserved (normal, softirq, irq, etc).
  *
  * Returns the time stamp associated with the current event.
  * If the event has an extended time stamp, then that is used as
  * the time stamp to return.
  * In the highly unlikely case that the event was nested more than
  * the max nesting, then the write_stamp of the buffer is returned,
  * otherwise  current time is returned, but that really neither of
  * the last two cases should ever happen.
  */
 u64 ring_buffer_event_time_stamp(struct trace_buffer *buffer,
                  struct ring_buffer_event *event)
 {
     struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[smp_processor_id()];
     unsigned int nest;
     u64 ts;
 
     /* If the event includes an absolute time, then just use that */
     if (event->type_len == RINGBUF_TYPE_TIME_STAMP) {
         ts = rb_event_time_stamp(event);
         return rb_fix_abs_ts(ts, cpu_buffer->tail_page->page->time_stamp);
     }
 
     nest = local_read(&cpu_buffer->committing);
     verify_event(cpu_buffer, event);
     if (WARN_ON_ONCE(!nest))
         goto fail;
 
     /* Read the current saved nesting level time stamp */
     if (likely(--nest < MAX_NEST))
         return cpu_buffer->event_stamp[nest];
 
     /* Shouldn't happen, warn if it does */
     WARN_ONCE(1, "nest (%d) greater than max", nest);
 
  fail:
     /* Can only fail on 32 bit */
     if (!rb_time_read(&cpu_buffer->write_stamp, &ts))
         /* Screw it, just read the current time */
         ts = rb_time_stamp(cpu_buffer->buffer);
 
     return ts;
 }
 
 /**
  * ring_buffer_nr_pages - get the number of buffer pages in the ring buffer
  * @buffer: The ring_buffer to get the number of pages from
  * @cpu: The cpu of the ring_buffer to get the number of pages from
  *
  * Returns the number of pages used by a per_cpu buffer of the ring buffer.
  */
 size_t ring_buffer_nr_pages(struct trace_buffer *buffer, int cpu)
 {
     return buffer->buffers[cpu]->nr_pages;
 }
 
 /**
  * ring_buffer_nr_pages_dirty - get the number of used pages in the ring buffer
  * @buffer: The ring_buffer to get the number of pages from
  * @cpu: The cpu of the ring_buffer to get the number of pages from
  *
  * Returns the number of pages that have content in the ring buffer.
  */
 size_t ring_buffer_nr_dirty_pages(struct trace_buffer *buffer, int cpu)
 {
     size_t read;
     size_t cnt;
 
     read = local_read(&buffer->buffers[cpu]->pages_read);
     cnt = local_read(&buffer->buffers[cpu]->pages_touched);
     /* The reader can read an empty page, but not more than that */
     if (cnt < read) {
         WARN_ON_ONCE(read > cnt + 1);
         return 0;
     }
 
     return cnt - read;
 }
 
 /*
  * rb_wake_up_waiters - wake up tasks waiting for ring buffer input
  *
  * Schedules a delayed work to wake up any task that is blocked on the
  * ring buffer waiters queue.
  */
 static void rb_wake_up_waiters(struct irq_work *work)
 {
     struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work, work);
 
     wake_up_all(&rbwork->waiters);
     if (rbwork->wakeup_full) {
         rbwork->wakeup_full = false;
         wake_up_all(&rbwork->full_waiters);
     }
 }
 
 /**
  * ring_buffer_wait - wait for input to the ring buffer
  * @buffer: buffer to wait on
  * @cpu: the cpu buffer to wait on
  * @full: wait until the percentage of pages are available, if @cpu != RING_BUFFER_ALL_CPUS
  *
  * If @cpu == RING_BUFFER_ALL_CPUS then the task will wake up as soon
  * as data is added to any of the @buffer's cpu buffers. Otherwise
  * it will wait for data to be added to a specific cpu buffer.
  */
 int ring_buffer_wait(struct trace_buffer *buffer, int cpu, int full)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     DEFINE_WAIT(wait);
     struct rb_irq_work *work;
     int ret = 0;
 
     /*
      * Depending on what the caller is waiting for, either any
      * data in any cpu buffer, or a specific buffer, put the
      * caller on the appropriate wait queue.
      */
     if (cpu == RING_BUFFER_ALL_CPUS) {
         work = &buffer->irq_work;
         /* Full only makes sense on per cpu reads */
         full = 0;
     } else {
         if (!cpumask_test_cpu(cpu, buffer->cpumask))
             return -ENODEV;
         cpu_buffer = buffer->buffers[cpu];
         work = &cpu_buffer->irq_work;
     }
 
 
     while (true) {
         if (full)
             prepare_to_wait(&work->full_waiters, &wait, TASK_INTERRUPTIBLE);
         else
             prepare_to_wait(&work->waiters, &wait, TASK_INTERRUPTIBLE);
 
         /*
          * The events can happen in critical sections where
          * checking a work queue can cause deadlocks.
          * After adding a task to the queue, this flag is set
          * only to notify events to try to wake up the queue
          * using irq_work.
          *
          * We don't clear it even if the buffer is no longer
          * empty. The flag only causes the next event to run
          * irq_work to do the work queue wake up. The worse
          * that can happen if we race with !trace_empty() is that
          * an event will cause an irq_work to try to wake up
          * an empty queue.
          *
          * There's no reason to protect this flag either, as
          * the work queue and irq_work logic will do the necessary
          * synchronization for the wake ups. The only thing
          * that is necessary is that the wake up happens after
          * a task has been queued. It's OK for spurious wake ups.
          */
         if (full)
             work->full_waiters_pending = true;
         else
             work->waiters_pending = true;
 
         if (signal_pending(current)) {
             ret = -EINTR;
             break;
         }
 
         if (cpu == RING_BUFFER_ALL_CPUS && !ring_buffer_empty(buffer))
             break;
 
         if (cpu != RING_BUFFER_ALL_CPUS &&
             !ring_buffer_empty_cpu(buffer, cpu)) {
             unsigned long flags;
             bool pagebusy;
             size_t nr_pages;
             size_t dirty;
 
             if (!full)
                 break;
 
             raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
             pagebusy = cpu_buffer->reader_page == cpu_buffer->commit_page;
             nr_pages = cpu_buffer->nr_pages;
             dirty = ring_buffer_nr_dirty_pages(buffer, cpu);
             if (!cpu_buffer->shortest_full ||
                 cpu_buffer->shortest_full < full)
                 cpu_buffer->shortest_full = full;
             raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
             if (!pagebusy &&
                 (!nr_pages || (dirty * 100) > full * nr_pages))
                 break;
         }
 
         schedule();
     }
 
     if (full)
         finish_wait(&work->full_waiters, &wait);
     else
         finish_wait(&work->waiters, &wait);
 
     return ret;
 }
 
 /**
  * ring_buffer_poll_wait - poll on buffer input
  * @buffer: buffer to wait on
  * @cpu: the cpu buffer to wait on
  * @filp: the file descriptor
  * @poll_table: The poll descriptor
  *
  * If @cpu == RING_BUFFER_ALL_CPUS then the task will wake up as soon
  * as data is added to any of the @buffer's cpu buffers. Otherwise
  * it will wait for data to be added to a specific cpu buffer.
  *
  * Returns EPOLLIN | EPOLLRDNORM if data exists in the buffers,
  * zero otherwise.
  */
 __poll_t ring_buffer_poll_wait(struct trace_buffer *buffer, int cpu,
               struct file *filp, poll_table *poll_table)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     struct rb_irq_work *work;
 
     if (cpu == RING_BUFFER_ALL_CPUS)
         work = &buffer->irq_work;
     else {
         if (!cpumask_test_cpu(cpu, buffer->cpumask))
             return -EINVAL;
 
         cpu_buffer = buffer->buffers[cpu];
         work = &cpu_buffer->irq_work;
     }
 
     poll_wait(filp, &work->waiters, poll_table);
     work->waiters_pending = true;
     /*
      * There's a tight race between setting the waiters_pending and
      * checking if the ring buffer is empty.  Once the waiters_pending bit
      * is set, the next event will wake the task up, but we can get stuck
      * if there's only a single event in.
      *
      * FIXME: Ideally, we need a memory barrier on the writer side as well,
      * but adding a memory barrier to all events will cause too much of a
      * performance hit in the fast path.  We only need a memory barrier when
      * the buffer goes from empty to having content.  But as this race is
      * extremely small, and it's not a problem if another event comes in, we
      * will fix it later.
      */
     smp_mb();
 
     if ((cpu == RING_BUFFER_ALL_CPUS && !ring_buffer_empty(buffer)) ||
         (cpu != RING_BUFFER_ALL_CPUS && !ring_buffer_empty_cpu(buffer, cpu)))
         return EPOLLIN | EPOLLRDNORM;
     return 0;
 }
 
 /* buffer may be either ring_buffer or ring_buffer_per_cpu */
 #define RB_WARN_ON(b, cond)                     \
     ({                              \
         int _____ret = unlikely(cond);              \
         if (_____ret) {                     \
             if (__same_type(*(b), struct ring_buffer_per_cpu)) { \
                 struct ring_buffer_per_cpu *__b =   \
                     (void *)b;          \
                 atomic_inc(&__b->buffer->record_disabled); \
             } else                      \
                 atomic_inc(&b->record_disabled);    \
             WARN_ON(1);                 \
         }                           \
         _____ret;                       \
     })
 
 /* Up this if you want to test the TIME_EXTENTS and normalization */
 #define DEBUG_SHIFT 0
 
 static inline u64 rb_time_stamp(struct trace_buffer *buffer)
 {
     u64 ts;
 
     /* Skip retpolines :-( */
     if (IS_ENABLED(CONFIG_RETPOLINE) && likely(buffer->clock == trace_clock_local))
         ts = trace_clock_local();
     else
         ts = buffer->clock();
 
     /* shift to debug/test normalization and TIME_EXTENTS */
     return ts << DEBUG_SHIFT;
 }
 
 u64 ring_buffer_time_stamp(struct trace_buffer *buffer)
 {
     u64 time;
 
     preempt_disable_notrace();
     time = rb_time_stamp(buffer);
     preempt_enable_notrace();
 
     return time;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_time_stamp);
 
 void ring_buffer_normalize_time_stamp(struct trace_buffer *buffer,
                       int cpu, u64 *ts)
 {
     /* Just stupid testing the normalize function and deltas */
     *ts >>= DEBUG_SHIFT;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_normalize_time_stamp);
 
 /*
  * Making the ring buffer lockless makes things tricky.
  * Although writes only happen on the CPU that they are on,
  * and they only need to worry about interrupts. Reads can
  * happen on any CPU.
  *
  * The reader page is always off the ring buffer, but when the
  * reader finishes with a page, it needs to swap its page with
  * a new one from the buffer. The reader needs to take from
  * the head (writes go to the tail). But if a writer is in overwrite
  * mode and wraps, it must push the head page forward.
  *
  * Here lies the problem.
  *
  * The reader must be careful to replace only the head page, and
  * not another one. As described at the top of the file in the
  * ASCII art, the reader sets its old page to point to the next
  * page after head. It then sets the page after head to point to
  * the old reader page. But if the writer moves the head page
  * during this operation, the reader could end up with the tail.
  *
  * We use cmpxchg to help prevent this race. We also do something
  * special with the page before head. We set the LSB to 1.
  *
  * When the writer must push the page forward, it will clear the
  * bit that points to the head page, move the head, and then set
  * the bit that points to the new head page.
  *
  * We also don't want an interrupt coming in and moving the head
  * page on another writer. Thus we use the second LSB to catch
  * that too. Thus:
  *
  * head->list->prev->next        bit 1          bit 0
  *                              -------        -------
  * Normal page                     0              0
  * Points to head page             0              1
  * New head page                   1              0
  *
  * Note we can not trust the prev pointer of the head page, because:
  *
  * +----+       +-----+        +-----+
  * |    |------>|  T  |---X--->|  N  |
  * |    |<------|     |        |     |
  * +----+       +-----+        +-----+
  *   ^                           ^ |
  *   |          +-----+          | |
  *   +----------|  R  |----------+ |
  *              |     |<-----------+
  *              +-----+
  *
  * Key:  ---X-->  HEAD flag set in pointer
  *         T      Tail page
  *         R      Reader page
  *         N      Next page
  *
  * (see __rb_reserve_next() to see where this happens)
  *
  *  What the above shows is that the reader just swapped out
  *  the reader page with a page in the buffer, but before it
  *  could make the new header point back to the new page added
  *  it was preempted by a writer. The writer moved forward onto
  *  the new page added by the reader and is about to move forward
  *  again.
  *
  *  You can see, it is legitimate for the previous pointer of
  *  the head (or any page) not to point back to itself. But only
  *  temporarily.
  */
 
 #define RB_PAGE_NORMAL      0UL
 #define RB_PAGE_HEAD        1UL
 #define RB_PAGE_UPDATE      2UL
 
 
 #define RB_FLAG_MASK        3UL
 
 /* PAGE_MOVED is not part of the mask */
 #define RB_PAGE_MOVED       4UL
 
 /*
  * rb_list_head - remove any bit
  */
 static struct list_head *rb_list_head(struct list_head *list)
 {
     unsigned long val = (unsigned long)list;
 
     return (struct list_head *)(val & ~RB_FLAG_MASK);
 }
 
 /*
  * rb_is_head_page - test if the given page is the head page
  *
  * Because the reader may move the head_page pointer, we can
  * not trust what the head page is (it may be pointing to
  * the reader page). But if the next page is a header page,
  * its flags will be non zero.
  */
 static inline int
 rb_is_head_page(struct buffer_page *page, struct list_head *list)
 {
     unsigned long val;
 
     val = (unsigned long)list->next;
 
     if ((val & ~RB_FLAG_MASK) != (unsigned long)&page->list)
         return RB_PAGE_MOVED;
 
     return val & RB_FLAG_MASK;
 }
 
 /*
  * rb_is_reader_page
  *
  * The unique thing about the reader page, is that, if the
  * writer is ever on it, the previous pointer never points
  * back to the reader page.
  */
 static bool rb_is_reader_page(struct buffer_page *page)
 {
     struct list_head *list = page->list.prev;
 
     return rb_list_head(list->next) != &page->list;
 }
 
 /*
  * rb_set_list_to_head - set a list_head to be pointing to head.
  */
 static void rb_set_list_to_head(struct list_head *list)
 {
     unsigned long *ptr;
 
     ptr = (unsigned long *)&list->next;
     *ptr |= RB_PAGE_HEAD;
     *ptr &= ~RB_PAGE_UPDATE;
 }
 
 /*
  * rb_head_page_activate - sets up head page
  */
 static void rb_head_page_activate(struct ring_buffer_per_cpu *cpu_buffer)
 {
     struct buffer_page *head;
 
     head = cpu_buffer->head_page;
     if (!head)
         return;
 
     /*
      * Set the previous list pointer to have the HEAD flag.
      */
     rb_set_list_to_head(head->list.prev);
 }
 
 static void rb_list_head_clear(struct list_head *list)
 {
     unsigned long *ptr = (unsigned long *)&list->next;
 
     *ptr &= ~RB_FLAG_MASK;
 }
 
 /*
  * rb_head_page_deactivate - clears head page ptr (for free list)
  */
 static void
 rb_head_page_deactivate(struct ring_buffer_per_cpu *cpu_buffer)
 {
     struct list_head *hd;
 
     /* Go through the whole list and clear any pointers found. */
     rb_list_head_clear(cpu_buffer->pages);
 
     list_for_each(hd, cpu_buffer->pages)
         rb_list_head_clear(hd);
 }
 
 static int rb_head_page_set(struct ring_buffer_per_cpu *cpu_buffer,
                 struct buffer_page *head,
                 struct buffer_page *prev,
                 int old_flag, int new_flag)
 {
     struct list_head *list;
     unsigned long val = (unsigned long)&head->list;
     unsigned long ret;
 
     list = &prev->list;
 
     val &= ~RB_FLAG_MASK;
 
     ret = cmpxchg((unsigned long *)&list->next,
               val | old_flag, val | new_flag);
 
     /* check if the reader took the page */
     if ((ret & ~RB_FLAG_MASK) != val)
         return RB_PAGE_MOVED;
 
     return ret & RB_FLAG_MASK;
 }
 
 static int rb_head_page_set_update(struct ring_buffer_per_cpu *cpu_buffer,
                    struct buffer_page *head,
                    struct buffer_page *prev,
                    int old_flag)
 {
     return rb_head_page_set(cpu_buffer, head, prev,
                 old_flag, RB_PAGE_UPDATE);
 }
 
 static int rb_head_page_set_head(struct ring_buffer_per_cpu *cpu_buffer,
                  struct buffer_page *head,
                  struct buffer_page *prev,
                  int old_flag)
 {
     return rb_head_page_set(cpu_buffer, head, prev,
                 old_flag, RB_PAGE_HEAD);
 }
 
 static int rb_head_page_set_normal(struct ring_buffer_per_cpu *cpu_buffer,
                    struct buffer_page *head,
                    struct buffer_page *prev,
                    int old_flag)
 {
     return rb_head_page_set(cpu_buffer, head, prev,
                 old_flag, RB_PAGE_NORMAL);
 }
 
 static inline void rb_inc_page(struct buffer_page **bpage)
 {
     struct list_head *p = rb_list_head((*bpage)->list.next);
 
     *bpage = list_entry(p, struct buffer_page, list);
 }
 
 static struct buffer_page *
 rb_set_head_page(struct ring_buffer_per_cpu *cpu_buffer)
 {
     struct buffer_page *head;
     struct buffer_page *page;
     struct list_head *list;
     int i;
 
     if (RB_WARN_ON(cpu_buffer, !cpu_buffer->head_page))
         return NULL;
 
     /* sanity check */
     list = cpu_buffer->pages;
     if (RB_WARN_ON(cpu_buffer, rb_list_head(list->prev->next) != list))
         return NULL;
 
     page = head = cpu_buffer->head_page;
     /*
      * It is possible that the writer moves the header behind
      * where we started, and we miss in one loop.
      * A second loop should grab the header, but we'll do
      * three loops just because I'm paranoid.
      */
     for (i = 0; i < 3; i++) {
         do {
             if (rb_is_head_page(page, page->list.prev)) {
                 cpu_buffer->head_page = page;
                 return page;
             }
             rb_inc_page(&page);
         } while (page != head);
     }
 
     RB_WARN_ON(cpu_buffer, 1);
 
     return NULL;
 }
 
 static int rb_head_page_replace(struct buffer_page *old,
                 struct buffer_page *new)
 {
     unsigned long *ptr = (unsigned long *)&old->list.prev->next;
     unsigned long val;
     unsigned long ret;
 
     val = *ptr & ~RB_FLAG_MASK;
     val |= RB_PAGE_HEAD;
 
     ret = cmpxchg(ptr, val, (unsigned long)&new->list);
 
     return ret == val;
 }
 
 /*
  * rb_tail_page_update - move the tail page forward
  */
 static void rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer,
                    struct buffer_page *tail_page,
                    struct buffer_page *next_page)
 {
     unsigned long old_entries;
     unsigned long old_write;
 
     /*
      * The tail page now needs to be moved forward.
      *
      * We need to reset the tail page, but without messing
      * with possible erasing of data brought in by interrupts
      * that have moved the tail page and are currently on it.
      *
      * We add a counter to the write field to denote this.
      */
     old_write = local_add_return(RB_WRITE_INTCNT, &next_page->write);
     old_entries = local_add_return(RB_WRITE_INTCNT, &next_page->entries);
 
     local_inc(&cpu_buffer->pages_touched);
     /*
      * Just make sure we have seen our old_write and synchronize
      * with any interrupts that come in.
      */
     barrier();
 
     /*
      * If the tail page is still the same as what we think
      * it is, then it is up to us to update the tail
      * pointer.
      */
     if (tail_page == READ_ONCE(cpu_buffer->tail_page)) {
         /* Zero the write counter */
         unsigned long val = old_write & ~RB_WRITE_MASK;
         unsigned long eval = old_entries & ~RB_WRITE_MASK;
 
         /*
          * This will only succeed if an interrupt did
          * not come in and change it. In which case, we
          * do not want to modify it.
          *
          * We add (void) to let the compiler know that we do not care
          * about the return value of these functions. We use the
          * cmpxchg to only update if an interrupt did not already
          * do it for us. If the cmpxchg fails, we don't care.
          */
         (void)local_cmpxchg(&next_page->write, old_write, val);
         (void)local_cmpxchg(&next_page->entries, old_entries, eval);
 
         /*
          * No need to worry about races with clearing out the commit.
          * it only can increment when a commit takes place. But that
          * only happens in the outer most nested commit.
          */
         local_set(&next_page->page->commit, 0);
 
         /* Again, either we update tail_page or an interrupt does */
         (void)cmpxchg(&cpu_buffer->tail_page, tail_page, next_page);
     }
 }
 
 static int rb_check_bpage(struct ring_buffer_per_cpu *cpu_buffer,
               struct buffer_page *bpage)
 {
     unsigned long val = (unsigned long)bpage;
 
     if (RB_WARN_ON(cpu_buffer, val & RB_FLAG_MASK))
         return 1;
 
     return 0;
 }
 
 /**
  * rb_check_list - make sure a pointer to a list has the last bits zero
  */
 static int rb_check_list(struct ring_buffer_per_cpu *cpu_buffer,
              struct list_head *list)
 {
     if (RB_WARN_ON(cpu_buffer, rb_list_head(list->prev) != list->prev))
         return 1;
     if (RB_WARN_ON(cpu_buffer, rb_list_head(list->next) != list->next))
         return 1;
     return 0;
 }
 
 /**
  * rb_check_pages - integrity check of buffer pages
  * @cpu_buffer: CPU buffer with pages to test
  *
  * As a safety measure we check to make sure the data pages have not
  * been corrupted.
  */
 static int rb_check_pages(struct ring_buffer_per_cpu *cpu_buffer)
 {
     struct list_head *head = cpu_buffer->pages;
     struct buffer_page *bpage, *tmp;
 
     /* Reset the head page if it exists */
     if (cpu_buffer->head_page)
         rb_set_head_page(cpu_buffer);
 
     rb_head_page_deactivate(cpu_buffer);
 
     if (RB_WARN_ON(cpu_buffer, head->next->prev != head))
         return -1;
     if (RB_WARN_ON(cpu_buffer, head->prev->next != head))
         return -1;
 
     if (rb_check_list(cpu_buffer, head))
         return -1;
 
     list_for_each_entry_safe(bpage, tmp, head, list) {
         if (RB_WARN_ON(cpu_buffer,
                    bpage->list.next->prev != &bpage->list))
             return -1;
         if (RB_WARN_ON(cpu_buffer,
                    bpage->list.prev->next != &bpage->list))
             return -1;
         if (rb_check_list(cpu_buffer, &bpage->list))
             return -1;
     }
 
     rb_head_page_activate(cpu_buffer);
 
     return 0;
 }
 
 static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
         long nr_pages, struct list_head *pages)
 {
     struct buffer_page *bpage, *tmp;
     bool user_thread = current->mm != NULL;
     gfp_t mflags;
     long i;
 
     /*
      * Check if the available memory is there first.
      * Note, si_mem_available() only gives us a rough estimate of available
      * memory. It may not be accurate. But we don't care, we just want
      * to prevent doing any allocation when it is obvious that it is
      * not going to succeed.
      */
     i = si_mem_available();
     if (i < nr_pages)
         return -ENOMEM;
 
     /*
      * __GFP_RETRY_MAYFAIL flag makes sure that the allocation fails
      * gracefully without invoking oom-killer and the system is not
      * destabilized.
      */
     mflags = GFP_KERNEL | __GFP_RETRY_MAYFAIL;
 
     /*
      * If a user thread allocates too much, and si_mem_available()
      * reports there's enough memory, even though there is not.
      * Make sure the OOM killer kills this thread. This can happen
      * even with RETRY_MAYFAIL because another task may be doing
      * an allocation after this task has taken all memory.
      * This is the task the OOM killer needs to take out during this
      * loop, even if it was triggered by an allocation somewhere else.
      */
     if (user_thread)
         set_current_oom_origin();
     for (i = 0; i < nr_pages; i++) {
         struct page *page;
 
         bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
                     mflags, cpu_to_node(cpu_buffer->cpu));
         if (!bpage)
             goto free_pages;
 
         rb_check_bpage(cpu_buffer, bpage);
 
         list_add(&bpage->list, pages);
 
         page = alloc_pages_node(cpu_to_node(cpu_buffer->cpu), mflags, 0);
         if (!page)
             goto free_pages;
         bpage->page = page_address(page);
         rb_init_page(bpage->page);
 
         if (user_thread && fatal_signal_pending(current))
             goto free_pages;
     }
     if (user_thread)
         clear_current_oom_origin();
 
     return 0;
 
 free_pages:
     list_for_each_entry_safe(bpage, tmp, pages, list) {
         list_del_init(&bpage->list);
         free_buffer_page(bpage);
     }
     if (user_thread)
         clear_current_oom_origin();
 
     return -ENOMEM;
 }
 
 static int rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
                  unsigned long nr_pages)
 {
     LIST_HEAD(pages);
 
     WARN_ON(!nr_pages);
 
     if (__rb_allocate_pages(cpu_buffer, nr_pages, &pages))
         return -ENOMEM;
 
     /*
      * The ring buffer page list is a circular list that does not
      * start and end with a list head. All page list items point to
      * other pages.
      */
     cpu_buffer->pages = pages.next;
     list_del(&pages);
 
     cpu_buffer->nr_pages = nr_pages;
 
     rb_check_pages(cpu_buffer);
 
     return 0;
 }
 
 static struct ring_buffer_per_cpu *
 rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     struct buffer_page *bpage;
     struct page *page;
     int ret;
 
     cpu_buffer = kzalloc_node(ALIGN(sizeof(*cpu_buffer), cache_line_size()),
                   GFP_KERNEL, cpu_to_node(cpu));
     if (!cpu_buffer)
         return NULL;
 
     cpu_buffer->cpu = cpu;
     cpu_buffer->buffer = buffer;
     raw_spin_lock_init(&cpu_buffer->reader_lock);
     lockdep_set_class(&cpu_buffer->reader_lock, buffer->reader_lock_key);
     cpu_buffer->lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
     INIT_WORK(&cpu_buffer->update_pages_work, update_pages_handler);
     init_completion(&cpu_buffer->update_done);
     init_irq_work(&cpu_buffer->irq_work.work, rb_wake_up_waiters);
     init_waitqueue_head(&cpu_buffer->irq_work.waiters);
     init_waitqueue_head(&cpu_buffer->irq_work.full_waiters);
 
     bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
                 GFP_KERNEL, cpu_to_node(cpu));
     if (!bpage)
         goto fail_free_buffer;
 
     rb_check_bpage(cpu_buffer, bpage);
 
     cpu_buffer->reader_page = bpage;
     page = alloc_pages_node(cpu_to_node(cpu), GFP_KERNEL, 0);
     if (!page)
         goto fail_free_reader;
     bpage->page = page_address(page);
     rb_init_page(bpage->page);
 
     INIT_LIST_HEAD(&cpu_buffer->reader_page->list);
     INIT_LIST_HEAD(&cpu_buffer->new_pages);
 
     ret = rb_allocate_pages(cpu_buffer, nr_pages);
     if (ret < 0)
         goto fail_free_reader;
 
     cpu_buffer->head_page
         = list_entry(cpu_buffer->pages, struct buffer_page, list);
     cpu_buffer->tail_page = cpu_buffer->commit_page = cpu_buffer->head_page;
 
     rb_head_page_activate(cpu_buffer);
 
     return cpu_buffer;
 
  fail_free_reader:
     free_buffer_page(cpu_buffer->reader_page);
 
  fail_free_buffer:
     kfree(cpu_buffer);
     return NULL;
 }
 
 static void rb_free_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer)
 {
     struct list_head *head = cpu_buffer->pages;
     struct buffer_page *bpage, *tmp;
 
     free_buffer_page(cpu_buffer->reader_page);
 
     rb_head_page_deactivate(cpu_buffer);
 
     if (head) {
         list_for_each_entry_safe(bpage, tmp, head, list) {
             list_del_init(&bpage->list);
             free_buffer_page(bpage);
         }
         bpage = list_entry(head, struct buffer_page, list);
         free_buffer_page(bpage);
     }
 
     kfree(cpu_buffer);
 }
 
 /**
  * __ring_buffer_alloc - allocate a new ring_buffer
  * @size: the size in bytes per cpu that is needed.
  * @flags: attributes to set for the ring buffer.
  * @key: ring buffer reader_lock_key.
  *
  * Currently the only flag that is available is the RB_FL_OVERWRITE
  * flag. This flag means that the buffer will overwrite old data
  * when the buffer wraps. If this flag is not set, the buffer will
  * drop data when the tail hits the head.
  */
 struct trace_buffer *__ring_buffer_alloc(unsigned long size, unsigned flags,
                     struct lock_class_key *key)
 {
     struct trace_buffer *buffer;
     long nr_pages;
     int bsize;
     int cpu;
     int ret;
 
     /* keep it in its own cache line */
     buffer = kzalloc(ALIGN(sizeof(*buffer), cache_line_size()),
              GFP_KERNEL);
     if (!buffer)
         return NULL;
 
     if (!zalloc_cpumask_var(&buffer->cpumask, GFP_KERNEL))
         goto fail_free_buffer;
 
     nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
     buffer->flags = flags;
     buffer->clock = trace_clock_local;
     buffer->reader_lock_key = key;
 
     init_irq_work(&buffer->irq_work.work, rb_wake_up_waiters);
     init_waitqueue_head(&buffer->irq_work.waiters);
 
     /* need at least two pages */
     if (nr_pages < 2)
         nr_pages = 2;
 
     buffer->cpus = nr_cpu_ids;
 
     bsize = sizeof(void *) * nr_cpu_ids;
     buffer->buffers = kzalloc(ALIGN(bsize, cache_line_size()),
                   GFP_KERNEL);
     if (!buffer->buffers)
         goto fail_free_cpumask;
 
     cpu = raw_smp_processor_id();
     cpumask_set_cpu(cpu, buffer->cpumask);
     buffer->buffers[cpu] = rb_allocate_cpu_buffer(buffer, nr_pages, cpu);
     if (!buffer->buffers[cpu])
         goto fail_free_buffers;
 
     ret = cpuhp_state_add_instance(CPUHP_TRACE_RB_PREPARE, &buffer->node);
     if (ret < 0)
         goto fail_free_buffers;
 
     mutex_init(&buffer->mutex);
 
     return buffer;
 
  fail_free_buffers:
     for_each_buffer_cpu(buffer, cpu) {
         if (buffer->buffers[cpu])
             rb_free_cpu_buffer(buffer->buffers[cpu]);
     }
     kfree(buffer->buffers);
 
  fail_free_cpumask:
     free_cpumask_var(buffer->cpumask);
 
  fail_free_buffer:
     kfree(buffer);
     return NULL;
 }
 EXPORT_SYMBOL_GPL(__ring_buffer_alloc);
 
 /**
  * ring_buffer_free - free a ring buffer.
  * @buffer: the buffer to free.
  */
 void
 ring_buffer_free(struct trace_buffer *buffer)
 {
     int cpu;
 
     cpuhp_state_remove_instance(CPUHP_TRACE_RB_PREPARE, &buffer->node);
 
     for_each_buffer_cpu(buffer, cpu)
         rb_free_cpu_buffer(buffer->buffers[cpu]);
 
     kfree(buffer->buffers);
     free_cpumask_var(buffer->cpumask);
 
     kfree(buffer);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_free);
 
 void ring_buffer_set_clock(struct trace_buffer *buffer,
                u64 (*clock)(void))
 {
     buffer->clock = clock;
 }
 
 void ring_buffer_set_time_stamp_abs(struct trace_buffer *buffer, bool abs)
 {
     buffer->time_stamp_abs = abs;
 }
 
 bool ring_buffer_time_stamp_abs(struct trace_buffer *buffer)
 {
     return buffer->time_stamp_abs;
 }
 
 static void rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer);
 
 static inline unsigned long rb_page_entries(struct buffer_page *bpage)
 {
     return local_read(&bpage->entries) & RB_WRITE_MASK;
 }
 
 static inline unsigned long rb_page_write(struct buffer_page *bpage)
 {
     return local_read(&bpage->write) & RB_WRITE_MASK;
 }
 
 static int
 rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages)
 {
     struct list_head *tail_page, *to_remove, *next_page;
     struct buffer_page *to_remove_page, *tmp_iter_page;
     struct buffer_page *last_page, *first_page;
     unsigned long nr_removed;
     unsigned long head_bit;
     int page_entries;
 
     head_bit = 0;
 
     raw_spin_lock_irq(&cpu_buffer->reader_lock);
     atomic_inc(&cpu_buffer->record_disabled);
     /*
      * We don't race with the readers since we have acquired the reader
      * lock. We also don't race with writers after disabling recording.
      * This makes it easy to figure out the first and the last page to be
      * removed from the list. We unlink all the pages in between including
      * the first and last pages. This is done in a busy loop so that we
      * lose the least number of traces.
      * The pages are freed after we restart recording and unlock readers.
      */
     tail_page = &cpu_buffer->tail_page->list;
 
     /*
      * tail page might be on reader page, we remove the next page
      * from the ring buffer
      */
     if (cpu_buffer->tail_page == cpu_buffer->reader_page)
         tail_page = rb_list_head(tail_page->next);
     to_remove = tail_page;
 
     /* start of pages to remove */
     first_page = list_entry(rb_list_head(to_remove->next),
                 struct buffer_page, list);
 
     for (nr_removed = 0; nr_removed < nr_pages; nr_removed++) {
         to_remove = rb_list_head(to_remove)->next;
         head_bit |= (unsigned long)to_remove & RB_PAGE_HEAD;
     }
 
     next_page = rb_list_head(to_remove)->next;
 
     /*
      * Now we remove all pages between tail_page and next_page.
      * Make sure that we have head_bit value preserved for the
      * next page
      */
     tail_page->next = (struct list_head *)((unsigned long)next_page |
                         head_bit);
     next_page = rb_list_head(next_page);
     next_page->prev = tail_page;
 
     /* make sure pages points to a valid page in the ring buffer */
     cpu_buffer->pages = next_page;
 
     /* update head page */
     if (head_bit)
         cpu_buffer->head_page = list_entry(next_page,
                         struct buffer_page, list);
 
     /*
      * change read pointer to make sure any read iterators reset
      * themselves
      */
     cpu_buffer->read = 0;
 
     /* pages are removed, resume tracing and then free the pages */
     atomic_dec(&cpu_buffer->record_disabled);
     raw_spin_unlock_irq(&cpu_buffer->reader_lock);
 
     RB_WARN_ON(cpu_buffer, list_empty(cpu_buffer->pages));
 
     /* last buffer page to remove */
     last_page = list_entry(rb_list_head(to_remove), struct buffer_page,
                 list);
     tmp_iter_page = first_page;
 
     do {
         cond_resched();
 
         to_remove_page = tmp_iter_page;
         rb_inc_page(&tmp_iter_page);
 
         /* update the counters */
         page_entries = rb_page_entries(to_remove_page);
         if (page_entries) {
             /*
              * If something was added to this page, it was full
              * since it is not the tail page. So we deduct the
              * bytes consumed in ring buffer from here.
              * Increment overrun to account for the lost events.
              */
             local_add(page_entries, &cpu_buffer->overrun);
             local_sub(BUF_PAGE_SIZE, &cpu_buffer->entries_bytes);
         }
 
         /*
          * We have already removed references to this list item, just
          * free up the buffer_page and its page
          */
         free_buffer_page(to_remove_page);
         nr_removed--;
 
     } while (to_remove_page != last_page);
 
     RB_WARN_ON(cpu_buffer, nr_removed);
 
     return nr_removed == 0;
 }
 
 static int
 rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer)
 {
     struct list_head *pages = &cpu_buffer->new_pages;
     int retries, success;
 
     raw_spin_lock_irq(&cpu_buffer->reader_lock);
     /*
      * We are holding the reader lock, so the reader page won't be swapped
      * in the ring buffer. Now we are racing with the writer trying to
      * move head page and the tail page.
      * We are going to adapt the reader page update process where:
      * 1. We first splice the start and end of list of new pages between
      *    the head page and its previous page.
      * 2. We cmpxchg the prev_page->next to point from head page to the
      *    start of new pages list.
      * 3. Finally, we update the head->prev to the end of new list.
      *
      * We will try this process 10 times, to make sure that we don't keep
      * spinning.
      */
     retries = 10;
     success = 0;
     while (retries--) {
         struct list_head *head_page, *prev_page, *r;
         struct list_head *last_page, *first_page;
         struct list_head *head_page_with_bit;
 
         head_page = &rb_set_head_page(cpu_buffer)->list;
         if (!head_page)
             break;
         prev_page = head_page->prev;
 
         first_page = pages->next;
         last_page  = pages->prev;
 
         head_page_with_bit = (struct list_head *)
                      ((unsigned long)head_page | RB_PAGE_HEAD);
 
         last_page->next = head_page_with_bit;
         first_page->prev = prev_page;
 
         r = cmpxchg(&prev_page->next, head_page_with_bit, first_page);
 
         if (r == head_page_with_bit) {
             /*
              * yay, we replaced the page pointer to our new list,
              * now, we just have to update to head page's prev
              * pointer to point to end of list
              */
             head_page->prev = last_page;
             success = 1;
             break;
         }
     }
 
     if (success)
         INIT_LIST_HEAD(pages);
     /*
      * If we weren't successful in adding in new pages, warn and stop
      * tracing
      */
     RB_WARN_ON(cpu_buffer, !success);
     raw_spin_unlock_irq(&cpu_buffer->reader_lock);
 
     /* free pages if they weren't inserted */
     if (!success) {
         struct buffer_page *bpage, *tmp;
         list_for_each_entry_safe(bpage, tmp, &cpu_buffer->new_pages,
                      list) {
             list_del_init(&bpage->list);
             free_buffer_page(bpage);
         }
     }
     return success;
 }
 
 static void rb_update_pages(struct ring_buffer_per_cpu *cpu_buffer)
 {
     int success;
 
     if (cpu_buffer->nr_pages_to_update > 0)
         success = rb_insert_pages(cpu_buffer);
     else
         success = rb_remove_pages(cpu_buffer,
                     -cpu_buffer->nr_pages_to_update);
 
     if (success)
         cpu_buffer->nr_pages += cpu_buffer->nr_pages_to_update;
 }
 
 static void update_pages_handler(struct work_struct *work)
 {
     struct ring_buffer_per_cpu *cpu_buffer = container_of(work,
             struct ring_buffer_per_cpu, update_pages_work);
     rb_update_pages(cpu_buffer);
     complete(&cpu_buffer->update_done);
 }
 
 /**
  * ring_buffer_resize - resize the ring buffer
  * @buffer: the buffer to resize.
  * @size: the new size.
  * @cpu_id: the cpu buffer to resize
  *
  * Minimum size is 2 * BUF_PAGE_SIZE.
  *
  * Returns 0 on success and < 0 on failure.
  */
 int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
             int cpu_id)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     unsigned long nr_pages;
     int cpu, err;
 
     /*
      * Always succeed at resizing a non-existent buffer:
      */
     if (!buffer)
         return 0;
 
     /* Make sure the requested buffer exists */
     if (cpu_id != RING_BUFFER_ALL_CPUS &&
         !cpumask_test_cpu(cpu_id, buffer->cpumask))
         return 0;
 
     nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
 
     /* we need a minimum of two pages */
     if (nr_pages < 2)
         nr_pages = 2;
 
     /* prevent another thread from changing buffer sizes */
     mutex_lock(&buffer->mutex);
 
 
     if (cpu_id == RING_BUFFER_ALL_CPUS) {
         /*
          * Don't succeed if resizing is disabled, as a reader might be
          * manipulating the ring buffer and is expecting a sane state while
          * this is true.
          */
         for_each_buffer_cpu(buffer, cpu) {
             cpu_buffer = buffer->buffers[cpu];
             if (atomic_read(&cpu_buffer->resize_disabled)) {
                 err = -EBUSY;
                 goto out_err_unlock;
             }
         }
 
         /* calculate the pages to update */
         for_each_buffer_cpu(buffer, cpu) {
             cpu_buffer = buffer->buffers[cpu];
 
             cpu_buffer->nr_pages_to_update = nr_pages -
                             cpu_buffer->nr_pages;
             /*
              * nothing more to do for removing pages or no update
              */
             if (cpu_buffer->nr_pages_to_update <= 0)
                 continue;
             /*
              * to add pages, make sure all new pages can be
              * allocated without receiving ENOMEM
              */
             INIT_LIST_HEAD(&cpu_buffer->new_pages);
             if (__rb_allocate_pages(cpu_buffer, cpu_buffer->nr_pages_to_update,
                         &cpu_buffer->new_pages)) {
                 /* not enough memory for new pages */
                 err = -ENOMEM;
                 goto out_err;
             }
         }
 
         cpus_read_lock();
         /*
          * Fire off all the required work handlers
          * We can't schedule on offline CPUs, but it's not necessary
          * since we can change their buffer sizes without any race.
          */
         for_each_buffer_cpu(buffer, cpu) {
             cpu_buffer = buffer->buffers[cpu];
             if (!cpu_buffer->nr_pages_to_update)
                 continue;
 
             /* Can't run something on an offline CPU. */
             if (!cpu_online(cpu)) {
                 rb_update_pages(cpu_buffer);
                 cpu_buffer->nr_pages_to_update = 0;
             } else {
                 schedule_work_on(cpu,
                         &cpu_buffer->update_pages_work);
             }
         }
 
         /* wait for all the updates to complete */
         for_each_buffer_cpu(buffer, cpu) {
             cpu_buffer = buffer->buffers[cpu];
             if (!cpu_buffer->nr_pages_to_update)
                 continue;
 
             if (cpu_online(cpu))
                 wait_for_completion(&cpu_buffer->update_done);
             cpu_buffer->nr_pages_to_update = 0;
         }
 
         cpus_read_unlock();
     } else {
         cpu_buffer = buffer->buffers[cpu_id];
 
         if (nr_pages == cpu_buffer->nr_pages)
             goto out;
 
         /*
          * Don't succeed if resizing is disabled, as a reader might be
          * manipulating the ring buffer and is expecting a sane state while
          * this is true.
          */
         if (atomic_read(&cpu_buffer->resize_disabled)) {
             err = -EBUSY;
             goto out_err_unlock;
         }
 
         cpu_buffer->nr_pages_to_update = nr_pages -
                         cpu_buffer->nr_pages;
 
         INIT_LIST_HEAD(&cpu_buffer->new_pages);
         if (cpu_buffer->nr_pages_to_update > 0 &&
             __rb_allocate_pages(cpu_buffer, cpu_buffer->nr_pages_to_update,
                         &cpu_buffer->new_pages)) {
             err = -ENOMEM;
             goto out_err;
         }
 
         cpus_read_lock();
 
         /* Can't run something on an offline CPU. */
         if (!cpu_online(cpu_id))
             rb_update_pages(cpu_buffer);
         else {
             schedule_work_on(cpu_id,
                      &cpu_buffer->update_pages_work);
             wait_for_completion(&cpu_buffer->update_done);
         }
 
         cpu_buffer->nr_pages_to_update = 0;
         cpus_read_unlock();
     }
 
  out:
     /*
      * The ring buffer resize can happen with the ring buffer
      * enabled, so that the update disturbs the tracing as little
      * as possible. But if the buffer is disabled, we do not need
      * to worry about that, and we can take the time to verify
      * that the buffer is not corrupt.
      */
     if (atomic_read(&buffer->record_disabled)) {
         atomic_inc(&buffer->record_disabled);
         /*
          * Even though the buffer was disabled, we must make sure
          * that it is truly disabled before calling rb_check_pages.
          * There could have been a race between checking
          * record_disable and incrementing it.
          */
         synchronize_rcu();
         for_each_buffer_cpu(buffer, cpu) {
             cpu_buffer = buffer->buffers[cpu];
             rb_check_pages(cpu_buffer);
         }
         atomic_dec(&buffer->record_disabled);
     }
 
     mutex_unlock(&buffer->mutex);
     return 0;
 
  out_err:
     for_each_buffer_cpu(buffer, cpu) {
         struct buffer_page *bpage, *tmp;
 
         cpu_buffer = buffer->buffers[cpu];
         cpu_buffer->nr_pages_to_update = 0;
 
         if (list_empty(&cpu_buffer->new_pages))
             continue;
 
         list_for_each_entry_safe(bpage, tmp, &cpu_buffer->new_pages,
                     list) {
             list_del_init(&bpage->list);
             free_buffer_page(bpage);
         }
     }
  out_err_unlock:
     mutex_unlock(&buffer->mutex);
     return err;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_resize);
 
 void ring_buffer_change_overwrite(struct trace_buffer *buffer, int val)
 {
     mutex_lock(&buffer->mutex);
     if (val)
         buffer->flags |= RB_FL_OVERWRITE;
     else
         buffer->flags &= ~RB_FL_OVERWRITE;
     mutex_unlock(&buffer->mutex);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_change_overwrite);
 
 static __always_inline void *__rb_page_index(struct buffer_page *bpage, unsigned index)
 {
     return bpage->page->data + index;
 }
 
 static __always_inline struct ring_buffer_event *
 rb_reader_event(struct ring_buffer_per_cpu *cpu_buffer)
 {
     return __rb_page_index(cpu_buffer->reader_page,
                    cpu_buffer->reader_page->read);
 }
 
 static __always_inline unsigned rb_page_commit(struct buffer_page *bpage)
 {
     return local_read(&bpage->page->commit);
 }
 
 static struct ring_buffer_event *
 rb_iter_head_event(struct ring_buffer_iter *iter)
 {
     struct ring_buffer_event *event;
     struct buffer_page *iter_head_page = iter->head_page;
     unsigned long commit;
     unsigned length;
 
     if (iter->head != iter->next_event)
         return iter->event;
 
     /*
      * When the writer goes across pages, it issues a cmpxchg which
      * is a mb(), which will synchronize with the rmb here.
      * (see rb_tail_page_update() and __rb_reserve_next())
      */
     commit = rb_page_commit(iter_head_page);
     smp_rmb();
     event = __rb_page_index(iter_head_page, iter->head);
     length = rb_event_length(event);
 
     /*
      * READ_ONCE() doesn't work on functions and we don't want the
      * compiler doing any crazy optimizations with length.
      */
     barrier();
 
     if ((iter->head + length) > commit || length > BUF_MAX_DATA_SIZE)
         /* Writer corrupted the read? */
         goto reset;
 
     memcpy(iter->event, event, length);
     /*
      * If the page stamp is still the same after this rmb() then the
      * event was safely copied without the writer entering the page.
      */
     smp_rmb();
 
     /* Make sure the page didn't change since we read this */
     if (iter->page_stamp != iter_head_page->page->time_stamp ||
         commit > rb_page_commit(iter_head_page))
         goto reset;
 
     iter->next_event = iter->head + length;
     return iter->event;
  reset:
     /* Reset to the beginning */
     iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
     iter->head = 0;
     iter->next_event = 0;
     iter->missed_events = 1;
     return NULL;
 }
 
 /* Size is determined by what has been committed */
 static __always_inline unsigned rb_page_size(struct buffer_page *bpage)
 {
     return rb_page_commit(bpage);
 }
 
 static __always_inline unsigned
 rb_commit_index(struct ring_buffer_per_cpu *cpu_buffer)
 {
     return rb_page_commit(cpu_buffer->commit_page);
 }
 
 static __always_inline unsigned
 rb_event_index(struct ring_buffer_event *event)
 {
     unsigned long addr = (unsigned long)event;
 
     return (addr & ~PAGE_MASK) - BUF_PAGE_HDR_SIZE;
 }
 
 static void rb_inc_iter(struct ring_buffer_iter *iter)
 {
     struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
 
     /*
      * The iterator could be on the reader page (it starts there).
      * But the head could have moved, since the reader was
      * found. Check for this case and assign the iterator
      * to the head page instead of next.
      */
     if (iter->head_page == cpu_buffer->reader_page)
         iter->head_page = rb_set_head_page(cpu_buffer);
     else
         rb_inc_page(&iter->head_page);
 
     iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
     iter->head = 0;
     iter->next_event = 0;
 }
 
 /*
  * rb_handle_head_page - writer hit the head page
  *
  * Returns: +1 to retry page
  *           0 to continue
  *          -1 on error
  */
 static int
 rb_handle_head_page(struct ring_buffer_per_cpu *cpu_buffer,
             struct buffer_page *tail_page,
             struct buffer_page *next_page)
 {
     struct buffer_page *new_head;
     int entries;
     int type;
     int ret;
 
     entries = rb_page_entries(next_page);
 
     /*
      * The hard part is here. We need to move the head
      * forward, and protect against both readers on
      * other CPUs and writers coming in via interrupts.
      */
     type = rb_head_page_set_update(cpu_buffer, next_page, tail_page,
                        RB_PAGE_HEAD);
 
     /*
      * type can be one of four:
      *  NORMAL - an interrupt already moved it for us
      *  HEAD   - we are the first to get here.
      *  UPDATE - we are the interrupt interrupting
      *           a current move.
      *  MOVED  - a reader on another CPU moved the next
      *           pointer to its reader page. Give up
      *           and try again.
      */
 
     switch (type) {
     case RB_PAGE_HEAD:
         /*
          * We changed the head to UPDATE, thus
          * it is our responsibility to update
          * the counters.
          */
         local_add(entries, &cpu_buffer->overrun);
         local_sub(BUF_PAGE_SIZE, &cpu_buffer->entries_bytes);
 
         /*
          * The entries will be zeroed out when we move the
          * tail page.
          */
 
         /* still more to do */
         break;
 
     case RB_PAGE_UPDATE:
         /*
          * This is an interrupt that interrupt the
          * previous update. Still more to do.
          */
         break;
     case RB_PAGE_NORMAL:
         /*
          * An interrupt came in before the update
          * and processed this for us.
          * Nothing left to do.
          */
         return 1;
     case RB_PAGE_MOVED:
         /*
          * The reader is on another CPU and just did
          * a swap with our next_page.
          * Try again.
          */
         return 1;
     default:
         RB_WARN_ON(cpu_buffer, 1); /* WTF??? */
         return -1;
     }
 
     /*
      * Now that we are here, the old head pointer is
      * set to UPDATE. This will keep the reader from
      * swapping the head page with the reader page.
      * The reader (on another CPU) will spin till
      * we are finished.
      *
      * We just need to protect against interrupts
      * doing the job. We will set the next pointer
      * to HEAD. After that, we set the old pointer
      * to NORMAL, but only if it was HEAD before.
      * otherwise we are an interrupt, and only
      * want the outer most commit to reset it.
      */
     new_head = next_page;
     rb_inc_page(&new_head);
 
     ret = rb_head_page_set_head(cpu_buffer, new_head, next_page,
                     RB_PAGE_NORMAL);
 
     /*
      * Valid returns are:
      *  HEAD   - an interrupt came in and already set it.
      *  NORMAL - One of two things:
      *            1) We really set it.
      *            2) A bunch of interrupts came in and moved
      *               the page forward again.
      */
     switch (ret) {
     case RB_PAGE_HEAD:
     case RB_PAGE_NORMAL:
         /* OK */
         break;
     default:
         RB_WARN_ON(cpu_buffer, 1);
         return -1;
     }
 
     /*
      * It is possible that an interrupt came in,
      * set the head up, then more interrupts came in
      * and moved it again. When we get back here,
      * the page would have been set to NORMAL but we
      * just set it back to HEAD.
      *
      * How do you detect this? Well, if that happened
      * the tail page would have moved.
      */
     if (ret == RB_PAGE_NORMAL) {
         struct buffer_page *buffer_tail_page;
 
         buffer_tail_page = READ_ONCE(cpu_buffer->tail_page);
         /*
          * If the tail had moved passed next, then we need
          * to reset the pointer.
          */
         if (buffer_tail_page != tail_page &&
             buffer_tail_page != next_page)
             rb_head_page_set_normal(cpu_buffer, new_head,
                         next_page,
                         RB_PAGE_HEAD);
     }
 
     /*
      * If this was the outer most commit (the one that
      * changed the original pointer from HEAD to UPDATE),
      * then it is up to us to reset it to NORMAL.
      */
     if (type == RB_PAGE_HEAD) {
         ret = rb_head_page_set_normal(cpu_buffer, next_page,
                           tail_page,
                           RB_PAGE_UPDATE);
         if (RB_WARN_ON(cpu_buffer,
                    ret != RB_PAGE_UPDATE))
             return -1;
     }
 
     return 0;
 }
 
 static inline void
 rb_reset_tail(struct ring_buffer_per_cpu *cpu_buffer,
           unsigned long tail, struct rb_event_info *info)
 {
     struct buffer_page *tail_page = info->tail_page;
     struct ring_buffer_event *event;
     unsigned long length = info->length;
 
     /*
      * Only the event that crossed the page boundary
      * must fill the old tail_page with padding.
      */
     if (tail >= BUF_PAGE_SIZE) {
         /*
          * If the page was filled, then we still need
          * to update the real_end. Reset it to zero
          * and the reader will ignore it.
          */
         if (tail == BUF_PAGE_SIZE)
             tail_page->real_end = 0;
 
         local_sub(length, &tail_page->write);
         return;
     }
 
     event = __rb_page_index(tail_page, tail);
 
     /* account for padding bytes */
     local_add(BUF_PAGE_SIZE - tail, &cpu_buffer->entries_bytes);
 
     /*
      * Save the original length to the meta data.
      * This will be used by the reader to add lost event
      * counter.
      */
     tail_page->real_end = tail;
 
     /*
      * If this event is bigger than the minimum size, then
      * we need to be careful that we don't subtract the
      * write counter enough to allow another writer to slip
      * in on this page.
      * We put in a discarded commit instead, to make sure
      * that this space is not used again.
      *
      * If we are less than the minimum size, we don't need to
      * worry about it.
      */
     if (tail > (BUF_PAGE_SIZE - RB_EVNT_MIN_SIZE)) {
         /* No room for any events */
 
         /* Mark the rest of the page with padding */
         rb_event_set_padding(event);
 
         /* Set the write back to the previous setting */
         local_sub(length, &tail_page->write);
         return;
     }
 
     /* Put in a discarded event */
     event->array[0] = (BUF_PAGE_SIZE - tail) - RB_EVNT_HDR_SIZE;
     event->type_len = RINGBUF_TYPE_PADDING;
     /* time delta must be non zero */
     event->time_delta = 1;
 
     /* Set write to end of buffer */
     length = (tail + length) - BUF_PAGE_SIZE;
     local_sub(length, &tail_page->write);
 }
 
 static inline void rb_end_commit(struct ring_buffer_per_cpu *cpu_buffer);
 
 /*
  * This is the slow path, force gcc not to inline it.
  */
 static noinline struct ring_buffer_event *
 rb_move_tail(struct ring_buffer_per_cpu *cpu_buffer,
          unsigned long tail, struct rb_event_info *info)
 {
     struct buffer_page *tail_page = info->tail_page;
     struct buffer_page *commit_page = cpu_buffer->commit_page;
     struct trace_buffer *buffer = cpu_buffer->buffer;
     struct buffer_page *next_page;
     int ret;
 
     next_page = tail_page;
 
     rb_inc_page(&next_page);
 
     /*
      * If for some reason, we had an interrupt storm that made
      * it all the way around the buffer, bail, and warn
      * about it.
      */
     if (unlikely(next_page == commit_page)) {
         local_inc(&cpu_buffer->commit_overrun);
         goto out_reset;
     }
 
     /*
      * This is where the fun begins!
      *
      * We are fighting against races between a reader that
      * could be on another CPU trying to swap its reader
      * page with the buffer head.
      *
      * We are also fighting against interrupts coming in and
      * moving the head or tail on us as well.
      *
      * If the next page is the head page then we have filled
      * the buffer, unless the commit page is still on the
      * reader page.
      */
     if (rb_is_head_page(next_page, &tail_page->list)) {
 
         /*
          * If the commit is not on the reader page, then
          * move the header page.
          */
         if (!rb_is_reader_page(cpu_buffer->commit_page)) {
             /*
              * If we are not in overwrite mode,
              * this is easy, just stop here.
              */
             if (!(buffer->flags & RB_FL_OVERWRITE)) {
                 local_inc(&cpu_buffer->dropped_events);
                 goto out_reset;
             }
 
             ret = rb_handle_head_page(cpu_buffer,
                           tail_page,
                           next_page);
             if (ret < 0)
                 goto out_reset;
             if (ret)
                 goto out_again;
         } else {
             /*
              * We need to be careful here too. The
              * commit page could still be on the reader
              * page. We could have a small buffer, and
              * have filled up the buffer with events
              * from interrupts and such, and wrapped.
              *
              * Note, if the tail page is also on the
              * reader_page, we let it move out.
              */
             if (unlikely((cpu_buffer->commit_page !=
                       cpu_buffer->tail_page) &&
                      (cpu_buffer->commit_page ==
                       cpu_buffer->reader_page))) {
                 local_inc(&cpu_buffer->commit_overrun);
                 goto out_reset;
             }
         }
     }
 
     rb_tail_page_update(cpu_buffer, tail_page, next_page);
 
  out_again:
 
     rb_reset_tail(cpu_buffer, tail, info);
 
     /* Commit what we have for now. */
     rb_end_commit(cpu_buffer);
     /* rb_end_commit() decs committing */
     local_inc(&cpu_buffer->committing);
 
     /* fail and let the caller try again */
     return ERR_PTR(-EAGAIN);
 
  out_reset:
     /* reset write */
     rb_reset_tail(cpu_buffer, tail, info);
 
     return NULL;
 }
 
 /* Slow path */
 static struct ring_buffer_event *
 rb_add_time_stamp(struct ring_buffer_event *event, u64 delta, bool abs)
 {
     if (abs)
         event->type_len = RINGBUF_TYPE_TIME_STAMP;
     else
         event->type_len = RINGBUF_TYPE_TIME_EXTEND;
 
     /* Not the first event on the page, or not delta? */
     if (abs || rb_event_index(event)) {
         event->time_delta = delta & TS_MASK;
         event->array[0] = delta >> TS_SHIFT;
     } else {
         /* nope, just zero it */
         event->time_delta = 0;
         event->array[0] = 0;
     }
 
     return skip_time_extend(event);
 }
 
 #ifndef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
 static inline bool sched_clock_stable(void)
 {
     return true;
 }
 #endif
 
 static void
 rb_check_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
            struct rb_event_info *info)
 {
     u64 write_stamp;
 
     WARN_ONCE(1, "Delta way too big! %llu ts=%llu before=%llu after=%llu write stamp=%llu\n%s",
           (unsigned long long)info->delta,
           (unsigned long long)info->ts,
           (unsigned long long)info->before,
           (unsigned long long)info->after,
           (unsigned long long)(rb_time_read(&cpu_buffer->write_stamp, &write_stamp) ? write_stamp : 0),
           sched_clock_stable() ? "" :
           "If you just came from a suspend/resume,\n"
           "please switch to the trace global clock:\n"
           "  echo global > /sys/kernel/debug/tracing/trace_clock\n"
           "or add trace_clock=global to the kernel command line\n");
 }
 
 static void rb_add_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
                       struct ring_buffer_event **event,
                       struct rb_event_info *info,
                       u64 *delta,
                       unsigned int *length)
 {
     bool abs = info->add_timestamp &
         (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE);
 
     if (unlikely(info->delta > (1ULL << 59))) {
         /*
          * Some timers can use more than 59 bits, and when a timestamp
          * is added to the buffer, it will lose those bits.
          */
         if (abs && (info->ts & TS_MSB)) {
             info->delta &= ABS_TS_MASK;
 
         /* did the clock go backwards */
         } else if (info->before == info->after && info->before > info->ts) {
             /* not interrupted */
             static int once;
 
             /*
              * This is possible with a recalibrating of the TSC.
              * Do not produce a call stack, but just report it.
              */
             if (!once) {
                 once++;
                 pr_warn("Ring buffer clock went backwards: %llu -> %llu\n",
                     info->before, info->ts);
             }
         } else
             rb_check_timestamp(cpu_buffer, info);
         if (!abs)
             info->delta = 0;
     }
     *event = rb_add_time_stamp(*event, info->delta, abs);
     *length -= RB_LEN_TIME_EXTEND;
     *delta = 0;
 }
 
 /**
  * rb_update_event - update event type and data
  * @cpu_buffer: The per cpu buffer of the @event
  * @event: the event to update
  * @info: The info to update the @event with (contains length and delta)
  *
  * Update the type and data fields of the @event. The length
  * is the actual size that is written to the ring buffer,
  * and with this, we can determine what to place into the
  * data field.
  */
 static void
 rb_update_event(struct ring_buffer_per_cpu *cpu_buffer,
         struct ring_buffer_event *event,
         struct rb_event_info *info)
 {
     unsigned length = info->length;
     u64 delta = info->delta;
     unsigned int nest = local_read(&cpu_buffer->committing) - 1;
 
     if (!WARN_ON_ONCE(nest >= MAX_NEST))
         cpu_buffer->event_stamp[nest] = info->ts;
 
     /*
      * If we need to add a timestamp, then we
      * add it to the start of the reserved space.
      */
     if (unlikely(info->add_timestamp))
         rb_add_timestamp(cpu_buffer, &event, info, &delta, &length);
 
     event->time_delta = delta;
     length -= RB_EVNT_HDR_SIZE;
     if (length > RB_MAX_SMALL_DATA || RB_FORCE_8BYTE_ALIGNMENT) {
         event->type_len = 0;
         event->array[0] = length;
     } else
         event->type_len = DIV_ROUND_UP(length, RB_ALIGNMENT);
 }
 
 static unsigned rb_calculate_event_length(unsigned length)
 {
     struct ring_buffer_event event; /* Used only for sizeof array */
 
     /* zero length can cause confusions */
     if (!length)
         length++;
 
     if (length > RB_MAX_SMALL_DATA || RB_FORCE_8BYTE_ALIGNMENT)
         length += sizeof(event.array[0]);
 
     length += RB_EVNT_HDR_SIZE;
     length = ALIGN(length, RB_ARCH_ALIGNMENT);
 
     /*
      * In case the time delta is larger than the 27 bits for it
      * in the header, we need to add a timestamp. If another
      * event comes in when trying to discard this one to increase
      * the length, then the timestamp will be added in the allocated
      * space of this event. If length is bigger than the size needed
      * for the TIME_EXTEND, then padding has to be used. The events
      * length must be either RB_LEN_TIME_EXTEND, or greater than or equal
      * to RB_LEN_TIME_EXTEND + 8, as 8 is the minimum size for padding.
      * As length is a multiple of 4, we only need to worry if it
      * is 12 (RB_LEN_TIME_EXTEND + 4).
      */
     if (length == RB_LEN_TIME_EXTEND + RB_ALIGNMENT)
         length += RB_ALIGNMENT;
 
     return length;
 }
 
 static u64 rb_time_delta(struct ring_buffer_event *event)
 {
     switch (event->type_len) {
     case RINGBUF_TYPE_PADDING:
         return 0;
 
     case RINGBUF_TYPE_TIME_EXTEND:
         return rb_event_time_stamp(event);
 
     case RINGBUF_TYPE_TIME_STAMP:
         return 0;
 
     case RINGBUF_TYPE_DATA:
         return event->time_delta;
     default:
         return 0;
     }
 }
 
 static inline int
 rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
           struct ring_buffer_event *event)
 {
     unsigned long new_index, old_index;
     struct buffer_page *bpage;
     unsigned long index;
     unsigned long addr;
     u64 write_stamp;
     u64 delta;
 
     new_index = rb_event_index(event);
     old_index = new_index + rb_event_ts_length(event);
     addr = (unsigned long)event;
     addr &= PAGE_MASK;
 
     bpage = READ_ONCE(cpu_buffer->tail_page);
 
     delta = rb_time_delta(event);
 
     if (!rb_time_read(&cpu_buffer->write_stamp, &write_stamp))
         return 0;
 
     /* Make sure the write stamp is read before testing the location */
     barrier();
 
     if (bpage->page == (void *)addr && rb_page_write(bpage) == old_index) {
         unsigned long write_mask =
             local_read(&bpage->write) & ~RB_WRITE_MASK;
         unsigned long event_length = rb_event_length(event);
 
         /* Something came in, can't discard */
         if (!rb_time_cmpxchg(&cpu_buffer->write_stamp,
                        write_stamp, write_stamp - delta))
             return 0;
 
         /*
          * It's possible that the event time delta is zero
          * (has the same time stamp as the previous event)
          * in which case write_stamp and before_stamp could
          * be the same. In such a case, force before_stamp
          * to be different than write_stamp. It doesn't
          * matter what it is, as long as its different.
          */
         if (!delta)
             rb_time_set(&cpu_buffer->before_stamp, 0);
 
         /*
          * If an event were to come in now, it would see that the
          * write_stamp and the before_stamp are different, and assume
          * that this event just added itself before updating
          * the write stamp. The interrupting event will fix the
          * write stamp for us, and use the before stamp as its delta.
          */
 
         /*
          * This is on the tail page. It is possible that
          * a write could come in and move the tail page
          * and write to the next page. That is fine
          * because we just shorten what is on this page.
          */
         old_index += write_mask;
         new_index += write_mask;
         index = local_cmpxchg(&bpage->write, old_index, new_index);
         if (index == old_index) {
             /* update counters */
             local_sub(event_length, &cpu_buffer->entries_bytes);
             return 1;
         }
     }
 
     /* could not discard */
     return 0;
 }
 
 static void rb_start_commit(struct ring_buffer_per_cpu *cpu_buffer)
 {
     local_inc(&cpu_buffer->committing);
     local_inc(&cpu_buffer->commits);
 }
 
 static __always_inline void
 rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer)
 {
     unsigned long max_count;
 
     /*
      * We only race with interrupts and NMIs on this CPU.
      * If we own the commit event, then we can commit
      * all others that interrupted us, since the interruptions
      * are in stack format (they finish before they come
      * back to us). This allows us to do a simple loop to
      * assign the commit to the tail.
      */
  again:
     max_count = cpu_buffer->nr_pages * 100;
 
     while (cpu_buffer->commit_page != READ_ONCE(cpu_buffer->tail_page)) {
         if (RB_WARN_ON(cpu_buffer, !(--max_count)))
             return;
         if (RB_WARN_ON(cpu_buffer,
                    rb_is_reader_page(cpu_buffer->tail_page)))
             return;
         local_set(&cpu_buffer->commit_page->page->commit,
               rb_page_write(cpu_buffer->commit_page));
         rb_inc_page(&cpu_buffer->commit_page);
         /* add barrier to keep gcc from optimizing too much */
         barrier();
     }
     while (rb_commit_index(cpu_buffer) !=
            rb_page_write(cpu_buffer->commit_page)) {
 
         local_set(&cpu_buffer->commit_page->page->commit,
               rb_page_write(cpu_buffer->commit_page));
         RB_WARN_ON(cpu_buffer,
                local_read(&cpu_buffer->commit_page->page->commit) &
                ~RB_WRITE_MASK);
         barrier();
     }
 
     /* again, keep gcc from optimizing */
     barrier();
 
     /*
      * If an interrupt came in just after the first while loop
      * and pushed the tail page forward, we will be left with
      * a dangling commit that will never go forward.
      */
     if (unlikely(cpu_buffer->commit_page != READ_ONCE(cpu_buffer->tail_page)))
         goto again;
 }
 
 static __always_inline void rb_end_commit(struct ring_buffer_per_cpu *cpu_buffer)
 {
     unsigned long commits;
 
     if (RB_WARN_ON(cpu_buffer,
                !local_read(&cpu_buffer->committing)))
         return;
 
  again:
     commits = local_read(&cpu_buffer->commits);
     /* synchronize with interrupts */
     barrier();
     if (local_read(&cpu_buffer->committing) == 1)
         rb_set_commit_to_write(cpu_buffer);
 
     local_dec(&cpu_buffer->committing);
 
     /* synchronize with interrupts */
     barrier();
 
     /*
      * Need to account for interrupts coming in between the
      * updating of the commit page and the clearing of the
      * committing counter.
      */
     if (unlikely(local_read(&cpu_buffer->commits) != commits) &&
         !local_read(&cpu_buffer->committing)) {
         local_inc(&cpu_buffer->committing);
         goto again;
     }
 }
 
 static inline void rb_event_discard(struct ring_buffer_event *event)
 {
     if (extended_time(event))
         event = skip_time_extend(event);
 
     /* array[0] holds the actual length for the discarded event */
     event->array[0] = rb_event_data_length(event) - RB_EVNT_HDR_SIZE;
     event->type_len = RINGBUF_TYPE_PADDING;
     /* time delta must be non zero */
     if (!event->time_delta)
         event->time_delta = 1;
 }
 
 static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer,
               struct ring_buffer_event *event)
 {
     local_inc(&cpu_buffer->entries);
     rb_end_commit(cpu_buffer);
 }
 
 static __always_inline void
 rb_wakeups(struct trace_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer)
 {
     size_t nr_pages;
     size_t dirty;
     size_t full;
 
     if (buffer->irq_work.waiters_pending) {
         buffer->irq_work.waiters_pending = false;
         /* irq_work_queue() supplies it's own memory barriers */
         irq_work_queue(&buffer->irq_work.work);
     }
 
     if (cpu_buffer->irq_work.waiters_pending) {
         cpu_buffer->irq_work.waiters_pending = false;
         /* irq_work_queue() supplies it's own memory barriers */
         irq_work_queue(&cpu_buffer->irq_work.work);
     }
 
     if (cpu_buffer->last_pages_touch == local_read(&cpu_buffer->pages_touched))
         return;
 
     if (cpu_buffer->reader_page == cpu_buffer->commit_page)
         return;
 
     if (!cpu_buffer->irq_work.full_waiters_pending)
         return;
 
     cpu_buffer->last_pages_touch = local_read(&cpu_buffer->pages_touched);
 
     full = cpu_buffer->shortest_full;
     nr_pages = cpu_buffer->nr_pages;
     dirty = ring_buffer_nr_dirty_pages(buffer, cpu_buffer->cpu);
     if (full && nr_pages && (dirty * 100) <= full * nr_pages)
         return;
 
     cpu_buffer->irq_work.wakeup_full = true;
     cpu_buffer->irq_work.full_waiters_pending = false;
     /* irq_work_queue() supplies it's own memory barriers */
     irq_work_queue(&cpu_buffer->irq_work.work);
 }
 
 #ifdef CONFIG_RING_BUFFER_RECORD_RECURSION
 # define do_ring_buffer_record_recursion()  \
     do_ftrace_record_recursion(_THIS_IP_, _RET_IP_)
 #else
 # define do_ring_buffer_record_recursion() do { } while (0)
 #endif
 
 /*
  * The lock and unlock are done within a preempt disable section.
  * The current_context per_cpu variable can only be modified
  * by the current task between lock and unlock. But it can
  * be modified more than once via an interrupt. To pass this
  * information from the lock to the unlock without having to
  * access the 'in_interrupt()' functions again (which do show
  * a bit of overhead in something as critical as function tracing,
  * we use a bitmask trick.
  *
  *  bit 1 =  NMI context
  *  bit 2 =  IRQ context
  *  bit 3 =  SoftIRQ context
  *  bit 4 =  normal context.
  *
  * This works because this is the order of contexts that can
  * preempt other contexts. A SoftIRQ never preempts an IRQ
  * context.
  *
  * When the context is determined, the corresponding bit is
  * checked and set (if it was set, then a recursion of that context
  * happened).
  *
  * On unlock, we need to clear this bit. To do so, just subtract
  * 1 from the current_context and AND it to itself.
  *
  * (binary)
  *  101 - 1 = 100
  *  101 & 100 = 100 (clearing bit zero)
  *
  *  1010 - 1 = 1001
  *  1010 & 1001 = 1000 (clearing bit 1)
  *
  * The least significant bit can be cleared this way, and it
  * just so happens that it is the same bit corresponding to
  * the current context.
  *
  * Now the TRANSITION bit breaks the above slightly. The TRANSITION bit
  * is set when a recursion is detected at the current context, and if
  * the TRANSITION bit is already set, it will fail the recursion.
  * This is needed because there's a lag between the changing of
  * interrupt context and updating the preempt count. In this case,
  * a false positive will be found. To handle this, one extra recursion
  * is allowed, and this is done by the TRANSITION bit. If the TRANSITION
  * bit is already set, then it is considered a recursion and the function
  * ends. Otherwise, the TRANSITION bit is set, and that bit is returned.
  *
  * On the trace_recursive_unlock(), the TRANSITION bit will be the first
  * to be cleared. Even if it wasn't the context that set it. That is,
  * if an interrupt comes in while NORMAL bit is set and the ring buffer
  * is called before preempt_count() is updated, since the check will
  * be on the NORMAL bit, the TRANSITION bit will then be set. If an
  * NMI then comes in, it will set the NMI bit, but when the NMI code
  * does the trace_recursive_unlock() it will clear the TRANSITION bit
  * and leave the NMI bit set. But this is fine, because the interrupt
  * code that set the TRANSITION bit will then clear the NMI bit when it
  * calls trace_recursive_unlock(). If another NMI comes in, it will
  * set the TRANSITION bit and continue.
  *
  * Note: The TRANSITION bit only handles a single transition between context.
  */
 
 static __always_inline int
 trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer)
 {
     unsigned int val = cpu_buffer->current_context;
     int bit = interrupt_context_level();
 
     bit = RB_CTX_NORMAL - bit;
 
     if (unlikely(val & (1 << (bit + cpu_buffer->nest)))) {
         /*
          * It is possible that this was called by transitioning
          * between interrupt context, and preempt_count() has not
          * been updated yet. In this case, use the TRANSITION bit.
          */
         bit = RB_CTX_TRANSITION;
         if (val & (1 << (bit + cpu_buffer->nest))) {
             do_ring_buffer_record_recursion();
             return 1;
         }
     }
 
     val |= (1 << (bit + cpu_buffer->nest));
     cpu_buffer->current_context = val;
 
     return 0;
 }
 
 static __always_inline void
 trace_recursive_unlock(struct ring_buffer_per_cpu *cpu_buffer)
 {
     cpu_buffer->current_context &=
         cpu_buffer->current_context - (1 << cpu_buffer->nest);
 }
 
 /* The recursive locking above uses 5 bits */
 #define NESTED_BITS 5
 
 /**
  * ring_buffer_nest_start - Allow to trace while nested
  * @buffer: The ring buffer to modify
  *
  * The ring buffer has a safety mechanism to prevent recursion.
  * But there may be a case where a trace needs to be done while
  * tracing something else. In this case, calling this function
  * will allow this function to nest within a currently active
  * ring_buffer_lock_reserve().
  *
  * Call this function before calling another ring_buffer_lock_reserve() and
  * call ring_buffer_nest_end() after the nested ring_buffer_unlock_commit().
  */
 void ring_buffer_nest_start(struct trace_buffer *buffer)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     int cpu;
 
     /* Enabled by ring_buffer_nest_end() */
     preempt_disable_notrace();
     cpu = raw_smp_processor_id();
     cpu_buffer = buffer->buffers[cpu];
     /* This is the shift value for the above recursive locking */
     cpu_buffer->nest += NESTED_BITS;
 }
 
 /**
  * ring_buffer_nest_end - Allow to trace while nested
  * @buffer: The ring buffer to modify
  *
  * Must be called after ring_buffer_nest_start() and after the
  * ring_buffer_unlock_commit().
  */
 void ring_buffer_nest_end(struct trace_buffer *buffer)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     int cpu;
 
     /* disabled by ring_buffer_nest_start() */
     cpu = raw_smp_processor_id();
     cpu_buffer = buffer->buffers[cpu];
     /* This is the shift value for the above recursive locking */
     cpu_buffer->nest -= NESTED_BITS;
     preempt_enable_notrace();
 }
 
 /**
  * ring_buffer_unlock_commit - commit a reserved
  * @buffer: The buffer to commit to
  * @event: The event pointer to commit.
  *
  * This commits the data to the ring buffer, and releases any locks held.
  *
  * Must be paired with ring_buffer_lock_reserve.
  */
 int ring_buffer_unlock_commit(struct trace_buffer *buffer,
                   struct ring_buffer_event *event)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     int cpu = raw_smp_processor_id();
 
     cpu_buffer = buffer->buffers[cpu];
 
     rb_commit(cpu_buffer, event);
 
     rb_wakeups(buffer, cpu_buffer);
 
     trace_recursive_unlock(cpu_buffer);
 
     preempt_enable_notrace();
 
     return 0;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_unlock_commit);
 
 /* Special value to validate all deltas on a page. */
 #define CHECK_FULL_PAGE     1L
 
 #ifdef CONFIG_RING_BUFFER_VALIDATE_TIME_DELTAS
 static void dump_buffer_page(struct buffer_data_page *bpage,
                  struct rb_event_info *info,
                  unsigned long tail)
 {
     struct ring_buffer_event *event;
     u64 ts, delta;
     int e;
 
     ts = bpage->time_stamp;
     pr_warn("  [%lld] PAGE TIME STAMP\n", ts);
 
     for (e = 0; e < tail; e += rb_event_length(event)) {
 
         event = (struct ring_buffer_event *)(bpage->data + e);
 
         switch (event->type_len) {
 
         case RINGBUF_TYPE_TIME_EXTEND:
             delta = rb_event_time_stamp(event);
             ts += delta;
             pr_warn("  [%lld] delta:%lld TIME EXTEND\n", ts, delta);
             break;
 
         case RINGBUF_TYPE_TIME_STAMP:
             delta = rb_event_time_stamp(event);
             ts = rb_fix_abs_ts(delta, ts);
             pr_warn("  [%lld] absolute:%lld TIME STAMP\n", ts, delta);
             break;
 
         case RINGBUF_TYPE_PADDING:
             ts += event->time_delta;
             pr_warn("  [%lld] delta:%d PADDING\n", ts, event->time_delta);
             break;
 
         case RINGBUF_TYPE_DATA:
             ts += event->time_delta;
             pr_warn("  [%lld] delta:%d\n", ts, event->time_delta);
             break;
 
         default:
             break;
         }
     }
 }
 
 static DEFINE_PER_CPU(atomic_t, checking);
 static atomic_t ts_dump;
 
 /*
  * Check if the current event time stamp matches the deltas on
  * the buffer page.
  */
 static void check_buffer(struct ring_buffer_per_cpu *cpu_buffer,
              struct rb_event_info *info,
              unsigned long tail)
 {
     struct ring_buffer_event *event;
     struct buffer_data_page *bpage;
     u64 ts, delta;
     bool full = false;
     int e;
 
     bpage = info->tail_page->page;
 
     if (tail == CHECK_FULL_PAGE) {
         full = true;
         tail = local_read(&bpage->commit);
     } else if (info->add_timestamp &
            (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE)) {
         /* Ignore events with absolute time stamps */
         return;
     }
 
     /*
      * Do not check the first event (skip possible extends too).
      * Also do not check if previous events have not been committed.
      */
     if (tail <= 8 || tail > local_read(&bpage->commit))
         return;
 
     /*
      * If this interrupted another event, 
      */
     if (atomic_inc_return(this_cpu_ptr(&checking)) != 1)
         goto out;
 
     ts = bpage->time_stamp;
 
     for (e = 0; e < tail; e += rb_event_length(event)) {
 
         event = (struct ring_buffer_event *)(bpage->data + e);
 
         switch (event->type_len) {
 
         case RINGBUF_TYPE_TIME_EXTEND:
             delta = rb_event_time_stamp(event);
             ts += delta;
             break;
 
         case RINGBUF_TYPE_TIME_STAMP:
             delta = rb_event_time_stamp(event);
             ts = rb_fix_abs_ts(delta, ts);
             break;
 
         case RINGBUF_TYPE_PADDING:
             if (event->time_delta == 1)
                 break;
             fallthrough;
         case RINGBUF_TYPE_DATA:
             ts += event->time_delta;
             break;
 
         default:
             RB_WARN_ON(cpu_buffer, 1);
         }
     }
     if ((full && ts > info->ts) ||
         (!full && ts + info->delta != info->ts)) {
         /* If another report is happening, ignore this one */
         if (atomic_inc_return(&ts_dump) != 1) {
             atomic_dec(&ts_dump);
             goto out;
         }
         atomic_inc(&cpu_buffer->record_disabled);
         /* There's some cases in boot up that this can happen */
         WARN_ON_ONCE(system_state != SYSTEM_BOOTING);
         pr_warn("[CPU: %d]TIME DOES NOT MATCH expected:%lld actual:%lld delta:%lld before:%lld after:%lld%s\n",
             cpu_buffer->cpu,
             ts + info->delta, info->ts, info->delta,
             info->before, info->after,
             full ? " (full)" : "");
         dump_buffer_page(bpage, info, tail);
         atomic_dec(&ts_dump);
         /* Do not re-enable checking */
         return;
     }
 out:
     atomic_dec(this_cpu_ptr(&checking));
 }
 #else
 static inline void check_buffer(struct ring_buffer_per_cpu *cpu_buffer,
              struct rb_event_info *info,
              unsigned long tail)
 {
 }
 #endif /* CONFIG_RING_BUFFER_VALIDATE_TIME_DELTAS */
 
 static struct ring_buffer_event *
 __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
           struct rb_event_info *info)
 {
     struct ring_buffer_event *event;
     struct buffer_page *tail_page;
     unsigned long tail, write, w;
     bool a_ok;
     bool b_ok;
 
     /* Don't let the compiler play games with cpu_buffer->tail_page */
     tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
 
  /*A*/  w = local_read(&tail_page->write) & RB_WRITE_MASK;
     barrier();
     b_ok = rb_time_read(&cpu_buffer->before_stamp, &info->before);
     a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after);
     barrier();
     info->ts = rb_time_stamp(cpu_buffer->buffer);
 
     if ((info->add_timestamp & RB_ADD_STAMP_ABSOLUTE)) {
         info->delta = info->ts;
     } else {
         /*
          * If interrupting an event time update, we may need an
          * absolute timestamp.
          * Don't bother if this is the start of a new page (w == 0).
          */
         if (unlikely(!a_ok || !b_ok || (info->before != info->after && w))) {
             info->add_timestamp |= RB_ADD_STAMP_FORCE | RB_ADD_STAMP_EXTEND;
             info->length += RB_LEN_TIME_EXTEND;
         } else {
             info->delta = info->ts - info->after;
             if (unlikely(test_time_stamp(info->delta))) {
                 info->add_timestamp |= RB_ADD_STAMP_EXTEND;
                 info->length += RB_LEN_TIME_EXTEND;
             }
         }
     }
 
  /*B*/  rb_time_set(&cpu_buffer->before_stamp, info->ts);
 
  /*C*/  write = local_add_return(info->length, &tail_page->write);
 
     /* set write to only the index of the write */
     write &= RB_WRITE_MASK;
 
     tail = write - info->length;
 
     /* See if we shot pass the end of this buffer page */
     if (unlikely(write > BUF_PAGE_SIZE)) {
         /* before and after may now different, fix it up*/
         b_ok = rb_time_read(&cpu_buffer->before_stamp, &info->before);
         a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after);
         if (a_ok && b_ok && info->before != info->after)
             (void)rb_time_cmpxchg(&cpu_buffer->before_stamp,
                           info->before, info->after);
         if (a_ok && b_ok)
             check_buffer(cpu_buffer, info, CHECK_FULL_PAGE);
         return rb_move_tail(cpu_buffer, tail, info);
     }
 
     if (likely(tail == w)) {
         u64 save_before;
         bool s_ok;
 
         /* Nothing interrupted us between A and C */
  /*D*/      rb_time_set(&cpu_buffer->write_stamp, info->ts);
         barrier();
  /*E*/      s_ok = rb_time_read(&cpu_buffer->before_stamp, &save_before);
         RB_WARN_ON(cpu_buffer, !s_ok);
         if (likely(!(info->add_timestamp &
                  (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE))))
             /* This did not interrupt any time update */
             info->delta = info->ts - info->after;
         else
             /* Just use full timestamp for interrupting event */
             info->delta = info->ts;
         barrier();
         check_buffer(cpu_buffer, info, tail);
         if (unlikely(info->ts != save_before)) {
             /* SLOW PATH - Interrupted between C and E */
 
             a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after);
             RB_WARN_ON(cpu_buffer, !a_ok);
 
             /* Write stamp must only go forward */
             if (save_before > info->after) {
                 /*
                  * We do not care about the result, only that
                  * it gets updated atomically.
                  */
                 (void)rb_time_cmpxchg(&cpu_buffer->write_stamp,
                               info->after, save_before);
             }
         }
     } else {
         u64 ts;
         /* SLOW PATH - Interrupted between A and C */
         a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after);
         /* Was interrupted before here, write_stamp must be valid */
         RB_WARN_ON(cpu_buffer, !a_ok);
         ts = rb_time_stamp(cpu_buffer->buffer);
         barrier();
  /*E*/      if (write == (local_read(&tail_page->write) & RB_WRITE_MASK) &&
             info->after < ts &&
             rb_time_cmpxchg(&cpu_buffer->write_stamp,
                     info->after, ts)) {
             /* Nothing came after this event between C and E */
             info->delta = ts - info->after;
         } else {
             /*
              * Interrupted between C and E:
              * Lost the previous events time stamp. Just set the
              * delta to zero, and this will be the same time as
              * the event this event interrupted. And the events that
              * came after this will still be correct (as they would
              * have built their delta on the previous event.
              */
             info->delta = 0;
         }
         info->ts = ts;
         info->add_timestamp &= ~RB_ADD_STAMP_FORCE;
     }
 
     /*
      * If this is the first commit on the page, then it has the same
      * timestamp as the page itself.
      */
     if (unlikely(!tail && !(info->add_timestamp &
                 (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE))))
         info->delta = 0;
 
     /* We reserved something on the buffer */
 
     event = __rb_page_index(tail_page, tail);
     rb_update_event(cpu_buffer, event, info);
 
     local_inc(&tail_page->entries);
 
     /*
      * If this is the first commit on the page, then update
      * its timestamp.
      */
     if (unlikely(!tail))
         tail_page->page->time_stamp = info->ts;
 
     /* account for these added bytes */
     local_add(info->length, &cpu_buffer->entries_bytes);
 
     return event;
 }
 
 static __always_inline struct ring_buffer_event *
 rb_reserve_next_event(struct trace_buffer *buffer,
               struct ring_buffer_per_cpu *cpu_buffer,
               unsigned long length)
 {
     struct ring_buffer_event *event;
     struct rb_event_info info;
     int nr_loops = 0;
     int add_ts_default;
 
     rb_start_commit(cpu_buffer);
     /* The commit page can not change after this */
 
 #ifdef CONFIG_RING_BUFFER_ALLOW_SWAP
     /*
      * Due to the ability to swap a cpu buffer from a buffer
      * it is possible it was swapped before we committed.
      * (committing stops a swap). We check for it here and
      * if it happened, we have to fail the write.
      */
     barrier();
     if (unlikely(READ_ONCE(cpu_buffer->buffer) != buffer)) {
         local_dec(&cpu_buffer->committing);
         local_dec(&cpu_buffer->commits);
         return NULL;
     }
 #endif
 
     info.length = rb_calculate_event_length(length);
 
     if (ring_buffer_time_stamp_abs(cpu_buffer->buffer)) {
         add_ts_default = RB_ADD_STAMP_ABSOLUTE;
         info.length += RB_LEN_TIME_EXTEND;
     } else {
         add_ts_default = RB_ADD_STAMP_NONE;
     }
 
  again:
     info.add_timestamp = add_ts_default;
     info.delta = 0;
 
     /*
      * We allow for interrupts to reenter here and do a trace.
      * If one does, it will cause this original code to loop
      * back here. Even with heavy interrupts happening, this
      * should only happen a few times in a row. If this happens
      * 1000 times in a row, there must be either an interrupt
      * storm or we have something buggy.
      * Bail!
      */
     if (RB_WARN_ON(cpu_buffer, ++nr_loops > 1000))
         goto out_fail;
 
     event = __rb_reserve_next(cpu_buffer, &info);
 
     if (unlikely(PTR_ERR(event) == -EAGAIN)) {
         if (info.add_timestamp & (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_EXTEND))
             info.length -= RB_LEN_TIME_EXTEND;
         goto again;
     }
 
     if (likely(event))
         return event;
  out_fail:
     rb_end_commit(cpu_buffer);
     return NULL;
 }
 
 /**
  * ring_buffer_lock_reserve - reserve a part of the buffer
  * @buffer: the ring buffer to reserve from
  * @length: the length of the data to reserve (excluding event header)
  *
  * Returns a reserved event on the ring buffer to copy directly to.
  * The user of this interface will need to get the body to write into
  * and can use the ring_buffer_event_data() interface.
  *
  * The length is the length of the data needed, not the event length
  * which also includes the event header.
  *
  * Must be paired with ring_buffer_unlock_commit, unless NULL is returned.
  * If NULL is returned, then nothing has been allocated or locked.
  */
 struct ring_buffer_event *
 ring_buffer_lock_reserve(struct trace_buffer *buffer, unsigned long length)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     struct ring_buffer_event *event;
     int cpu;
 
     /* If we are tracing schedule, we don't want to recurse */
     preempt_disable_notrace();
 
     if (unlikely(atomic_read(&buffer->record_disabled)))
         goto out;
 
     cpu = raw_smp_processor_id();
 
     if (unlikely(!cpumask_test_cpu(cpu, buffer->cpumask)))
         goto out;
 
     cpu_buffer = buffer->buffers[cpu];
 
     if (unlikely(atomic_read(&cpu_buffer->record_disabled)))
         goto out;
 
     if (unlikely(length > BUF_MAX_DATA_SIZE))
         goto out;
 
     if (unlikely(trace_recursive_lock(cpu_buffer)))
         goto out;
 
     event = rb_reserve_next_event(buffer, cpu_buffer, length);
     if (!event)
         goto out_unlock;
 
     return event;
 
  out_unlock:
     trace_recursive_unlock(cpu_buffer);
  out:
     preempt_enable_notrace();
     return NULL;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_lock_reserve);
 
 /*
  * Decrement the entries to the page that an event is on.
  * The event does not even need to exist, only the pointer
  * to the page it is on. This may only be called before the commit
  * takes place.
  */
 static inline void
 rb_decrement_entry(struct ring_buffer_per_cpu *cpu_buffer,
            struct ring_buffer_event *event)
 {
     unsigned long addr = (unsigned long)event;
     struct buffer_page *bpage = cpu_buffer->commit_page;
     struct buffer_page *start;
 
     addr &= PAGE_MASK;
 
     /* Do the likely case first */
     if (likely(bpage->page == (void *)addr)) {
         local_dec(&bpage->entries);
         return;
     }
 
     /*
      * Because the commit page may be on the reader page we
      * start with the next page and check the end loop there.
      */
     rb_inc_page(&bpage);
     start = bpage;
     do {
         if (bpage->page == (void *)addr) {
             local_dec(&bpage->entries);
             return;
         }
         rb_inc_page(&bpage);
     } while (bpage != start);
 
     /* commit not part of this buffer?? */
     RB_WARN_ON(cpu_buffer, 1);
 }
 
 /**
  * ring_buffer_discard_commit - discard an event that has not been committed
  * @buffer: the ring buffer
  * @event: non committed event to discard
  *
  * Sometimes an event that is in the ring buffer needs to be ignored.
  * This function lets the user discard an event in the ring buffer
  * and then that event will not be read later.
  *
  * This function only works if it is called before the item has been
  * committed. It will try to free the event from the ring buffer
  * if another event has not been added behind it.
  *
  * If another event has been added behind it, it will set the event
  * up as discarded, and perform the commit.
  *
  * If this function is called, do not call ring_buffer_unlock_commit on
  * the event.
  */
 void ring_buffer_discard_commit(struct trace_buffer *buffer,
                 struct ring_buffer_event *event)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     int cpu;
 
     /* The event is discarded regardless */
     rb_event_discard(event);
 
     cpu = smp_processor_id();
     cpu_buffer = buffer->buffers[cpu];
 
     /*
      * This must only be called if the event has not been
      * committed yet. Thus we can assume that preemption
      * is still disabled.
      */
     RB_WARN_ON(buffer, !local_read(&cpu_buffer->committing));
 
     rb_decrement_entry(cpu_buffer, event);
     if (rb_try_to_discard(cpu_buffer, event))
         goto out;
 
  out:
     rb_end_commit(cpu_buffer);
 
     trace_recursive_unlock(cpu_buffer);
 
     preempt_enable_notrace();
 
 }
 EXPORT_SYMBOL_GPL(ring_buffer_discard_commit);
 
 /**
  * ring_buffer_write - write data to the buffer without reserving
  * @buffer: The ring buffer to write to.
  * @length: The length of the data being written (excluding the event header)
  * @data: The data to write to the buffer.
  *
  * This is like ring_buffer_lock_reserve and ring_buffer_unlock_commit as
  * one function. If you already have the data to write to the buffer, it
  * may be easier to simply call this function.
  *
  * Note, like ring_buffer_lock_reserve, the length is the length of the data
  * and not the length of the event which would hold the header.
  */
 int ring_buffer_write(struct trace_buffer *buffer,
               unsigned long length,
               void *data)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     struct ring_buffer_event *event;
     void *body;
     int ret = -EBUSY;
     int cpu;
 
     preempt_disable_notrace();
 
     if (atomic_read(&buffer->record_disabled))
         goto out;
 
     cpu = raw_smp_processor_id();
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         goto out;
 
     cpu_buffer = buffer->buffers[cpu];
 
     if (atomic_read(&cpu_buffer->record_disabled))
         goto out;
 
     if (length > BUF_MAX_DATA_SIZE)
         goto out;
 
     if (unlikely(trace_recursive_lock(cpu_buffer)))
         goto out;
 
     event = rb_reserve_next_event(buffer, cpu_buffer, length);
     if (!event)
         goto out_unlock;
 
     body = rb_event_data(event);
 
     memcpy(body, data, length);
 
     rb_commit(cpu_buffer, event);
 
     rb_wakeups(buffer, cpu_buffer);
 
     ret = 0;
 
  out_unlock:
     trace_recursive_unlock(cpu_buffer);
 
  out:
     preempt_enable_notrace();
 
     return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_write);
 
 static bool rb_per_cpu_empty(struct ring_buffer_per_cpu *cpu_buffer)
 {
     struct buffer_page *reader = cpu_buffer->reader_page;
     struct buffer_page *head = rb_set_head_page(cpu_buffer);
     struct buffer_page *commit = cpu_buffer->commit_page;
 
     /* In case of error, head will be NULL */
     if (unlikely(!head))
         return true;
 
     /* Reader should exhaust content in reader page */
     if (reader->read != rb_page_commit(reader))
         return false;
 
     /*
      * If writers are committing on the reader page, knowing all
      * committed content has been read, the ring buffer is empty.
      */
     if (commit == reader)
         return true;
 
     /*
      * If writers are committing on a page other than reader page
      * and head page, there should always be content to read.
      */
     if (commit != head)
         return false;
 
     /*
      * Writers are committing on the head page, we just need
      * to care about there're committed data, and the reader will
      * swap reader page with head page when it is to read data.
      */
     return rb_page_commit(commit) == 0;
 }
 
 /**
  * ring_buffer_record_disable - stop all writes into the buffer
  * @buffer: The ring buffer to stop writes to.
  *
  * This prevents all writes to the buffer. Any attempt to write
  * to the buffer after this will fail and return NULL.
  *
  * The caller should call synchronize_rcu() after this.
  */
 void ring_buffer_record_disable(struct trace_buffer *buffer)
 {
     atomic_inc(&buffer->record_disabled);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_record_disable);
 
 /**
  * ring_buffer_record_enable - enable writes to the buffer
  * @buffer: The ring buffer to enable writes
  *
  * Note, multiple disables will need the same number of enables
  * to truly enable the writing (much like preempt_disable).
  */
 void ring_buffer_record_enable(struct trace_buffer *buffer)
 {
     atomic_dec(&buffer->record_disabled);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_record_enable);
 
 /**
  * ring_buffer_record_off - stop all writes into the buffer
  * @buffer: The ring buffer to stop writes to.
  *
  * This prevents all writes to the buffer. Any attempt to write
  * to the buffer after this will fail and return NULL.
  *
  * This is different than ring_buffer_record_disable() as
  * it works like an on/off switch, where as the disable() version
  * must be paired with a enable().
  */
 void ring_buffer_record_off(struct trace_buffer *buffer)
 {
     unsigned int rd;
     unsigned int new_rd;
 
     do {
         rd = atomic_read(&buffer->record_disabled);
         new_rd = rd | RB_BUFFER_OFF;
     } while (atomic_cmpxchg(&buffer->record_disabled, rd, new_rd) != rd);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_record_off);
 
 /**
  * ring_buffer_record_on - restart writes into the buffer
  * @buffer: The ring buffer to start writes to.
  *
  * This enables all writes to the buffer that was disabled by
  * ring_buffer_record_off().
  *
  * This is different than ring_buffer_record_enable() as
  * it works like an on/off switch, where as the enable() version
  * must be paired with a disable().
  */
 void ring_buffer_record_on(struct trace_buffer *buffer)
 {
     unsigned int rd;
     unsigned int new_rd;
 
     do {
         rd = atomic_read(&buffer->record_disabled);
         new_rd = rd & ~RB_BUFFER_OFF;
     } while (atomic_cmpxchg(&buffer->record_disabled, rd, new_rd) != rd);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_record_on);
 
 /**
  * ring_buffer_record_is_on - return true if the ring buffer can write
  * @buffer: The ring buffer to see if write is enabled
  *
  * Returns true if the ring buffer is in a state that it accepts writes.
  */
 bool ring_buffer_record_is_on(struct trace_buffer *buffer)
 {
     return !atomic_read(&buffer->record_disabled);
 }
 
 /**
  * ring_buffer_record_is_set_on - return true if the ring buffer is set writable
  * @buffer: The ring buffer to see if write is set enabled
  *
  * Returns true if the ring buffer is set writable by ring_buffer_record_on().
  * Note that this does NOT mean it is in a writable state.
  *
  * It may return true when the ring buffer has been disabled by
  * ring_buffer_record_disable(), as that is a temporary disabling of
  * the ring buffer.
  */
 bool ring_buffer_record_is_set_on(struct trace_buffer *buffer)
 {
     return !(atomic_read(&buffer->record_disabled) & RB_BUFFER_OFF);
 }
 
 /**
  * ring_buffer_record_disable_cpu - stop all writes into the cpu_buffer
  * @buffer: The ring buffer to stop writes to.
  * @cpu: The CPU buffer to stop
  *
  * This prevents all writes to the buffer. Any attempt to write
  * to the buffer after this will fail and return NULL.
  *
  * The caller should call synchronize_rcu() after this.
  */
 void ring_buffer_record_disable_cpu(struct trace_buffer *buffer, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return;
 
     cpu_buffer = buffer->buffers[cpu];
     atomic_inc(&cpu_buffer->record_disabled);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_record_disable_cpu);
 
 /**
  * ring_buffer_record_enable_cpu - enable writes to the buffer
  * @buffer: The ring buffer to enable writes
  * @cpu: The CPU to enable.
  *
  * Note, multiple disables will need the same number of enables
  * to truly enable the writing (much like preempt_disable).
  */
 void ring_buffer_record_enable_cpu(struct trace_buffer *buffer, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return;
 
     cpu_buffer = buffer->buffers[cpu];
     atomic_dec(&cpu_buffer->record_disabled);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_record_enable_cpu);
 
 /*
  * The total entries in the ring buffer is the running counter
  * of entries entered into the ring buffer, minus the sum of
  * the entries read from the ring buffer and the number of
  * entries that were overwritten.
  */
 static inline unsigned long
 rb_num_of_entries(struct ring_buffer_per_cpu *cpu_buffer)
 {
     return local_read(&cpu_buffer->entries) -
         (local_read(&cpu_buffer->overrun) + cpu_buffer->read);
 }
 
 /**
  * ring_buffer_oldest_event_ts - get the oldest event timestamp from the buffer
  * @buffer: The ring buffer
  * @cpu: The per CPU buffer to read from.
  */
 u64 ring_buffer_oldest_event_ts(struct trace_buffer *buffer, int cpu)
 {
     unsigned long flags;
     struct ring_buffer_per_cpu *cpu_buffer;
     struct buffer_page *bpage;
     u64 ret = 0;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return 0;
 
     cpu_buffer = buffer->buffers[cpu];
     raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
     /*
      * if the tail is on reader_page, oldest time stamp is on the reader
      * page
      */
     if (cpu_buffer->tail_page == cpu_buffer->reader_page)
         bpage = cpu_buffer->reader_page;
     else
         bpage = rb_set_head_page(cpu_buffer);
     if (bpage)
         ret = bpage->page->time_stamp;
     raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 
     return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_oldest_event_ts);
 
 /**
  * ring_buffer_bytes_cpu - get the number of bytes consumed in a cpu buffer
  * @buffer: The ring buffer
  * @cpu: The per CPU buffer to read from.
  */
 unsigned long ring_buffer_bytes_cpu(struct trace_buffer *buffer, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     unsigned long ret;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return 0;
 
     cpu_buffer = buffer->buffers[cpu];
     ret = local_read(&cpu_buffer->entries_bytes) - cpu_buffer->read_bytes;
 
     return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_bytes_cpu);
 
 /**
  * ring_buffer_entries_cpu - get the number of entries in a cpu buffer
  * @buffer: The ring buffer
  * @cpu: The per CPU buffer to get the entries from.
  */
 unsigned long ring_buffer_entries_cpu(struct trace_buffer *buffer, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return 0;
 
     cpu_buffer = buffer->buffers[cpu];
 
     return rb_num_of_entries(cpu_buffer);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_entries_cpu);
 
 /**
  * ring_buffer_overrun_cpu - get the number of overruns caused by the ring
  * buffer wrapping around (only if RB_FL_OVERWRITE is on).
  * @buffer: The ring buffer
  * @cpu: The per CPU buffer to get the number of overruns from
  */
 unsigned long ring_buffer_overrun_cpu(struct trace_buffer *buffer, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     unsigned long ret;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return 0;
 
     cpu_buffer = buffer->buffers[cpu];
     ret = local_read(&cpu_buffer->overrun);
 
     return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_overrun_cpu);
 
 /**
  * ring_buffer_commit_overrun_cpu - get the number of overruns caused by
  * commits failing due to the buffer wrapping around while there are uncommitted
  * events, such as during an interrupt storm.
  * @buffer: The ring buffer
  * @cpu: The per CPU buffer to get the number of overruns from
  */
 unsigned long
 ring_buffer_commit_overrun_cpu(struct trace_buffer *buffer, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     unsigned long ret;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return 0;
 
     cpu_buffer = buffer->buffers[cpu];
     ret = local_read(&cpu_buffer->commit_overrun);
 
     return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_commit_overrun_cpu);
 
 /**
  * ring_buffer_dropped_events_cpu - get the number of dropped events caused by
  * the ring buffer filling up (only if RB_FL_OVERWRITE is off).
  * @buffer: The ring buffer
  * @cpu: The per CPU buffer to get the number of overruns from
  */
 unsigned long
 ring_buffer_dropped_events_cpu(struct trace_buffer *buffer, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     unsigned long ret;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return 0;
 
     cpu_buffer = buffer->buffers[cpu];
     ret = local_read(&cpu_buffer->dropped_events);
 
     return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_dropped_events_cpu);
 
 /**
  * ring_buffer_read_events_cpu - get the number of events successfully read
  * @buffer: The ring buffer
  * @cpu: The per CPU buffer to get the number of events read
  */
 unsigned long
 ring_buffer_read_events_cpu(struct trace_buffer *buffer, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return 0;
 
     cpu_buffer = buffer->buffers[cpu];
     return cpu_buffer->read;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_read_events_cpu);
 
 /**
  * ring_buffer_entries - get the number of entries in a buffer
  * @buffer: The ring buffer
  *
  * Returns the total number of entries in the ring buffer
  * (all CPU entries)
  */
 unsigned long ring_buffer_entries(struct trace_buffer *buffer)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     unsigned long entries = 0;
     int cpu;
 
     /* if you care about this being correct, lock the buffer */
     for_each_buffer_cpu(buffer, cpu) {
         cpu_buffer = buffer->buffers[cpu];
         entries += rb_num_of_entries(cpu_buffer);
     }
 
     return entries;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_entries);
 
 /**
  * ring_buffer_overruns - get the number of overruns in buffer
  * @buffer: The ring buffer
  *
  * Returns the total number of overruns in the ring buffer
  * (all CPU entries)
  */
 unsigned long ring_buffer_overruns(struct trace_buffer *buffer)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     unsigned long overruns = 0;
     int cpu;
 
     /* if you care about this being correct, lock the buffer */
     for_each_buffer_cpu(buffer, cpu) {
         cpu_buffer = buffer->buffers[cpu];
         overruns += local_read(&cpu_buffer->overrun);
     }
 
     return overruns;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_overruns);
 
 static void rb_iter_reset(struct ring_buffer_iter *iter)
 {
     struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
 
     /* Iterator usage is expected to have record disabled */
     iter->head_page = cpu_buffer->reader_page;
     iter->head = cpu_buffer->reader_page->read;
     iter->next_event = iter->head;
 
     iter->cache_reader_page = iter->head_page;
     iter->cache_read = cpu_buffer->read;
 
     if (iter->head) {
         iter->read_stamp = cpu_buffer->read_stamp;
         iter->page_stamp = cpu_buffer->reader_page->page->time_stamp;
     } else {
         iter->read_stamp = iter->head_page->page->time_stamp;
         iter->page_stamp = iter->read_stamp;
     }
 }
 
 /**
  * ring_buffer_iter_reset - reset an iterator
  * @iter: The iterator to reset
  *
  * Resets the iterator, so that it will start from the beginning
  * again.
  */
 void ring_buffer_iter_reset(struct ring_buffer_iter *iter)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     unsigned long flags;
 
     if (!iter)
         return;
 
     cpu_buffer = iter->cpu_buffer;
 
     raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
     rb_iter_reset(iter);
     raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_iter_reset);
 
 /**
  * ring_buffer_iter_empty - check if an iterator has no more to read
  * @iter: The iterator to check
  */
 int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     struct buffer_page *reader;
     struct buffer_page *head_page;
     struct buffer_page *commit_page;
     struct buffer_page *curr_commit_page;
     unsigned commit;
     u64 curr_commit_ts;
     u64 commit_ts;
 
     cpu_buffer = iter->cpu_buffer;
     reader = cpu_buffer->reader_page;
     head_page = cpu_buffer->head_page;
     commit_page = cpu_buffer->commit_page;
     commit_ts = commit_page->page->time_stamp;
 
     /*
      * When the writer goes across pages, it issues a cmpxchg which
      * is a mb(), which will synchronize with the rmb here.
      * (see rb_tail_page_update())
      */
     smp_rmb();
     commit = rb_page_commit(commit_page);
     /* We want to make sure that the commit page doesn't change */
     smp_rmb();
 
     /* Make sure commit page didn't change */
     curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
     curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
 
     /* If the commit page changed, then there's more data */
     if (curr_commit_page != commit_page ||
         curr_commit_ts != commit_ts)
         return 0;
 
     /* Still racy, as it may return a false positive, but that's OK */
     return ((iter->head_page == commit_page && iter->head >= commit) ||
         (iter->head_page == reader && commit_page == head_page &&
          head_page->read == commit &&
          iter->head == rb_page_commit(cpu_buffer->reader_page)));
 }
 EXPORT_SYMBOL_GPL(ring_buffer_iter_empty);
 
 static void
 rb_update_read_stamp(struct ring_buffer_per_cpu *cpu_buffer,
              struct ring_buffer_event *event)
 {
     u64 delta;
 
     switch (event->type_len) {
     case RINGBUF_TYPE_PADDING:
         return;
 
     case RINGBUF_TYPE_TIME_EXTEND:
         delta = rb_event_time_stamp(event);
         cpu_buffer->read_stamp += delta;
         return;
 
     case RINGBUF_TYPE_TIME_STAMP:
         delta = rb_event_time_stamp(event);
         delta = rb_fix_abs_ts(delta, cpu_buffer->read_stamp);
         cpu_buffer->read_stamp = delta;
         return;
 
     case RINGBUF_TYPE_DATA:
         cpu_buffer->read_stamp += event->time_delta;
         return;
 
     default:
         RB_WARN_ON(cpu_buffer, 1);
     }
     return;
 }
 
 static void
 rb_update_iter_read_stamp(struct ring_buffer_iter *iter,
               struct ring_buffer_event *event)
 {
     u64 delta;
 
     switch (event->type_len) {
     case RINGBUF_TYPE_PADDING:
         return;
 
     case RINGBUF_TYPE_TIME_EXTEND:
         delta = rb_event_time_stamp(event);
         iter->read_stamp += delta;
         return;
 
     case RINGBUF_TYPE_TIME_STAMP:
         delta = rb_event_time_stamp(event);
         delta = rb_fix_abs_ts(delta, iter->read_stamp);
         iter->read_stamp = delta;
         return;
 
     case RINGBUF_TYPE_DATA:
         iter->read_stamp += event->time_delta;
         return;
 
     default:
         RB_WARN_ON(iter->cpu_buffer, 1);
     }
     return;
 }
 
 static struct buffer_page *
 rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
 {
     struct buffer_page *reader = NULL;
     unsigned long overwrite;
     unsigned long flags;
     int nr_loops = 0;
     int ret;
 
     local_irq_save(flags);
     arch_spin_lock(&cpu_buffer->lock);
 
  again:
     /*
      * This should normally only loop twice. But because the
      * start of the reader inserts an empty page, it causes
      * a case where we will loop three times. There should be no
      * reason to loop four times (that I know of).
      */
     if (RB_WARN_ON(cpu_buffer, ++nr_loops > 3)) {
         reader = NULL;
         goto out;
     }
 
     reader = cpu_buffer->reader_page;
 
     /* If there's more to read, return this page */
     if (cpu_buffer->reader_page->read < rb_page_size(reader))
         goto out;
 
     /* Never should we have an index greater than the size */
     if (RB_WARN_ON(cpu_buffer,
                cpu_buffer->reader_page->read > rb_page_size(reader)))
         goto out;
 
     /* check if we caught up to the tail */
     reader = NULL;
     if (cpu_buffer->commit_page == cpu_buffer->reader_page)
         goto out;
 
     /* Don't bother swapping if the ring buffer is empty */
     if (rb_num_of_entries(cpu_buffer) == 0)
         goto out;
 
     /*
      * Reset the reader page to size zero.
      */
     local_set(&cpu_buffer->reader_page->write, 0);
     local_set(&cpu_buffer->reader_page->entries, 0);
     local_set(&cpu_buffer->reader_page->page->commit, 0);
     cpu_buffer->reader_page->real_end = 0;
 
  spin:
     /*
      * Splice the empty reader page into the list around the head.
      */
     reader = rb_set_head_page(cpu_buffer);
     if (!reader)
         goto out;
     cpu_buffer->reader_page->list.next = rb_list_head(reader->list.next);
     cpu_buffer->reader_page->list.prev = reader->list.prev;
 
     /*
      * cpu_buffer->pages just needs to point to the buffer, it
      *  has no specific buffer page to point to. Lets move it out
      *  of our way so we don't accidentally swap it.
      */
     cpu_buffer->pages = reader->list.prev;
 
     /* The reader page will be pointing to the new head */
     rb_set_list_to_head(&cpu_buffer->reader_page->list);
 
     /*
      * We want to make sure we read the overruns after we set up our
      * pointers to the next object. The writer side does a
      * cmpxchg to cross pages which acts as the mb on the writer
      * side. Note, the reader will constantly fail the swap
      * while the writer is updating the pointers, so this
      * guarantees that the overwrite recorded here is the one we
      * want to compare with the last_overrun.
      */
     smp_mb();
     overwrite = local_read(&(cpu_buffer->overrun));
 
     /*
      * Here's the tricky part.
      *
      * We need to move the pointer past the header page.
      * But we can only do that if a writer is not currently
      * moving it. The page before the header page has the
      * flag bit '1' set if it is pointing to the page we want.
      * but if the writer is in the process of moving it
      * than it will be '2' or already moved '0'.
      */
 
     ret = rb_head_page_replace(reader, cpu_buffer->reader_page);
 
     /*
      * If we did not convert it, then we must try again.
      */
     if (!ret)
         goto spin;
 
     /*
      * Yay! We succeeded in replacing the page.
      *
      * Now make the new head point back to the reader page.
      */
     rb_list_head(reader->list.next)->prev = &cpu_buffer->reader_page->list;
     rb_inc_page(&cpu_buffer->head_page);
 
     local_inc(&cpu_buffer->pages_read);
 
     /* Finally update the reader page to the new head */
     cpu_buffer->reader_page = reader;
     cpu_buffer->reader_page->read = 0;
 
     if (overwrite != cpu_buffer->last_overrun) {
         cpu_buffer->lost_events = overwrite - cpu_buffer->last_overrun;
         cpu_buffer->last_overrun = overwrite;
     }
 
     goto again;
 
  out:
     /* Update the read_stamp on the first event */
     if (reader && reader->read == 0)
         cpu_buffer->read_stamp = reader->page->time_stamp;
 
     arch_spin_unlock(&cpu_buffer->lock);
     local_irq_restore(flags);
 
     return reader;
 }
 
 static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
 {
     struct ring_buffer_event *event;
     struct buffer_page *reader;
     unsigned length;
 
     reader = rb_get_reader_page(cpu_buffer);
 
     /* This function should not be called when buffer is empty */
     if (RB_WARN_ON(cpu_buffer, !reader))
         return;
 
     event = rb_reader_event(cpu_buffer);
 
     if (event->type_len <= RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
         cpu_buffer->read++;
 
     rb_update_read_stamp(cpu_buffer, event);
 
     length = rb_event_length(event);
     cpu_buffer->reader_page->read += length;
 }
 
 static void rb_advance_iter(struct ring_buffer_iter *iter)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
 
     cpu_buffer = iter->cpu_buffer;
 
     /* If head == next_event then we need to jump to the next event */
     if (iter->head == iter->next_event) {
         /* If the event gets overwritten again, there's nothing to do */
         if (rb_iter_head_event(iter) == NULL)
             return;
     }
 
     iter->head = iter->next_event;
 
     /*
      * Check if we are at the end of the buffer.
      */
     if (iter->next_event >= rb_page_size(iter->head_page)) {
         /* discarded commits can make the page empty */
         if (iter->head_page == cpu_buffer->commit_page)
             return;
         rb_inc_iter(iter);
         return;
     }
 
     rb_update_iter_read_stamp(iter, iter->event);
 }
 
 static int rb_lost_events(struct ring_buffer_per_cpu *cpu_buffer)
 {
     return cpu_buffer->lost_events;
 }
 
 static struct ring_buffer_event *
 rb_buffer_peek(struct ring_buffer_per_cpu *cpu_buffer, u64 *ts,
            unsigned long *lost_events)
 {
     struct ring_buffer_event *event;
     struct buffer_page *reader;
     int nr_loops = 0;
 
     if (ts)
         *ts = 0;
  again:
     /*
      * We repeat when a time extend is encountered.
      * Since the time extend is always attached to a data event,
      * we should never loop more than once.
      * (We never hit the following condition more than twice).
      */
     if (RB_WARN_ON(cpu_buffer, ++nr_loops > 2))
         return NULL;
 
     reader = rb_get_reader_page(cpu_buffer);
     if (!reader)
         return NULL;
 
     event = rb_reader_event(cpu_buffer);
 
     switch (event->type_len) {
     case RINGBUF_TYPE_PADDING:
         if (rb_null_event(event))
             RB_WARN_ON(cpu_buffer, 1);
         /*
          * Because the writer could be discarding every
          * event it creates (which would probably be bad)
          * if we were to go back to "again" then we may never
          * catch up, and will trigger the warn on, or lock
          * the box. Return the padding, and we will release
          * the current locks, and try again.
          */
         return event;
 
     case RINGBUF_TYPE_TIME_EXTEND:
         /* Internal data, OK to advance */
         rb_advance_reader(cpu_buffer);
         goto again;
 
     case RINGBUF_TYPE_TIME_STAMP:
         if (ts) {
             *ts = rb_event_time_stamp(event);
             *ts = rb_fix_abs_ts(*ts, reader->page->time_stamp);
             ring_buffer_normalize_time_stamp(cpu_buffer->buffer,
                              cpu_buffer->cpu, ts);
         }
         /* Internal data, OK to advance */
         rb_advance_reader(cpu_buffer);
         goto again;
 
     case RINGBUF_TYPE_DATA:
         if (ts && !(*ts)) {
             *ts = cpu_buffer->read_stamp + event->time_delta;
             ring_buffer_normalize_time_stamp(cpu_buffer->buffer,
                              cpu_buffer->cpu, ts);
         }
         if (lost_events)
             *lost_events = rb_lost_events(cpu_buffer);
         return event;
 
     default:
         RB_WARN_ON(cpu_buffer, 1);
     }
 
     return NULL;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_peek);
 
 static struct ring_buffer_event *
 rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
 {
     struct trace_buffer *buffer;
     struct ring_buffer_per_cpu *cpu_buffer;
     struct ring_buffer_event *event;
     int nr_loops = 0;
 
     if (ts)
         *ts = 0;
 
     cpu_buffer = iter->cpu_buffer;
     buffer = cpu_buffer->buffer;
 
     /*
      * Check if someone performed a consuming read to
      * the buffer. A consuming read invalidates the iterator
      * and we need to reset the iterator in this case.
      */
     if (unlikely(iter->cache_read != cpu_buffer->read ||
              iter->cache_reader_page != cpu_buffer->reader_page))
         rb_iter_reset(iter);
 
  again:
     if (ring_buffer_iter_empty(iter))
         return NULL;
 
     /*
      * As the writer can mess with what the iterator is trying
      * to read, just give up if we fail to get an event after
      * three tries. The iterator is not as reliable when reading
      * the ring buffer with an active write as the consumer is.
      * Do not warn if the three failures is reached.
      */
     if (++nr_loops > 3)
         return NULL;
 
     if (rb_per_cpu_empty(cpu_buffer))
         return NULL;
 
     if (iter->head >= rb_page_size(iter->head_page)) {
         rb_inc_iter(iter);
         goto again;
     }
 
     event = rb_iter_head_event(iter);
     if (!event)
         goto again;
 
     switch (event->type_len) {
     case RINGBUF_TYPE_PADDING:
         if (rb_null_event(event)) {
             rb_inc_iter(iter);
             goto again;
         }
         rb_advance_iter(iter);
         return event;
 
     case RINGBUF_TYPE_TIME_EXTEND:
         /* Internal data, OK to advance */
         rb_advance_iter(iter);
         goto again;
 
     case RINGBUF_TYPE_TIME_STAMP:
         if (ts) {
             *ts = rb_event_time_stamp(event);
             *ts = rb_fix_abs_ts(*ts, iter->head_page->page->time_stamp);
             ring_buffer_normalize_time_stamp(cpu_buffer->buffer,
                              cpu_buffer->cpu, ts);
         }
         /* Internal data, OK to advance */
         rb_advance_iter(iter);
         goto again;
 
     case RINGBUF_TYPE_DATA:
         if (ts && !(*ts)) {
             *ts = iter->read_stamp + event->time_delta;
             ring_buffer_normalize_time_stamp(buffer,
                              cpu_buffer->cpu, ts);
         }
         return event;
 
     default:
         RB_WARN_ON(cpu_buffer, 1);
     }
 
     return NULL;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_iter_peek);
 
 static inline bool rb_reader_lock(struct ring_buffer_per_cpu *cpu_buffer)
 {
     if (likely(!in_nmi())) {
         raw_spin_lock(&cpu_buffer->reader_lock);
         return true;
     }
 
     /*
      * If an NMI die dumps out the content of the ring buffer
      * trylock must be used to prevent a deadlock if the NMI
      * preempted a task that holds the ring buffer locks. If
      * we get the lock then all is fine, if not, then continue
      * to do the read, but this can corrupt the ring buffer,
      * so it must be permanently disabled from future writes.
      * Reading from NMI is a oneshot deal.
      */
     if (raw_spin_trylock(&cpu_buffer->reader_lock))
         return true;
 
     /* Continue without locking, but disable the ring buffer */
     atomic_inc(&cpu_buffer->record_disabled);
     return false;
 }
 
 static inline void
 rb_reader_unlock(struct ring_buffer_per_cpu *cpu_buffer, bool locked)
 {
     if (likely(locked))
         raw_spin_unlock(&cpu_buffer->reader_lock);
     return;
 }
 
 /**
  * ring_buffer_peek - peek at the next event to be read
  * @buffer: The ring buffer to read
  * @cpu: The cpu to peak at
  * @ts: The timestamp counter of this event.
  * @lost_events: a variable to store if events were lost (may be NULL)
  *
  * This will return the event that will be read next, but does
  * not consume the data.
  */
 struct ring_buffer_event *
 ring_buffer_peek(struct trace_buffer *buffer, int cpu, u64 *ts,
          unsigned long *lost_events)
 {
     struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
     struct ring_buffer_event *event;
     unsigned long flags;
     bool dolock;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return NULL;
 
  again:
     local_irq_save(flags);
     dolock = rb_reader_lock(cpu_buffer);
     event = rb_buffer_peek(cpu_buffer, ts, lost_events);
     if (event && event->type_len == RINGBUF_TYPE_PADDING)
         rb_advance_reader(cpu_buffer);
     rb_reader_unlock(cpu_buffer, dolock);
     local_irq_restore(flags);
 
     if (event && event->type_len == RINGBUF_TYPE_PADDING)
         goto again;
 
     return event;
 }
 
 /** ring_buffer_iter_dropped - report if there are dropped events
  * @iter: The ring buffer iterator
  *
  * Returns true if there was dropped events since the last peek.
  */
 bool ring_buffer_iter_dropped(struct ring_buffer_iter *iter)
 {
     bool ret = iter->missed_events != 0;
 
     iter->missed_events = 0;
     return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_iter_dropped);
 
 /**
  * ring_buffer_iter_peek - peek at the next event to be read
  * @iter: The ring buffer iterator
  * @ts: The timestamp counter of this event.
  *
  * This will return the event that will be read next, but does
  * not increment the iterator.
  */
 struct ring_buffer_event *
 ring_buffer_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
 {
     struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
     struct ring_buffer_event *event;
     unsigned long flags;
 
  again:
     raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
     event = rb_iter_peek(iter, ts);
     raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 
     if (event && event->type_len == RINGBUF_TYPE_PADDING)
         goto again;
 
     return event;
 }
 
 /**
  * ring_buffer_consume - return an event and consume it
  * @buffer: The ring buffer to get the next event from
  * @cpu: the cpu to read the buffer from
  * @ts: a variable to store the timestamp (may be NULL)
  * @lost_events: a variable to store if events were lost (may be NULL)
  *
  * Returns the next event in the ring buffer, and that event is consumed.
  * Meaning, that sequential reads will keep returning a different event,
  * and eventually empty the ring buffer if the producer is slower.
  */
 struct ring_buffer_event *
 ring_buffer_consume(struct trace_buffer *buffer, int cpu, u64 *ts,
             unsigned long *lost_events)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     struct ring_buffer_event *event = NULL;
     unsigned long flags;
     bool dolock;
 
  again:
     /* might be called in atomic */
     preempt_disable();
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         goto out;
 
     cpu_buffer = buffer->buffers[cpu];
     local_irq_save(flags);
     dolock = rb_reader_lock(cpu_buffer);
 
     event = rb_buffer_peek(cpu_buffer, ts, lost_events);
     if (event) {
         cpu_buffer->lost_events = 0;
         rb_advance_reader(cpu_buffer);
     }
 
     rb_reader_unlock(cpu_buffer, dolock);
     local_irq_restore(flags);
 
  out:
     preempt_enable();
 
     if (event && event->type_len == RINGBUF_TYPE_PADDING)
         goto again;
 
     return event;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_consume);
 
 /**
  * ring_buffer_read_prepare - Prepare for a non consuming read of the buffer
  * @buffer: The ring buffer to read from
  * @cpu: The cpu buffer to iterate over
  * @flags: gfp flags to use for memory allocation
  *
  * This performs the initial preparations necessary to iterate
  * through the buffer.  Memory is allocated, buffer recording
  * is disabled, and the iterator pointer is returned to the caller.
  *
  * Disabling buffer recording prevents the reading from being
  * corrupted. This is not a consuming read, so a producer is not
  * expected.
  *
  * After a sequence of ring_buffer_read_prepare calls, the user is
  * expected to make at least one call to ring_buffer_read_prepare_sync.
  * Afterwards, ring_buffer_read_start is invoked to get things going
  * for real.
  *
  * This overall must be paired with ring_buffer_read_finish.
  */
 struct ring_buffer_iter *
 ring_buffer_read_prepare(struct trace_buffer *buffer, int cpu, gfp_t flags)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     struct ring_buffer_iter *iter;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return NULL;
 
     iter = kzalloc(sizeof(*iter), flags);
     if (!iter)
         return NULL;
 
     iter->event = kmalloc(BUF_MAX_DATA_SIZE, flags);
     if (!iter->event) {
         kfree(iter);
         return NULL;
     }
 
     cpu_buffer = buffer->buffers[cpu];
 
     iter->cpu_buffer = cpu_buffer;
 
     atomic_inc(&cpu_buffer->resize_disabled);
 
     return iter;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_read_prepare);
 
 /**
  * ring_buffer_read_prepare_sync - Synchronize a set of prepare calls
  *
  * All previously invoked ring_buffer_read_prepare calls to prepare
  * iterators will be synchronized.  Afterwards, read_buffer_read_start
  * calls on those iterators are allowed.
  */
 void
 ring_buffer_read_prepare_sync(void)
 {
     synchronize_rcu();
 }
 EXPORT_SYMBOL_GPL(ring_buffer_read_prepare_sync);
 
 /**
  * ring_buffer_read_start - start a non consuming read of the buffer
  * @iter: The iterator returned by ring_buffer_read_prepare
  *
  * This finalizes the startup of an iteration through the buffer.
  * The iterator comes from a call to ring_buffer_read_prepare and
  * an intervening ring_buffer_read_prepare_sync must have been
  * performed.
  *
  * Must be paired with ring_buffer_read_finish.
  */
 void
 ring_buffer_read_start(struct ring_buffer_iter *iter)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     unsigned long flags;
 
     if (!iter)
         return;
 
     cpu_buffer = iter->cpu_buffer;
 
     raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
     arch_spin_lock(&cpu_buffer->lock);
     rb_iter_reset(iter);
     arch_spin_unlock(&cpu_buffer->lock);
     raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_read_start);
 
 /**
  * ring_buffer_read_finish - finish reading the iterator of the buffer
  * @iter: The iterator retrieved by ring_buffer_start
  *
  * This re-enables the recording to the buffer, and frees the
  * iterator.
  */
 void
 ring_buffer_read_finish(struct ring_buffer_iter *iter)
 {
     struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
     unsigned long flags;
 
     /*
      * Ring buffer is disabled from recording, here's a good place
      * to check the integrity of the ring buffer.
      * Must prevent readers from trying to read, as the check
      * clears the HEAD page and readers require it.
      */
     raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
     rb_check_pages(cpu_buffer);
     raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 
     atomic_dec(&cpu_buffer->resize_disabled);
     kfree(iter->event);
     kfree(iter);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_read_finish);
 
 /**
  * ring_buffer_iter_advance - advance the iterator to the next location
  * @iter: The ring buffer iterator
  *
  * Move the location of the iterator such that the next read will
  * be the next location of the iterator.
  */
 void ring_buffer_iter_advance(struct ring_buffer_iter *iter)
 {
     struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
     unsigned long flags;
 
     raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
 
     rb_advance_iter(iter);
 
     raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_iter_advance);
 
 /**
  * ring_buffer_size - return the size of the ring buffer (in bytes)
  * @buffer: The ring buffer.
  * @cpu: The CPU to get ring buffer size from.
  */
 unsigned long ring_buffer_size(struct trace_buffer *buffer, int cpu)
 {
     /*
      * Earlier, this method returned
      *  BUF_PAGE_SIZE * buffer->nr_pages
      * Since the nr_pages field is now removed, we have converted this to
      * return the per cpu buffer value.
      */
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return 0;
 
     return BUF_PAGE_SIZE * buffer->buffers[cpu]->nr_pages;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_size);
 
 static void
 rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
 {
     rb_head_page_deactivate(cpu_buffer);
 
     cpu_buffer->head_page
         = list_entry(cpu_buffer->pages, struct buffer_page, list);
     local_set(&cpu_buffer->head_page->write, 0);
     local_set(&cpu_buffer->head_page->entries, 0);
     local_set(&cpu_buffer->head_page->page->commit, 0);
 
     cpu_buffer->head_page->read = 0;
 
     cpu_buffer->tail_page = cpu_buffer->head_page;
     cpu_buffer->commit_page = cpu_buffer->head_page;
 
     INIT_LIST_HEAD(&cpu_buffer->reader_page->list);
     INIT_LIST_HEAD(&cpu_buffer->new_pages);
     local_set(&cpu_buffer->reader_page->write, 0);
     local_set(&cpu_buffer->reader_page->entries, 0);
     local_set(&cpu_buffer->reader_page->page->commit, 0);
     cpu_buffer->reader_page->read = 0;
 
     local_set(&cpu_buffer->entries_bytes, 0);
     local_set(&cpu_buffer->overrun, 0);
     local_set(&cpu_buffer->commit_overrun, 0);
     local_set(&cpu_buffer->dropped_events, 0);
     local_set(&cpu_buffer->entries, 0);
     local_set(&cpu_buffer->committing, 0);
     local_set(&cpu_buffer->commits, 0);
     local_set(&cpu_buffer->pages_touched, 0);
     local_set(&cpu_buffer->pages_read, 0);
     cpu_buffer->last_pages_touch = 0;
     cpu_buffer->shortest_full = 0;
     cpu_buffer->read = 0;
     cpu_buffer->read_bytes = 0;
 
     rb_time_set(&cpu_buffer->write_stamp, 0);
     rb_time_set(&cpu_buffer->before_stamp, 0);
 
     memset(cpu_buffer->event_stamp, 0, sizeof(cpu_buffer->event_stamp));
 
     cpu_buffer->lost_events = 0;
     cpu_buffer->last_overrun = 0;
 
     rb_head_page_activate(cpu_buffer);
 }
 
 /* Must have disabled the cpu buffer then done a synchronize_rcu */
 static void reset_disabled_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer)
 {
     unsigned long flags;
 
     raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
 
     if (RB_WARN_ON(cpu_buffer, local_read(&cpu_buffer->committing)))
         goto out;
 
     arch_spin_lock(&cpu_buffer->lock);
 
     rb_reset_cpu(cpu_buffer);
 
     arch_spin_unlock(&cpu_buffer->lock);
 
  out:
     raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 }
 
 /**
  * ring_buffer_reset_cpu - reset a ring buffer per CPU buffer
  * @buffer: The ring buffer to reset a per cpu buffer of
  * @cpu: The CPU buffer to be reset
  */
 void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return;
 
     /* prevent another thread from changing buffer sizes */
     mutex_lock(&buffer->mutex);
 
     atomic_inc(&cpu_buffer->resize_disabled);
     atomic_inc(&cpu_buffer->record_disabled);
 
     /* Make sure all commits have finished */
     synchronize_rcu();
 
     reset_disabled_cpu_buffer(cpu_buffer);
 
     atomic_dec(&cpu_buffer->record_disabled);
     atomic_dec(&cpu_buffer->resize_disabled);
 
     mutex_unlock(&buffer->mutex);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);
 
 /**
  * ring_buffer_reset_cpu - reset a ring buffer per CPU buffer
  * @buffer: The ring buffer to reset a per cpu buffer of
  * @cpu: The CPU buffer to be reset
  */
 void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     int cpu;
 
     /* prevent another thread from changing buffer sizes */
     mutex_lock(&buffer->mutex);
 
     for_each_online_buffer_cpu(buffer, cpu) {
         cpu_buffer = buffer->buffers[cpu];
 
         atomic_inc(&cpu_buffer->resize_disabled);
         atomic_inc(&cpu_buffer->record_disabled);
     }
 
     /* Make sure all commits have finished */
     synchronize_rcu();
 
     for_each_online_buffer_cpu(buffer, cpu) {
         cpu_buffer = buffer->buffers[cpu];
 
         reset_disabled_cpu_buffer(cpu_buffer);
 
         atomic_dec(&cpu_buffer->record_disabled);
         atomic_dec(&cpu_buffer->resize_disabled);
     }
 
     mutex_unlock(&buffer->mutex);
 }
 
 /**
  * ring_buffer_reset - reset a ring buffer
  * @buffer: The ring buffer to reset all cpu buffers
  */
 void ring_buffer_reset(struct trace_buffer *buffer)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     int cpu;
 
     /* prevent another thread from changing buffer sizes */
     mutex_lock(&buffer->mutex);
 
     for_each_buffer_cpu(buffer, cpu) {
         cpu_buffer = buffer->buffers[cpu];
 
         atomic_inc(&cpu_buffer->resize_disabled);
         atomic_inc(&cpu_buffer->record_disabled);
     }
 
     /* Make sure all commits have finished */
     synchronize_rcu();
 
     for_each_buffer_cpu(buffer, cpu) {
         cpu_buffer = buffer->buffers[cpu];
 
         reset_disabled_cpu_buffer(cpu_buffer);
 
         atomic_dec(&cpu_buffer->record_disabled);
         atomic_dec(&cpu_buffer->resize_disabled);
     }
 
     mutex_unlock(&buffer->mutex);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_reset);
 
 /**
  * rind_buffer_empty - is the ring buffer empty?
  * @buffer: The ring buffer to test
  */
 bool ring_buffer_empty(struct trace_buffer *buffer)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     unsigned long flags;
     bool dolock;
     int cpu;
     int ret;
 
     /* yes this is racy, but if you don't like the race, lock the buffer */
     for_each_buffer_cpu(buffer, cpu) {
         cpu_buffer = buffer->buffers[cpu];
         local_irq_save(flags);
         dolock = rb_reader_lock(cpu_buffer);
         ret = rb_per_cpu_empty(cpu_buffer);
         rb_reader_unlock(cpu_buffer, dolock);
         local_irq_restore(flags);
 
         if (!ret)
             return false;
     }
 
     return true;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_empty);
 
 /**
  * ring_buffer_empty_cpu - is a cpu buffer of a ring buffer empty?
  * @buffer: The ring buffer
  * @cpu: The CPU buffer to test
  */
 bool ring_buffer_empty_cpu(struct trace_buffer *buffer, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     unsigned long flags;
     bool dolock;
     int ret;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return true;
 
     cpu_buffer = buffer->buffers[cpu];
     local_irq_save(flags);
     dolock = rb_reader_lock(cpu_buffer);
     ret = rb_per_cpu_empty(cpu_buffer);
     rb_reader_unlock(cpu_buffer, dolock);
     local_irq_restore(flags);
 
     return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_empty_cpu);
 
 #ifdef CONFIG_RING_BUFFER_ALLOW_SWAP
 /**
  * ring_buffer_swap_cpu - swap a CPU buffer between two ring buffers
  * @buffer_a: One buffer to swap with
  * @buffer_b: The other buffer to swap with
  * @cpu: the CPU of the buffers to swap
  *
  * This function is useful for tracers that want to take a "snapshot"
  * of a CPU buffer and has another back up buffer lying around.
  * it is expected that the tracer handles the cpu buffer not being
  * used at the moment.
  */
 int ring_buffer_swap_cpu(struct trace_buffer *buffer_a,
              struct trace_buffer *buffer_b, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer_a;
     struct ring_buffer_per_cpu *cpu_buffer_b;
     int ret = -EINVAL;
 
     if (!cpumask_test_cpu(cpu, buffer_a->cpumask) ||
         !cpumask_test_cpu(cpu, buffer_b->cpumask))
         goto out;
 
     cpu_buffer_a = buffer_a->buffers[cpu];
     cpu_buffer_b = buffer_b->buffers[cpu];
 
     /* At least make sure the two buffers are somewhat the same */
     if (cpu_buffer_a->nr_pages != cpu_buffer_b->nr_pages)
         goto out;
 
     ret = -EAGAIN;
 
     if (atomic_read(&buffer_a->record_disabled))
         goto out;
 
     if (atomic_read(&buffer_b->record_disabled))
         goto out;
 
     if (atomic_read(&cpu_buffer_a->record_disabled))
         goto out;
 
     if (atomic_read(&cpu_buffer_b->record_disabled))
         goto out;
 
     /*
      * We can't do a synchronize_rcu here because this
      * function can be called in atomic context.
      * Normally this will be called from the same CPU as cpu.
      * If not it's up to the caller to protect this.
      */
     atomic_inc(&cpu_buffer_a->record_disabled);
     atomic_inc(&cpu_buffer_b->record_disabled);
 
     ret = -EBUSY;
     if (local_read(&cpu_buffer_a->committing))
         goto out_dec;
     if (local_read(&cpu_buffer_b->committing))
         goto out_dec;
 
     buffer_a->buffers[cpu] = cpu_buffer_b;
     buffer_b->buffers[cpu] = cpu_buffer_a;
 
     cpu_buffer_b->buffer = buffer_a;
     cpu_buffer_a->buffer = buffer_b;
 
     ret = 0;
 
 out_dec:
     atomic_dec(&cpu_buffer_a->record_disabled);
     atomic_dec(&cpu_buffer_b->record_disabled);
 out:
     return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_swap_cpu);
 #endif /* CONFIG_RING_BUFFER_ALLOW_SWAP */
 
 /**
  * ring_buffer_alloc_read_page - allocate a page to read from buffer
  * @buffer: the buffer to allocate for.
  * @cpu: the cpu buffer to allocate.
  *
  * This function is used in conjunction with ring_buffer_read_page.
  * When reading a full page from the ring buffer, these functions
  * can be used to speed up the process. The calling function should
  * allocate a few pages first with this function. Then when it
  * needs to get pages from the ring buffer, it passes the result
  * of this function into ring_buffer_read_page, which will swap
  * the page that was allocated, with the read page of the buffer.
  *
  * Returns:
  *  The page allocated, or ERR_PTR
  */
 void *ring_buffer_alloc_read_page(struct trace_buffer *buffer, int cpu)
 {
     struct ring_buffer_per_cpu *cpu_buffer;
     struct buffer_data_page *bpage = NULL;
     unsigned long flags;
     struct page *page;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         return ERR_PTR(-ENODEV);
 
     cpu_buffer = buffer->buffers[cpu];
     local_irq_save(flags);
     arch_spin_lock(&cpu_buffer->lock);
 
     if (cpu_buffer->free_page) {
         bpage = cpu_buffer->free_page;
         cpu_buffer->free_page = NULL;
     }
 
     arch_spin_unlock(&cpu_buffer->lock);
     local_irq_restore(flags);
 
     if (bpage)
         goto out;
 
     page = alloc_pages_node(cpu_to_node(cpu),
                 GFP_KERNEL | __GFP_NORETRY, 0);
     if (!page)
         return ERR_PTR(-ENOMEM);
 
     bpage = page_address(page);
 
  out:
     rb_init_page(bpage);
 
     return bpage;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_alloc_read_page);
 
 /**
  * ring_buffer_free_read_page - free an allocated read page
  * @buffer: the buffer the page was allocate for
  * @cpu: the cpu buffer the page came from
  * @data: the page to free
  *
  * Free a page allocated from ring_buffer_alloc_read_page.
  */
 void ring_buffer_free_read_page(struct trace_buffer *buffer, int cpu, void *data)
 {
     struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
     struct buffer_data_page *bpage = data;
     struct page *page = virt_to_page(bpage);
     unsigned long flags;
 
     /* If the page is still in use someplace else, we can't reuse it */
     if (page_ref_count(page) > 1)
         goto out;
 
     local_irq_save(flags);
     arch_spin_lock(&cpu_buffer->lock);
 
     if (!cpu_buffer->free_page) {
         cpu_buffer->free_page = bpage;
         bpage = NULL;
     }
 
     arch_spin_unlock(&cpu_buffer->lock);
     local_irq_restore(flags);
 
  out:
     free_page((unsigned long)bpage);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_free_read_page);
 
 /**
  * ring_buffer_read_page - extract a page from the ring buffer
  * @buffer: buffer to extract from
  * @data_page: the page to use allocated from ring_buffer_alloc_read_page
  * @len: amount to extract
  * @cpu: the cpu of the buffer to extract
  * @full: should the extraction only happen when the page is full.
  *
  * This function will pull out a page from the ring buffer and consume it.
  * @data_page must be the address of the variable that was returned
  * from ring_buffer_alloc_read_page. This is because the page might be used
  * to swap with a page in the ring buffer.
  *
  * for example:
  *  rpage = ring_buffer_alloc_read_page(buffer, cpu);
  *  if (IS_ERR(rpage))
  *      return PTR_ERR(rpage);
  *  ret = ring_buffer_read_page(buffer, &rpage, len, cpu, 0);
  *  if (ret >= 0)
  *      process_page(rpage, ret);
  *
  * When @full is set, the function will not return true unless
  * the writer is off the reader page.
  *
  * Note: it is up to the calling functions to handle sleeps and wakeups.
  *  The ring buffer can be used anywhere in the kernel and can not
  *  blindly call wake_up. The layer that uses the ring buffer must be
  *  responsible for that.
  *
  * Returns:
  *  >=0 if data has been transferred, returns the offset of consumed data.
  *  <0 if no data has been transferred.
  */
 int ring_buffer_read_page(struct trace_buffer *buffer,
               void **data_page, size_t len, int cpu, int full)
 {
     struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
     struct ring_buffer_event *event;
     struct buffer_data_page *bpage;
     struct buffer_page *reader;
     unsigned long missed_events;
     unsigned long flags;
     unsigned int commit;
     unsigned int read;
     u64 save_timestamp;
     int ret = -1;
 
     if (!cpumask_test_cpu(cpu, buffer->cpumask))
         goto out;
 
     /*
      * If len is not big enough to hold the page header, then
      * we can not copy anything.
      */
     if (len <= BUF_PAGE_HDR_SIZE)
         goto out;
 
     len -= BUF_PAGE_HDR_SIZE;
 
     if (!data_page)
         goto out;
 
     bpage = *data_page;
     if (!bpage)
         goto out;
 
     raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
 
     reader = rb_get_reader_page(cpu_buffer);
     if (!reader)
         goto out_unlock;
 
     event = rb_reader_event(cpu_buffer);
 
     read = reader->read;
     commit = rb_page_commit(reader);
 
     /* Check if any events were dropped */
     missed_events = cpu_buffer->lost_events;
 
     /*
      * If this page has been partially read or
      * if len is not big enough to read the rest of the page or
      * a writer is still on the page, then
      * we must copy the data from the page to the buffer.
      * Otherwise, we can simply swap the page with the one passed in.
      */
     if (read || (len < (commit - read)) ||
         cpu_buffer->reader_page == cpu_buffer->commit_page) {
         struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
         unsigned int rpos = read;
         unsigned int pos = 0;
         unsigned int size;
 
         if (full)
             goto out_unlock;
 
         if (len > (commit - read))
             len = (commit - read);
 
         /* Always keep the time extend and data together */
         size = rb_event_ts_length(event);
 
         if (len < size)
             goto out_unlock;
 
         /* save the current timestamp, since the user will need it */
         save_timestamp = cpu_buffer->read_stamp;
 
         /* Need to copy one event at a time */
         do {
             /* We need the size of one event, because
              * rb_advance_reader only advances by one event,
              * whereas rb_event_ts_length may include the size of
              * one or two events.
              * We have already ensured there's enough space if this
              * is a time extend. */
             size = rb_event_length(event);
             memcpy(bpage->data + pos, rpage->data + rpos, size);
 
             len -= size;
 
             rb_advance_reader(cpu_buffer);
             rpos = reader->read;
             pos += size;
 
             if (rpos >= commit)
                 break;
 
             event = rb_reader_event(cpu_buffer);
             /* Always keep the time extend and data together */
             size = rb_event_ts_length(event);
         } while (len >= size);
 
         /* update bpage */
         local_set(&bpage->commit, pos);
         bpage->time_stamp = save_timestamp;
 
         /* we copied everything to the beginning */
         read = 0;
     } else {
         /* update the entry counter */
         cpu_buffer->read += rb_page_entries(reader);
         cpu_buffer->read_bytes += BUF_PAGE_SIZE;
 
         /* swap the pages */
         rb_init_page(bpage);
         bpage = reader->page;
         reader->page = *data_page;
         local_set(&reader->write, 0);
         local_set(&reader->entries, 0);
         reader->read = 0;
         *data_page = bpage;
 
         /*
          * Use the real_end for the data size,
          * This gives us a chance to store the lost events
          * on the page.
          */
         if (reader->real_end)
             local_set(&bpage->commit, reader->real_end);
     }
     ret = read;
 
     cpu_buffer->lost_events = 0;
 
     commit = local_read(&bpage->commit);
     /*
      * Set a flag in the commit field if we lost events
      */
     if (missed_events) {
         /* If there is room at the end of the page to save the
          * missed events, then record it there.
          */
         if (BUF_PAGE_SIZE - commit >= sizeof(missed_events)) {
             memcpy(&bpage->data[commit], &missed_events,
                    sizeof(missed_events));
             local_add(RB_MISSED_STORED, &bpage->commit);
             commit += sizeof(missed_events);
         }
         local_add(RB_MISSED_EVENTS, &bpage->commit);
     }
 
     /*
      * This page may be off to user land. Zero it out here.
      */
     if (commit < BUF_PAGE_SIZE)
         memset(&bpage->data[commit], 0, BUF_PAGE_SIZE - commit);
 
  out_unlock:
     raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
 
  out:
     return ret;
 }
 EXPORT_SYMBOL_GPL(ring_buffer_read_page);
 
 /*
  * We only allocate new buffers, never free them if the CPU goes down.
  * If we were to free the buffer, then the user would lose any trace that was in
  * the buffer.
  */
 int trace_rb_cpu_prepare(unsigned int cpu, struct hlist_node *node)
 {
     struct trace_buffer *buffer;
     long nr_pages_same;
     int cpu_i;
     unsigned long nr_pages;
 
     buffer = container_of(node, struct trace_buffer, node);
     if (cpumask_test_cpu(cpu, buffer->cpumask))
         return 0;
 
     nr_pages = 0;
     nr_pages_same = 1;
     /* check if all cpu sizes are same */
     for_each_buffer_cpu(buffer, cpu_i) {
         /* fill in the size from first enabled cpu */
         if (nr_pages == 0)
             nr_pages = buffer->buffers[cpu_i]->nr_pages;
         if (nr_pages != buffer->buffers[cpu_i]->nr_pages) {
             nr_pages_same = 0;
             break;
         }
     }
     /* allocate minimum pages, user can later expand it */
     if (!nr_pages_same)
         nr_pages = 2;
     buffer->buffers[cpu] =
         rb_allocate_cpu_buffer(buffer, nr_pages, cpu);
     if (!buffer->buffers[cpu]) {
         WARN(1, "failed to allocate ring buffer on CPU %u\n",
              cpu);
         return -ENOMEM;
     }
     smp_wmb();
     cpumask_set_cpu(cpu, buffer->cpumask);
     return 0;
 }
 
 #ifdef CONFIG_RING_BUFFER_STARTUP_TEST
 /*
  * This is a basic integrity check of the ring buffer.
  * Late in the boot cycle this test will run when configured in.
  * It will kick off a thread per CPU that will go into a loop
  * writing to the per cpu ring buffer various sizes of data.
  * Some of the data will be large items, some small.
  *
  * Another thread is created that goes into a spin, sending out
  * IPIs to the other CPUs to also write into the ring buffer.
  * this is to test the nesting ability of the buffer.
  *
  * Basic stats are recorded and reported. If something in the
  * ring buffer should happen that's not expected, a big warning
  * is displayed and all ring buffers are disabled.
  */
 static struct task_struct *rb_threads[NR_CPUS] __initdata;
 
 struct rb_test_data {
     struct trace_buffer *buffer;
     unsigned long       events;
     unsigned long       bytes_written;
     unsigned long       bytes_alloc;
     unsigned long       bytes_dropped;
     unsigned long       events_nested;
     unsigned long       bytes_written_nested;
     unsigned long       bytes_alloc_nested;
     unsigned long       bytes_dropped_nested;
     int         min_size_nested;
     int         max_size_nested;
     int         max_size;
     int         min_size;
     int         cpu;
     int         cnt;
 };
 
 static struct rb_test_data rb_data[NR_CPUS] __initdata;
 
 /* 1 meg per cpu */
 #define RB_TEST_BUFFER_SIZE 1048576
 
 static char rb_string[] __initdata =
     "abcdefghijklmnopqrstuvwxyz1234567890!@#$%^&*()?+\\"
     "?+|:';\",.<>/?abcdefghijklmnopqrstuvwxyz1234567890"
     "!@#$%^&*()?+\\?+|:';\",.<>/?abcdefghijklmnopqrstuv";
 
 static bool rb_test_started __initdata;
 
 struct rb_item {
     int size;
     char str[];
 };
 
 static __init int rb_write_something(struct rb_test_data *data, bool nested)
 {
     struct ring_buffer_event *event;
     struct rb_item *item;
     bool started;
     int event_len;
     int size;
     int len;
     int cnt;
 
     /* Have nested writes different that what is written */
     cnt = data->cnt + (nested ? 27 : 0);
 
     /* Multiply cnt by ~e, to make some unique increment */
     size = (cnt * 68 / 25) % (sizeof(rb_string) - 1);
 
     len = size + sizeof(struct rb_item);
 
     started = rb_test_started;
     /* read rb_test_started before checking buffer enabled */
     smp_rmb();
 
     event = ring_buffer_lock_reserve(data->buffer, len);
     if (!event) {
         /* Ignore dropped events before test starts. */
         if (started) {
             if (nested)
                 data->bytes_dropped += len;
             else
                 data->bytes_dropped_nested += len;
         }
         return len;
     }
 
     event_len = ring_buffer_event_length(event);
 
     if (RB_WARN_ON(data->buffer, event_len < len))
         goto out;
 
     item = ring_buffer_event_data(event);
     item->size = size;
     memcpy(item->str, rb_string, size);
 
     if (nested) {
         data->bytes_alloc_nested += event_len;
         data->bytes_written_nested += len;
         data->events_nested++;
         if (!data->min_size_nested || len < data->min_size_nested)
             data->min_size_nested = len;
         if (len > data->max_size_nested)
             data->max_size_nested = len;
     } else {
         data->bytes_alloc += event_len;
         data->bytes_written += len;
         data->events++;
         if (!data->min_size || len < data->min_size)
             data->max_size = len;
         if (len > data->max_size)
             data->max_size = len;
     }
 
  out:
     ring_buffer_unlock_commit(data->buffer, event);
 
     return 0;
 }
 
 static __init int rb_test(void *arg)
 {
     struct rb_test_data *data = arg;
 
     while (!kthread_should_stop()) {
         rb_write_something(data, false);
         data->cnt++;
 
         set_current_state(TASK_INTERRUPTIBLE);
         /* Now sleep between a min of 100-300us and a max of 1ms */
         usleep_range(((data->cnt % 3) + 1) * 100, 1000);
     }
 
     return 0;
 }
 
 static __init void rb_ipi(void *ignore)
 {
     struct rb_test_data *data;
     int cpu = smp_processor_id();
 
     data = &rb_data[cpu];
     rb_write_something(data, true);
 }
 
 static __init int rb_hammer_test(void *arg)
 {
     while (!kthread_should_stop()) {
 
         /* Send an IPI to all cpus to write data! */
         smp_call_function(rb_ipi, NULL, 1);
         /* No sleep, but for non preempt, let others run */
         schedule();
     }
 
     return 0;
 }
 
 static __init int test_ringbuffer(void)
 {
     struct task_struct *rb_hammer;
     struct trace_buffer *buffer;
     int cpu;
     int ret = 0;
 
     if (security_locked_down(LOCKDOWN_TRACEFS)) {
         pr_warn("Lockdown is enabled, skipping ring buffer tests\n");
         return 0;
     }
 
     pr_info("Running ring buffer tests...\n");
 
     buffer = ring_buffer_alloc(RB_TEST_BUFFER_SIZE, RB_FL_OVERWRITE);
     if (WARN_ON(!buffer))
         return 0;
 
     /* Disable buffer so that threads can't write to it yet */
     ring_buffer_record_off(buffer);
 
     for_each_online_cpu(cpu) {
         rb_data[cpu].buffer = buffer;
         rb_data[cpu].cpu = cpu;
         rb_data[cpu].cnt = cpu;
         rb_threads[cpu] = kthread_run_on_cpu(rb_test, &rb_data[cpu],
                              cpu, "rbtester/%u");
         if (WARN_ON(IS_ERR(rb_threads[cpu]))) {
             pr_cont("FAILED\n");
             ret = PTR_ERR(rb_threads[cpu]);
             goto out_free;
         }
     }
 
     /* Now create the rb hammer! */
     rb_hammer = kthread_run(rb_hammer_test, NULL, "rbhammer");
     if (WARN_ON(IS_ERR(rb_hammer))) {
         pr_cont("FAILED\n");
         ret = PTR_ERR(rb_hammer);
         goto out_free;
     }
 
     ring_buffer_record_on(buffer);
     /*
      * Show buffer is enabled before setting rb_test_started.
      * Yes there's a small race window where events could be
      * dropped and the thread wont catch it. But when a ring
      * buffer gets enabled, there will always be some kind of
      * delay before other CPUs see it. Thus, we don't care about
      * those dropped events. We care about events dropped after
      * the threads see that the buffer is active.
      */
     smp_wmb();
     rb_test_started = true;
 
     set_current_state(TASK_INTERRUPTIBLE);
     /* Just run for 10 seconds */;
     schedule_timeout(10 * HZ);
 
     kthread_stop(rb_hammer);
 
  out_free:
     for_each_online_cpu(cpu) {
         if (!rb_threads[cpu])
             break;
         kthread_stop(rb_threads[cpu]);
     }
     if (ret) {
         ring_buffer_free(buffer);
         return ret;
     }
 
     /* Report! */
     pr_info("finished\n");
     for_each_online_cpu(cpu) {
         struct ring_buffer_event *event;
         struct rb_test_data *data = &rb_data[cpu];
         struct rb_item *item;
         unsigned long total_events;
         unsigned long total_dropped;
         unsigned long total_written;
         unsigned long total_alloc;
         unsigned long total_read = 0;
         unsigned long total_size = 0;
         unsigned long total_len = 0;
         unsigned long total_lost = 0;
         unsigned long lost;
         int big_event_size;
         int small_event_size;
 
         ret = -1;
 
         total_events = data->events + data->events_nested;
         total_written = data->bytes_written + data->bytes_written_nested;
         total_alloc = data->bytes_alloc + data->bytes_alloc_nested;
         total_dropped = data->bytes_dropped + data->bytes_dropped_nested;
 
         big_event_size = data->max_size + data->max_size_nested;
         small_event_size = data->min_size + data->min_size_nested;
 
         pr_info("CPU %d:\n", cpu);
         pr_info("              events:    %ld\n", total_events);
         pr_info("       dropped bytes:    %ld\n", total_dropped);
         pr_info("       alloced bytes:    %ld\n", total_alloc);
         pr_info("       written bytes:    %ld\n", total_written);
         pr_info("       biggest event:    %d\n", big_event_size);
         pr_info("      smallest event:    %d\n", small_event_size);
 
         if (RB_WARN_ON(buffer, total_dropped))
             break;
 
         ret = 0;
 
         while ((event = ring_buffer_consume(buffer, cpu, NULL, &lost))) {
             total_lost += lost;
             item = ring_buffer_event_data(event);
             total_len += ring_buffer_event_length(event);
             total_size += item->size + sizeof(struct rb_item);
             if (memcmp(&item->str[0], rb_string, item->size) != 0) {
                 pr_info("FAILED!\n");
                 pr_info("buffer had: %.*s\n", item->size, item->str);
                 pr_info("expected:   %.*s\n", item->size, rb_string);
                 RB_WARN_ON(buffer, 1);
                 ret = -1;
                 break;
             }
             total_read++;
         }
         if (ret)
             break;
 
         ret = -1;
 
         pr_info("         read events:   %ld\n", total_read);
         pr_info("         lost events:   %ld\n", total_lost);
         pr_info("        total events:   %ld\n", total_lost + total_read);
         pr_info("  recorded len bytes:   %ld\n", total_len);
         pr_info(" recorded size bytes:   %ld\n", total_size);
         if (total_lost) {
             pr_info(" With dropped events, record len and size may not match\n"
                 " alloced and written from above\n");
         } else {
             if (RB_WARN_ON(buffer, total_len != total_alloc ||
                        total_size != total_written))
                 break;
         }
         if (RB_WARN_ON(buffer, total_lost + total_read != total_events))
             break;
 
         ret = 0;
     }
     if (!ret)
         pr_info("Ring buffer PASSED!\n");
 
     ring_buffer_free(buffer);
     return 0;
 }
 
 late_initcall(test_ringbuffer);
 #endif /* CONFIG_RING_BUFFER_STARTUP_TEST */