OSCL-LXR linux-6.0.1/​kernel/​printk/​printk_ringbuffer.c	Source navigation Diff markup Identifier search General search
	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
 
 #include <linux/kernel.h>
 #include <linux/irqflags.h>
 #include <linux/string.h>
 #include <linux/errno.h>
 #include <linux/bug.h>
 #include "printk_ringbuffer.h"
 
 /**
  * DOC: printk_ringbuffer overview
  *
  * Data Structure
  * --------------
  * The printk_ringbuffer is made up of 3 internal ringbuffers:
  *
  *   desc_ring
  *     A ring of descriptors and their meta data (such as sequence number,
  *     timestamp, loglevel, etc.) as well as internal state information about
  *     the record and logical positions specifying where in the other
  *     ringbuffer the text strings are located.
  *
  *   text_data_ring
  *     A ring of data blocks. A data block consists of an unsigned long
  *     integer (ID) that maps to a desc_ring index followed by the text
  *     string of the record.
  *
  * The internal state information of a descriptor is the key element to allow
  * readers and writers to locklessly synchronize access to the data.
  *
  * Implementation
  * --------------
  *
  * Descriptor Ring
  * ~~~~~~~~~~~~~~~
  * The descriptor ring is an array of descriptors. A descriptor contains
  * essential meta data to track the data of a printk record using
  * blk_lpos structs pointing to associated text data blocks (see
  * "Data Rings" below). Each descriptor is assigned an ID that maps
  * directly to index values of the descriptor array and has a state. The ID
  * and the state are bitwise combined into a single descriptor field named
  * @state_var, allowing ID and state to be synchronously and atomically
  * updated.
  *
  * Descriptors have four states:
  *
  *   reserved
  *     A writer is modifying the record.
  *
  *   committed
  *     The record and all its data are written. A writer can reopen the
  *     descriptor (transitioning it back to reserved), but in the committed
  *     state the data is consistent.
  *
  *   finalized
  *     The record and all its data are complete and available for reading. A
  *     writer cannot reopen the descriptor.
  *
  *   reusable
  *     The record exists, but its text and/or meta data may no longer be
  *     available.
  *
  * Querying the @state_var of a record requires providing the ID of the
  * descriptor to query. This can yield a possible fifth (pseudo) state:
  *
  *   miss
  *     The descriptor being queried has an unexpected ID.
  *
  * The descriptor ring has a @tail_id that contains the ID of the oldest
  * descriptor and @head_id that contains the ID of the newest descriptor.
  *
  * When a new descriptor should be created (and the ring is full), the tail
  * descriptor is invalidated by first transitioning to the reusable state and
  * then invalidating all tail data blocks up to and including the data blocks
  * associated with the tail descriptor (for the text ring). Then
  * @tail_id is advanced, followed by advancing @head_id. And finally the
  * @state_var of the new descriptor is initialized to the new ID and reserved
  * state.
  *
  * The @tail_id can only be advanced if the new @tail_id would be in the
  * committed or reusable queried state. This makes it possible that a valid
  * sequence number of the tail is always available.
  *
  * Descriptor Finalization
  * ~~~~~~~~~~~~~~~~~~~~~~~
  * When a writer calls the commit function prb_commit(), record data is
  * fully stored and is consistent within the ringbuffer. However, a writer can
  * reopen that record, claiming exclusive access (as with prb_reserve()), and
  * modify that record. When finished, the writer must again commit the record.
  *
  * In order for a record to be made available to readers (and also become
  * recyclable for writers), it must be finalized. A finalized record cannot be
  * reopened and can never become "unfinalized". Record finalization can occur
  * in three different scenarios:
  *
  *   1) A writer can simultaneously commit and finalize its record by calling
  *      prb_final_commit() instead of prb_commit().
  *
  *   2) When a new record is reserved and the previous record has been
  *      committed via prb_commit(), that previous record is automatically
  *      finalized.
  *
  *   3) When a record is committed via prb_commit() and a newer record
  *      already exists, the record being committed is automatically finalized.
  *
  * Data Ring
  * ~~~~~~~~~
  * The text data ring is a byte array composed of data blocks. Data blocks are
  * referenced by blk_lpos structs that point to the logical position of the
  * beginning of a data block and the beginning of the next adjacent data
  * block. Logical positions are mapped directly to index values of the byte
  * array ringbuffer.
  *
  * Each data block consists of an ID followed by the writer data. The ID is
  * the identifier of a descriptor that is associated with the data block. A
  * given data block is considered valid if all of the following conditions
  * are met:
  *
  *   1) The descriptor associated with the data block is in the committed
  *      or finalized queried state.
  *
  *   2) The blk_lpos struct within the descriptor associated with the data
  *      block references back to the same data block.
  *
  *   3) The data block is within the head/tail logical position range.
  *
  * If the writer data of a data block would extend beyond the end of the
  * byte array, only the ID of the data block is stored at the logical
  * position and the full data block (ID and writer data) is stored at the
  * beginning of the byte array. The referencing blk_lpos will point to the
  * ID before the wrap and the next data block will be at the logical
  * position adjacent the full data block after the wrap.
  *
  * Data rings have a @tail_lpos that points to the beginning of the oldest
  * data block and a @head_lpos that points to the logical position of the
  * next (not yet existing) data block.
  *
  * When a new data block should be created (and the ring is full), tail data
  * blocks will first be invalidated by putting their associated descriptors
  * into the reusable state and then pushing the @tail_lpos forward beyond
  * them. Then the @head_lpos is pushed forward and is associated with a new
  * descriptor. If a data block is not valid, the @tail_lpos cannot be
  * advanced beyond it.
  *
  * Info Array
  * ~~~~~~~~~~
  * The general meta data of printk records are stored in printk_info structs,
  * stored in an array with the same number of elements as the descriptor ring.
  * Each info corresponds to the descriptor of the same index in the
  * descriptor ring. Info validity is confirmed by evaluating the corresponding
  * descriptor before and after loading the info.
  *
  * Usage
  * -----
  * Here are some simple examples demonstrating writers and readers. For the
  * examples a global ringbuffer (test_rb) is available (which is not the
  * actual ringbuffer used by printk)::
  *
  *  DEFINE_PRINTKRB(test_rb, 15, 5);
  *
  * This ringbuffer allows up to 32768 records (2 ^ 15) and has a size of
  * 1 MiB (2 ^ (15 + 5)) for text data.
  *
  * Sample writer code::
  *
  *  const char *textstr = "message text";
  *  struct prb_reserved_entry e;
  *  struct printk_record r;
  *
  *  // specify how much to allocate
  *  prb_rec_init_wr(&r, strlen(textstr) + 1);
  *
  *  if (prb_reserve(&e, &test_rb, &r)) {
  *      snprintf(r.text_buf, r.text_buf_size, "%s", textstr);
  *
  *      r.info->text_len = strlen(textstr);
  *      r.info->ts_nsec = local_clock();
  *      r.info->caller_id = printk_caller_id();
  *
  *      // commit and finalize the record
  *      prb_final_commit(&e);
  *  }
  *
  * Note that additional writer functions are available to extend a record
  * after it has been committed but not yet finalized. This can be done as
  * long as no new records have been reserved and the caller is the same.
  *
  * Sample writer code (record extending)::
  *
  *      // alternate rest of previous example
  *
  *      r.info->text_len = strlen(textstr);
  *      r.info->ts_nsec = local_clock();
  *      r.info->caller_id = printk_caller_id();
  *
  *      // commit the record (but do not finalize yet)
  *      prb_commit(&e);
  *  }
  *
  *  ...
  *
  *  // specify additional 5 bytes text space to extend
  *  prb_rec_init_wr(&r, 5);
  *
  *  // try to extend, but only if it does not exceed 32 bytes
  *  if (prb_reserve_in_last(&e, &test_rb, &r, printk_caller_id()), 32) {
  *      snprintf(&r.text_buf[r.info->text_len],
  *           r.text_buf_size - r.info->text_len, "hello");
  *
  *      r.info->text_len += 5;
  *
  *      // commit and finalize the record
  *      prb_final_commit(&e);
  *  }
  *
  * Sample reader code::
  *
  *  struct printk_info info;
  *  struct printk_record r;
  *  char text_buf[32];
  *  u64 seq;
  *
  *  prb_rec_init_rd(&r, &info, &text_buf[0], sizeof(text_buf));
  *
  *  prb_for_each_record(0, &test_rb, &seq, &r) {
  *      if (info.seq != seq)
  *          pr_warn("lost %llu records\n", info.seq - seq);
  *
  *      if (info.text_len > r.text_buf_size) {
  *          pr_warn("record %llu text truncated\n", info.seq);
  *          text_buf[r.text_buf_size - 1] = 0;
  *      }
  *
  *      pr_info("%llu: %llu: %s\n", info.seq, info.ts_nsec,
  *          &text_buf[0]);
  *  }
  *
  * Note that additional less convenient reader functions are available to
  * allow complex record access.
  *
  * ABA Issues
  * ~~~~~~~~~~
  * To help avoid ABA issues, descriptors are referenced by IDs (array index
  * values combined with tagged bits counting array wraps) and data blocks are
  * referenced by logical positions (array index values combined with tagged
  * bits counting array wraps). However, on 32-bit systems the number of
  * tagged bits is relatively small such that an ABA incident is (at least
  * theoretically) possible. For example, if 4 million maximally sized (1KiB)
  * printk messages were to occur in NMI context on a 32-bit system, the
  * interrupted context would not be able to recognize that the 32-bit integer
  * completely wrapped and thus represents a different data block than the one
  * the interrupted context expects.
  *
  * To help combat this possibility, additional state checking is performed
  * (such as using cmpxchg() even though set() would suffice). These extra
  * checks are commented as such and will hopefully catch any ABA issue that
  * a 32-bit system might experience.
  *
  * Memory Barriers
  * ~~~~~~~~~~~~~~~
  * Multiple memory barriers are used. To simplify proving correctness and
  * generating litmus tests, lines of code related to memory barriers
  * (loads, stores, and the associated memory barriers) are labeled::
  *
  *  LMM(function:letter)
  *
  * Comments reference the labels using only the "function:letter" part.
  *
  * The memory barrier pairs and their ordering are:
  *
  *   desc_reserve:D / desc_reserve:B
  *     push descriptor tail (id), then push descriptor head (id)
  *
  *   desc_reserve:D / data_push_tail:B
  *     push data tail (lpos), then set new descriptor reserved (state)
  *
  *   desc_reserve:D / desc_push_tail:C
  *     push descriptor tail (id), then set new descriptor reserved (state)
  *
  *   desc_reserve:D / prb_first_seq:C
  *     push descriptor tail (id), then set new descriptor reserved (state)
  *
  *   desc_reserve:F / desc_read:D
  *     set new descriptor id and reserved (state), then allow writer changes
  *
  *   data_alloc:A (or data_realloc:A) / desc_read:D
  *     set old descriptor reusable (state), then modify new data block area
  *
  *   data_alloc:A (or data_realloc:A) / data_push_tail:B
  *     push data tail (lpos), then modify new data block area
  *
  *   _prb_commit:B / desc_read:B
  *     store writer changes, then set new descriptor committed (state)
  *
  *   desc_reopen_last:A / _prb_commit:B
  *     set descriptor reserved (state), then read descriptor data
  *
  *   _prb_commit:B / desc_reserve:D
  *     set new descriptor committed (state), then check descriptor head (id)
  *
  *   data_push_tail:D / data_push_tail:A
  *     set descriptor reusable (state), then push data tail (lpos)
  *
  *   desc_push_tail:B / desc_reserve:D
  *     set descriptor reusable (state), then push descriptor tail (id)
  */
 
 #define DATA_SIZE(data_ring)        _DATA_SIZE((data_ring)->size_bits)
 #define DATA_SIZE_MASK(data_ring)   (DATA_SIZE(data_ring) - 1)
 
 #define DESCS_COUNT(desc_ring)      _DESCS_COUNT((desc_ring)->count_bits)
 #define DESCS_COUNT_MASK(desc_ring) (DESCS_COUNT(desc_ring) - 1)
 
 /* Determine the data array index from a logical position. */
 #define DATA_INDEX(data_ring, lpos) ((lpos) & DATA_SIZE_MASK(data_ring))
 
 /* Determine the desc array index from an ID or sequence number. */
 #define DESC_INDEX(desc_ring, n)    ((n) & DESCS_COUNT_MASK(desc_ring))
 
 /* Determine how many times the data array has wrapped. */
 #define DATA_WRAPS(data_ring, lpos) ((lpos) >> (data_ring)->size_bits)
 
 /* Determine if a logical position refers to a data-less block. */
 #define LPOS_DATALESS(lpos)     ((lpos) & 1UL)
 #define BLK_DATALESS(blk)       (LPOS_DATALESS((blk)->begin) && \
                      LPOS_DATALESS((blk)->next))
 
 /* Get the logical position at index 0 of the current wrap. */
 #define DATA_THIS_WRAP_START_LPOS(data_ring, lpos) \
 ((lpos) & ~DATA_SIZE_MASK(data_ring))
 
 /* Get the ID for the same index of the previous wrap as the given ID. */
 #define DESC_ID_PREV_WRAP(desc_ring, id) \
 DESC_ID((id) - DESCS_COUNT(desc_ring))
 
 /*
  * A data block: mapped directly to the beginning of the data block area
  * specified as a logical position within the data ring.
  *
  * @id:   the ID of the associated descriptor
  * @data: the writer data
  *
  * Note that the size of a data block is only known by its associated
  * descriptor.
  */
 struct prb_data_block {
     unsigned long   id;
     char        data[];
 };
 
 /*
  * Return the descriptor associated with @n. @n can be either a
  * descriptor ID or a sequence number.
  */
 static struct prb_desc *to_desc(struct prb_desc_ring *desc_ring, u64 n)
 {
     return &desc_ring->descs[DESC_INDEX(desc_ring, n)];
 }
 
 /*
  * Return the printk_info associated with @n. @n can be either a
  * descriptor ID or a sequence number.
  */
 static struct printk_info *to_info(struct prb_desc_ring *desc_ring, u64 n)
 {
     return &desc_ring->infos[DESC_INDEX(desc_ring, n)];
 }
 
 static struct prb_data_block *to_block(struct prb_data_ring *data_ring,
                        unsigned long begin_lpos)
 {
     return (void *)&data_ring->data[DATA_INDEX(data_ring, begin_lpos)];
 }
 
 /*
  * Increase the data size to account for data block meta data plus any
  * padding so that the adjacent data block is aligned on the ID size.
  */
 static unsigned int to_blk_size(unsigned int size)
 {
     struct prb_data_block *db = NULL;
 
     size += sizeof(*db);
     size = ALIGN(size, sizeof(db->id));
     return size;
 }
 
 /*
  * Sanity checker for reserve size. The ringbuffer code assumes that a data
  * block does not exceed the maximum possible size that could fit within the
  * ringbuffer. This function provides that basic size check so that the
  * assumption is safe.
  */
 static bool data_check_size(struct prb_data_ring *data_ring, unsigned int size)
 {
     struct prb_data_block *db = NULL;
 
     if (size == 0)
         return true;
 
     /*
      * Ensure the alignment padded size could possibly fit in the data
      * array. The largest possible data block must still leave room for
      * at least the ID of the next block.
      */
     size = to_blk_size(size);
     if (size > DATA_SIZE(data_ring) - sizeof(db->id))
         return false;
 
     return true;
 }
 
 /* Query the state of a descriptor. */
 static enum desc_state get_desc_state(unsigned long id,
                       unsigned long state_val)
 {
     if (id != DESC_ID(state_val))
         return desc_miss;
 
     return DESC_STATE(state_val);
 }
 
 /*
  * Get a copy of a specified descriptor and return its queried state. If the
  * descriptor is in an inconsistent state (miss or reserved), the caller can
  * only expect the descriptor's @state_var field to be valid.
  *
  * The sequence number and caller_id can be optionally retrieved. Like all
  * non-state_var data, they are only valid if the descriptor is in a
  * consistent state.
  */
 static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
                  unsigned long id, struct prb_desc *desc_out,
                  u64 *seq_out, u32 *caller_id_out)
 {
     struct printk_info *info = to_info(desc_ring, id);
     struct prb_desc *desc = to_desc(desc_ring, id);
     atomic_long_t *state_var = &desc->state_var;
     enum desc_state d_state;
     unsigned long state_val;
 
     /* Check the descriptor state. */
     state_val = atomic_long_read(state_var); /* LMM(desc_read:A) */
     d_state = get_desc_state(id, state_val);
     if (d_state == desc_miss || d_state == desc_reserved) {
         /*
          * The descriptor is in an inconsistent state. Set at least
          * @state_var so that the caller can see the details of
          * the inconsistent state.
          */
         goto out;
     }
 
     /*
      * Guarantee the state is loaded before copying the descriptor
      * content. This avoids copying obsolete descriptor content that might
      * not apply to the descriptor state. This pairs with _prb_commit:B.
      *
      * Memory barrier involvement:
      *
      * If desc_read:A reads from _prb_commit:B, then desc_read:C reads
      * from _prb_commit:A.
      *
      * Relies on:
      *
      * WMB from _prb_commit:A to _prb_commit:B
      *    matching
      * RMB from desc_read:A to desc_read:C
      */
     smp_rmb(); /* LMM(desc_read:B) */
 
     /*
      * Copy the descriptor data. The data is not valid until the
      * state has been re-checked. A memcpy() for all of @desc
      * cannot be used because of the atomic_t @state_var field.
      */
     if (desc_out) {
         memcpy(&desc_out->text_blk_lpos, &desc->text_blk_lpos,
                sizeof(desc_out->text_blk_lpos)); /* LMM(desc_read:C) */
     }
     if (seq_out)
         *seq_out = info->seq; /* also part of desc_read:C */
     if (caller_id_out)
         *caller_id_out = info->caller_id; /* also part of desc_read:C */
 
     /*
      * 1. Guarantee the descriptor content is loaded before re-checking
      *    the state. This avoids reading an obsolete descriptor state
      *    that may not apply to the copied content. This pairs with
      *    desc_reserve:F.
      *
      *    Memory barrier involvement:
      *
      *    If desc_read:C reads from desc_reserve:G, then desc_read:E
      *    reads from desc_reserve:F.
      *
      *    Relies on:
      *
      *    WMB from desc_reserve:F to desc_reserve:G
      *       matching
      *    RMB from desc_read:C to desc_read:E
      *
      * 2. Guarantee the record data is loaded before re-checking the
      *    state. This avoids reading an obsolete descriptor state that may
      *    not apply to the copied data. This pairs with data_alloc:A and
      *    data_realloc:A.
      *
      *    Memory barrier involvement:
      *
      *    If copy_data:A reads from data_alloc:B, then desc_read:E
      *    reads from desc_make_reusable:A.
      *
      *    Relies on:
      *
      *    MB from desc_make_reusable:A to data_alloc:B
      *       matching
      *    RMB from desc_read:C to desc_read:E
      *
      *    Note: desc_make_reusable:A and data_alloc:B can be different
      *          CPUs. However, the data_alloc:B CPU (which performs the
      *          full memory barrier) must have previously seen
      *          desc_make_reusable:A.
      */
     smp_rmb(); /* LMM(desc_read:D) */
 
     /*
      * The data has been copied. Return the current descriptor state,
      * which may have changed since the load above.
      */
     state_val = atomic_long_read(state_var); /* LMM(desc_read:E) */
     d_state = get_desc_state(id, state_val);
 out:
     if (desc_out)
         atomic_long_set(&desc_out->state_var, state_val);
     return d_state;
 }
 
 /*
  * Take a specified descriptor out of the finalized state by attempting
  * the transition from finalized to reusable. Either this context or some
  * other context will have been successful.
  */
 static void desc_make_reusable(struct prb_desc_ring *desc_ring,
                    unsigned long id)
 {
     unsigned long val_finalized = DESC_SV(id, desc_finalized);
     unsigned long val_reusable = DESC_SV(id, desc_reusable);
     struct prb_desc *desc = to_desc(desc_ring, id);
     atomic_long_t *state_var = &desc->state_var;
 
     atomic_long_cmpxchg_relaxed(state_var, val_finalized,
                     val_reusable); /* LMM(desc_make_reusable:A) */
 }
 
 /*
  * Given the text data ring, put the associated descriptor of each
  * data block from @lpos_begin until @lpos_end into the reusable state.
  *
  * If there is any problem making the associated descriptor reusable, either
  * the descriptor has not yet been finalized or another writer context has
  * already pushed the tail lpos past the problematic data block. Regardless,
  * on error the caller can re-load the tail lpos to determine the situation.
  */
 static bool data_make_reusable(struct printk_ringbuffer *rb,
                    unsigned long lpos_begin,
                    unsigned long lpos_end,
                    unsigned long *lpos_out)
 {
 
     struct prb_data_ring *data_ring = &rb->text_data_ring;
     struct prb_desc_ring *desc_ring = &rb->desc_ring;
     struct prb_data_block *blk;
     enum desc_state d_state;
     struct prb_desc desc;
     struct prb_data_blk_lpos *blk_lpos = &desc.text_blk_lpos;
     unsigned long id;
 
     /* Loop until @lpos_begin has advanced to or beyond @lpos_end. */
     while ((lpos_end - lpos_begin) - 1 < DATA_SIZE(data_ring)) {
         blk = to_block(data_ring, lpos_begin);
 
         /*
          * Load the block ID from the data block. This is a data race
          * against a writer that may have newly reserved this data
          * area. If the loaded value matches a valid descriptor ID,
          * the blk_lpos of that descriptor will be checked to make
          * sure it points back to this data block. If the check fails,
          * the data area has been recycled by another writer.
          */
         id = blk->id; /* LMM(data_make_reusable:A) */
 
         d_state = desc_read(desc_ring, id, &desc,
                     NULL, NULL); /* LMM(data_make_reusable:B) */
 
         switch (d_state) {
         case desc_miss:
         case desc_reserved:
         case desc_committed:
             return false;
         case desc_finalized:
             /*
              * This data block is invalid if the descriptor
              * does not point back to it.
              */
             if (blk_lpos->begin != lpos_begin)
                 return false;
             desc_make_reusable(desc_ring, id);
             break;
         case desc_reusable:
             /*
              * This data block is invalid if the descriptor
              * does not point back to it.
              */
             if (blk_lpos->begin != lpos_begin)
                 return false;
             break;
         }
 
         /* Advance @lpos_begin to the next data block. */
         lpos_begin = blk_lpos->next;
     }
 
     *lpos_out = lpos_begin;
     return true;
 }
 
 /*
  * Advance the data ring tail to at least @lpos. This function puts
  * descriptors into the reusable state if the tail is pushed beyond
  * their associated data block.
  */
 static bool data_push_tail(struct printk_ringbuffer *rb, unsigned long lpos)
 {
     struct prb_data_ring *data_ring = &rb->text_data_ring;
     unsigned long tail_lpos_new;
     unsigned long tail_lpos;
     unsigned long next_lpos;
 
     /* If @lpos is from a data-less block, there is nothing to do. */
     if (LPOS_DATALESS(lpos))
         return true;
 
     /*
      * Any descriptor states that have transitioned to reusable due to the
      * data tail being pushed to this loaded value will be visible to this
      * CPU. This pairs with data_push_tail:D.
      *
      * Memory barrier involvement:
      *
      * If data_push_tail:A reads from data_push_tail:D, then this CPU can
      * see desc_make_reusable:A.
      *
      * Relies on:
      *
      * MB from desc_make_reusable:A to data_push_tail:D
      *    matches
      * READFROM from data_push_tail:D to data_push_tail:A
      *    thus
      * READFROM from desc_make_reusable:A to this CPU
      */
     tail_lpos = atomic_long_read(&data_ring->tail_lpos); /* LMM(data_push_tail:A) */
 
     /*
      * Loop until the tail lpos is at or beyond @lpos. This condition
      * may already be satisfied, resulting in no full memory barrier
      * from data_push_tail:D being performed. However, since this CPU
      * sees the new tail lpos, any descriptor states that transitioned to
      * the reusable state must already be visible.
      */
     while ((lpos - tail_lpos) - 1 < DATA_SIZE(data_ring)) {
         /*
          * Make all descriptors reusable that are associated with
          * data blocks before @lpos.
          */
         if (!data_make_reusable(rb, tail_lpos, lpos, &next_lpos)) {
             /*
              * 1. Guarantee the block ID loaded in
              *    data_make_reusable() is performed before
              *    reloading the tail lpos. The failed
              *    data_make_reusable() may be due to a newly
              *    recycled data area causing the tail lpos to
              *    have been previously pushed. This pairs with
              *    data_alloc:A and data_realloc:A.
              *
              *    Memory barrier involvement:
              *
              *    If data_make_reusable:A reads from data_alloc:B,
              *    then data_push_tail:C reads from
              *    data_push_tail:D.
              *
              *    Relies on:
              *
              *    MB from data_push_tail:D to data_alloc:B
              *       matching
              *    RMB from data_make_reusable:A to
              *    data_push_tail:C
              *
              *    Note: data_push_tail:D and data_alloc:B can be
              *          different CPUs. However, the data_alloc:B
              *          CPU (which performs the full memory
              *          barrier) must have previously seen
              *          data_push_tail:D.
              *
              * 2. Guarantee the descriptor state loaded in
              *    data_make_reusable() is performed before
              *    reloading the tail lpos. The failed
              *    data_make_reusable() may be due to a newly
              *    recycled descriptor causing the tail lpos to
              *    have been previously pushed. This pairs with
              *    desc_reserve:D.
              *
              *    Memory barrier involvement:
              *
              *    If data_make_reusable:B reads from
              *    desc_reserve:F, then data_push_tail:C reads
              *    from data_push_tail:D.
              *
              *    Relies on:
              *
              *    MB from data_push_tail:D to desc_reserve:F
              *       matching
              *    RMB from data_make_reusable:B to
              *    data_push_tail:C
              *
              *    Note: data_push_tail:D and desc_reserve:F can
              *          be different CPUs. However, the
              *          desc_reserve:F CPU (which performs the
              *          full memory barrier) must have previously
              *          seen data_push_tail:D.
              */
             smp_rmb(); /* LMM(data_push_tail:B) */
 
             tail_lpos_new = atomic_long_read(&data_ring->tail_lpos
                             ); /* LMM(data_push_tail:C) */
             if (tail_lpos_new == tail_lpos)
                 return false;
 
             /* Another CPU pushed the tail. Try again. */
             tail_lpos = tail_lpos_new;
             continue;
         }
 
         /*
          * Guarantee any descriptor states that have transitioned to
          * reusable are stored before pushing the tail lpos. A full
          * memory barrier is needed since other CPUs may have made
          * the descriptor states reusable. This pairs with
          * data_push_tail:A.
          */
         if (atomic_long_try_cmpxchg(&data_ring->tail_lpos, &tail_lpos,
                         next_lpos)) { /* LMM(data_push_tail:D) */
             break;
         }
     }
 
     return true;
 }
 
 /*
  * Advance the desc ring tail. This function advances the tail by one
  * descriptor, thus invalidating the oldest descriptor. Before advancing
  * the tail, the tail descriptor is made reusable and all data blocks up to
  * and including the descriptor's data block are invalidated (i.e. the data
  * ring tail is pushed past the data block of the descriptor being made
  * reusable).
  */
 static bool desc_push_tail(struct printk_ringbuffer *rb,
                unsigned long tail_id)
 {
     struct prb_desc_ring *desc_ring = &rb->desc_ring;
     enum desc_state d_state;
     struct prb_desc desc;
 
     d_state = desc_read(desc_ring, tail_id, &desc, NULL, NULL);
 
     switch (d_state) {
     case desc_miss:
         /*
          * If the ID is exactly 1 wrap behind the expected, it is
          * in the process of being reserved by another writer and
          * must be considered reserved.
          */
         if (DESC_ID(atomic_long_read(&desc.state_var)) ==
             DESC_ID_PREV_WRAP(desc_ring, tail_id)) {
             return false;
         }
 
         /*
          * The ID has changed. Another writer must have pushed the
          * tail and recycled the descriptor already. Success is
          * returned because the caller is only interested in the
          * specified tail being pushed, which it was.
          */
         return true;
     case desc_reserved:
     case desc_committed:
         return false;
     case desc_finalized:
         desc_make_reusable(desc_ring, tail_id);
         break;
     case desc_reusable:
         break;
     }
 
     /*
      * Data blocks must be invalidated before their associated
      * descriptor can be made available for recycling. Invalidating
      * them later is not possible because there is no way to trust
      * data blocks once their associated descriptor is gone.
      */
 
     if (!data_push_tail(rb, desc.text_blk_lpos.next))
         return false;
 
     /*
      * Check the next descriptor after @tail_id before pushing the tail
      * to it because the tail must always be in a finalized or reusable
      * state. The implementation of prb_first_seq() relies on this.
      *
      * A successful read implies that the next descriptor is less than or
      * equal to @head_id so there is no risk of pushing the tail past the
      * head.
      */
     d_state = desc_read(desc_ring, DESC_ID(tail_id + 1), &desc,
                 NULL, NULL); /* LMM(desc_push_tail:A) */
 
     if (d_state == desc_finalized || d_state == desc_reusable) {
         /*
          * Guarantee any descriptor states that have transitioned to
          * reusable are stored before pushing the tail ID. This allows
          * verifying the recycled descriptor state. A full memory
          * barrier is needed since other CPUs may have made the
          * descriptor states reusable. This pairs with desc_reserve:D.
          */
         atomic_long_cmpxchg(&desc_ring->tail_id, tail_id,
                     DESC_ID(tail_id + 1)); /* LMM(desc_push_tail:B) */
     } else {
         /*
          * Guarantee the last state load from desc_read() is before
          * reloading @tail_id in order to see a new tail ID in the
          * case that the descriptor has been recycled. This pairs
          * with desc_reserve:D.
          *
          * Memory barrier involvement:
          *
          * If desc_push_tail:A reads from desc_reserve:F, then
          * desc_push_tail:D reads from desc_push_tail:B.
          *
          * Relies on:
          *
          * MB from desc_push_tail:B to desc_reserve:F
          *    matching
          * RMB from desc_push_tail:A to desc_push_tail:D
          *
          * Note: desc_push_tail:B and desc_reserve:F can be different
          *       CPUs. However, the desc_reserve:F CPU (which performs
          *       the full memory barrier) must have previously seen
          *       desc_push_tail:B.
          */
         smp_rmb(); /* LMM(desc_push_tail:C) */
 
         /*
          * Re-check the tail ID. The descriptor following @tail_id is
          * not in an allowed tail state. But if the tail has since
          * been moved by another CPU, then it does not matter.
          */
         if (atomic_long_read(&desc_ring->tail_id) == tail_id) /* LMM(desc_push_tail:D) */
             return false;
     }
 
     return true;
 }
 
 /* Reserve a new descriptor, invalidating the oldest if necessary. */
 static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
 {
     struct prb_desc_ring *desc_ring = &rb->desc_ring;
     unsigned long prev_state_val;
     unsigned long id_prev_wrap;
     struct prb_desc *desc;
     unsigned long head_id;
     unsigned long id;
 
     head_id = atomic_long_read(&desc_ring->head_id); /* LMM(desc_reserve:A) */
 
     do {
         id = DESC_ID(head_id + 1);
         id_prev_wrap = DESC_ID_PREV_WRAP(desc_ring, id);
 
         /*
          * Guarantee the head ID is read before reading the tail ID.
          * Since the tail ID is updated before the head ID, this
          * guarantees that @id_prev_wrap is never ahead of the tail
          * ID. This pairs with desc_reserve:D.
          *
          * Memory barrier involvement:
          *
          * If desc_reserve:A reads from desc_reserve:D, then
          * desc_reserve:C reads from desc_push_tail:B.
          *
          * Relies on:
          *
          * MB from desc_push_tail:B to desc_reserve:D
          *    matching
          * RMB from desc_reserve:A to desc_reserve:C
          *
          * Note: desc_push_tail:B and desc_reserve:D can be different
          *       CPUs. However, the desc_reserve:D CPU (which performs
          *       the full memory barrier) must have previously seen
          *       desc_push_tail:B.
          */
         smp_rmb(); /* LMM(desc_reserve:B) */
 
         if (id_prev_wrap == atomic_long_read(&desc_ring->tail_id
                             )) { /* LMM(desc_reserve:C) */
             /*
              * Make space for the new descriptor by
              * advancing the tail.
              */
             if (!desc_push_tail(rb, id_prev_wrap))
                 return false;
         }
 
         /*
          * 1. Guarantee the tail ID is read before validating the
          *    recycled descriptor state. A read memory barrier is
          *    sufficient for this. This pairs with desc_push_tail:B.
          *
          *    Memory barrier involvement:
          *
          *    If desc_reserve:C reads from desc_push_tail:B, then
          *    desc_reserve:E reads from desc_make_reusable:A.
          *
          *    Relies on:
          *
          *    MB from desc_make_reusable:A to desc_push_tail:B
          *       matching
          *    RMB from desc_reserve:C to desc_reserve:E
          *
          *    Note: desc_make_reusable:A and desc_push_tail:B can be
          *          different CPUs. However, the desc_push_tail:B CPU
          *          (which performs the full memory barrier) must have
          *          previously seen desc_make_reusable:A.
          *
          * 2. Guarantee the tail ID is stored before storing the head
          *    ID. This pairs with desc_reserve:B.
          *
          * 3. Guarantee any data ring tail changes are stored before
          *    recycling the descriptor. Data ring tail changes can
          *    happen via desc_push_tail()->data_push_tail(). A full
          *    memory barrier is needed since another CPU may have
          *    pushed the data ring tails. This pairs with
          *    data_push_tail:B.
          *
          * 4. Guarantee a new tail ID is stored before recycling the
          *    descriptor. A full memory barrier is needed since
          *    another CPU may have pushed the tail ID. This pairs
          *    with desc_push_tail:C and this also pairs with
          *    prb_first_seq:C.
          *
          * 5. Guarantee the head ID is stored before trying to
          *    finalize the previous descriptor. This pairs with
          *    _prb_commit:B.
          */
     } while (!atomic_long_try_cmpxchg(&desc_ring->head_id, &head_id,
                       id)); /* LMM(desc_reserve:D) */
 
     desc = to_desc(desc_ring, id);
 
     /*
      * If the descriptor has been recycled, verify the old state val.
      * See "ABA Issues" about why this verification is performed.
      */
     prev_state_val = atomic_long_read(&desc->state_var); /* LMM(desc_reserve:E) */
     if (prev_state_val &&
         get_desc_state(id_prev_wrap, prev_state_val) != desc_reusable) {
         WARN_ON_ONCE(1);
         return false;
     }
 
     /*
      * Assign the descriptor a new ID and set its state to reserved.
      * See "ABA Issues" about why cmpxchg() instead of set() is used.
      *
      * Guarantee the new descriptor ID and state is stored before making
      * any other changes. A write memory barrier is sufficient for this.
      * This pairs with desc_read:D.
      */
     if (!atomic_long_try_cmpxchg(&desc->state_var, &prev_state_val,
             DESC_SV(id, desc_reserved))) { /* LMM(desc_reserve:F) */
         WARN_ON_ONCE(1);
         return false;
     }
 
     /* Now data in @desc can be modified: LMM(desc_reserve:G) */
 
     *id_out = id;
     return true;
 }
 
 /* Determine the end of a data block. */
 static unsigned long get_next_lpos(struct prb_data_ring *data_ring,
                    unsigned long lpos, unsigned int size)
 {
     unsigned long begin_lpos;
     unsigned long next_lpos;
 
     begin_lpos = lpos;
     next_lpos = lpos + size;
 
     /* First check if the data block does not wrap. */
     if (DATA_WRAPS(data_ring, begin_lpos) == DATA_WRAPS(data_ring, next_lpos))
         return next_lpos;
 
     /* Wrapping data blocks store their data at the beginning. */
     return (DATA_THIS_WRAP_START_LPOS(data_ring, next_lpos) + size);
 }
 
 /*
  * Allocate a new data block, invalidating the oldest data block(s)
  * if necessary. This function also associates the data block with
  * a specified descriptor.
  */
 static char *data_alloc(struct printk_ringbuffer *rb, unsigned int size,
             struct prb_data_blk_lpos *blk_lpos, unsigned long id)
 {
     struct prb_data_ring *data_ring = &rb->text_data_ring;
     struct prb_data_block *blk;
     unsigned long begin_lpos;
     unsigned long next_lpos;
 
     if (size == 0) {
         /* Specify a data-less block. */
         blk_lpos->begin = NO_LPOS;
         blk_lpos->next = NO_LPOS;
         return NULL;
     }
 
     size = to_blk_size(size);
 
     begin_lpos = atomic_long_read(&data_ring->head_lpos);
 
     do {
         next_lpos = get_next_lpos(data_ring, begin_lpos, size);
 
         if (!data_push_tail(rb, next_lpos - DATA_SIZE(data_ring))) {
             /* Failed to allocate, specify a data-less block. */
             blk_lpos->begin = FAILED_LPOS;
             blk_lpos->next = FAILED_LPOS;
             return NULL;
         }
 
         /*
          * 1. Guarantee any descriptor states that have transitioned
          *    to reusable are stored before modifying the newly
          *    allocated data area. A full memory barrier is needed
          *    since other CPUs may have made the descriptor states
          *    reusable. See data_push_tail:A about why the reusable
          *    states are visible. This pairs with desc_read:D.
          *
          * 2. Guarantee any updated tail lpos is stored before
          *    modifying the newly allocated data area. Another CPU may
          *    be in data_make_reusable() and is reading a block ID
          *    from this area. data_make_reusable() can handle reading
          *    a garbage block ID value, but then it must be able to
          *    load a new tail lpos. A full memory barrier is needed
          *    since other CPUs may have updated the tail lpos. This
          *    pairs with data_push_tail:B.
          */
     } while (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &begin_lpos,
                       next_lpos)); /* LMM(data_alloc:A) */
 
     blk = to_block(data_ring, begin_lpos);
     blk->id = id; /* LMM(data_alloc:B) */
 
     if (DATA_WRAPS(data_ring, begin_lpos) != DATA_WRAPS(data_ring, next_lpos)) {
         /* Wrapping data blocks store their data at the beginning. */
         blk = to_block(data_ring, 0);
 
         /*
          * Store the ID on the wrapped block for consistency.
          * The printk_ringbuffer does not actually use it.
          */
         blk->id = id;
     }
 
     blk_lpos->begin = begin_lpos;
     blk_lpos->next = next_lpos;
 
     return &blk->data[0];
 }
 
 /*
  * Try to resize an existing data block associated with the descriptor
  * specified by @id. If the resized data block should become wrapped, it
  * copies the old data to the new data block. If @size yields a data block
  * with the same or less size, the data block is left as is.
  *
  * Fail if this is not the last allocated data block or if there is not
  * enough space or it is not possible make enough space.
  *
  * Return a pointer to the beginning of the entire data buffer or NULL on
  * failure.
  */
 static char *data_realloc(struct printk_ringbuffer *rb, unsigned int size,
               struct prb_data_blk_lpos *blk_lpos, unsigned long id)
 {
     struct prb_data_ring *data_ring = &rb->text_data_ring;
     struct prb_data_block *blk;
     unsigned long head_lpos;
     unsigned long next_lpos;
     bool wrapped;
 
     /* Reallocation only works if @blk_lpos is the newest data block. */
     head_lpos = atomic_long_read(&data_ring->head_lpos);
     if (head_lpos != blk_lpos->next)
         return NULL;
 
     /* Keep track if @blk_lpos was a wrapping data block. */
     wrapped = (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, blk_lpos->next));
 
     size = to_blk_size(size);
 
     next_lpos = get_next_lpos(data_ring, blk_lpos->begin, size);
 
     /* If the data block does not increase, there is nothing to do. */
     if (head_lpos - next_lpos < DATA_SIZE(data_ring)) {
         if (wrapped)
             blk = to_block(data_ring, 0);
         else
             blk = to_block(data_ring, blk_lpos->begin);
         return &blk->data[0];
     }
 
     if (!data_push_tail(rb, next_lpos - DATA_SIZE(data_ring)))
         return NULL;
 
     /* The memory barrier involvement is the same as data_alloc:A. */
     if (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &head_lpos,
                      next_lpos)) { /* LMM(data_realloc:A) */
         return NULL;
     }
 
     blk = to_block(data_ring, blk_lpos->begin);
 
     if (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, next_lpos)) {
         struct prb_data_block *old_blk = blk;
 
         /* Wrapping data blocks store their data at the beginning. */
         blk = to_block(data_ring, 0);
 
         /*
          * Store the ID on the wrapped block for consistency.
          * The printk_ringbuffer does not actually use it.
          */
         blk->id = id;
 
         if (!wrapped) {
             /*
              * Since the allocated space is now in the newly
              * created wrapping data block, copy the content
              * from the old data block.
              */
             memcpy(&blk->data[0], &old_blk->data[0],
                    (blk_lpos->next - blk_lpos->begin) - sizeof(blk->id));
         }
     }
 
     blk_lpos->next = next_lpos;
 
     return &blk->data[0];
 }
 
 /* Return the number of bytes used by a data block. */
 static unsigned int space_used(struct prb_data_ring *data_ring,
                    struct prb_data_blk_lpos *blk_lpos)
 {
     /* Data-less blocks take no space. */
     if (BLK_DATALESS(blk_lpos))
         return 0;
 
     if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next)) {
         /* Data block does not wrap. */
         return (DATA_INDEX(data_ring, blk_lpos->next) -
             DATA_INDEX(data_ring, blk_lpos->begin));
     }
 
     /*
      * For wrapping data blocks, the trailing (wasted) space is
      * also counted.
      */
     return (DATA_INDEX(data_ring, blk_lpos->next) +
         DATA_SIZE(data_ring) - DATA_INDEX(data_ring, blk_lpos->begin));
 }
 
 /*
  * Given @blk_lpos, return a pointer to the writer data from the data block
  * and calculate the size of the data part. A NULL pointer is returned if
  * @blk_lpos specifies values that could never be legal.
  *
  * This function (used by readers) performs strict validation on the lpos
  * values to possibly detect bugs in the writer code. A WARN_ON_ONCE() is
  * triggered if an internal error is detected.
  */
 static const char *get_data(struct prb_data_ring *data_ring,
                 struct prb_data_blk_lpos *blk_lpos,
                 unsigned int *data_size)
 {
     struct prb_data_block *db;
 
     /* Data-less data block description. */
     if (BLK_DATALESS(blk_lpos)) {
         if (blk_lpos->begin == NO_LPOS && blk_lpos->next == NO_LPOS) {
             *data_size = 0;
             return "";
         }
         return NULL;
     }
 
     /* Regular data block: @begin less than @next and in same wrap. */
     if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next) &&
         blk_lpos->begin < blk_lpos->next) {
         db = to_block(data_ring, blk_lpos->begin);
         *data_size = blk_lpos->next - blk_lpos->begin;
 
     /* Wrapping data block: @begin is one wrap behind @next. */
     } else if (DATA_WRAPS(data_ring, blk_lpos->begin + DATA_SIZE(data_ring)) ==
            DATA_WRAPS(data_ring, blk_lpos->next)) {
         db = to_block(data_ring, 0);
         *data_size = DATA_INDEX(data_ring, blk_lpos->next);
 
     /* Illegal block description. */
     } else {
         WARN_ON_ONCE(1);
         return NULL;
     }
 
     /* A valid data block will always be aligned to the ID size. */
     if (WARN_ON_ONCE(blk_lpos->begin != ALIGN(blk_lpos->begin, sizeof(db->id))) ||
         WARN_ON_ONCE(blk_lpos->next != ALIGN(blk_lpos->next, sizeof(db->id)))) {
         return NULL;
     }
 
     /* A valid data block will always have at least an ID. */
     if (WARN_ON_ONCE(*data_size < sizeof(db->id)))
         return NULL;
 
     /* Subtract block ID space from size to reflect data size. */
     *data_size -= sizeof(db->id);
 
     return &db->data[0];
 }
 
 /*
  * Attempt to transition the newest descriptor from committed back to reserved
  * so that the record can be modified by a writer again. This is only possible
  * if the descriptor is not yet finalized and the provided @caller_id matches.
  */
 static struct prb_desc *desc_reopen_last(struct prb_desc_ring *desc_ring,
                      u32 caller_id, unsigned long *id_out)
 {
     unsigned long prev_state_val;
     enum desc_state d_state;
     struct prb_desc desc;
     struct prb_desc *d;
     unsigned long id;
     u32 cid;
 
     id = atomic_long_read(&desc_ring->head_id);
 
     /*
      * To reduce unnecessarily reopening, first check if the descriptor
      * state and caller ID are correct.
      */
     d_state = desc_read(desc_ring, id, &desc, NULL, &cid);
     if (d_state != desc_committed || cid != caller_id)
         return NULL;
 
     d = to_desc(desc_ring, id);
 
     prev_state_val = DESC_SV(id, desc_committed);
 
     /*
      * Guarantee the reserved state is stored before reading any
      * record data. A full memory barrier is needed because @state_var
      * modification is followed by reading. This pairs with _prb_commit:B.
      *
      * Memory barrier involvement:
      *
      * If desc_reopen_last:A reads from _prb_commit:B, then
      * prb_reserve_in_last:A reads from _prb_commit:A.
      *
      * Relies on:
      *
      * WMB from _prb_commit:A to _prb_commit:B
      *    matching
      * MB If desc_reopen_last:A to prb_reserve_in_last:A
      */
     if (!atomic_long_try_cmpxchg(&d->state_var, &prev_state_val,
             DESC_SV(id, desc_reserved))) { /* LMM(desc_reopen_last:A) */
         return NULL;
     }
 
     *id_out = id;
     return d;
 }
 
 /**
  * prb_reserve_in_last() - Re-reserve and extend the space in the ringbuffer
  *                         used by the newest record.
  *
  * @e:         The entry structure to setup.
  * @rb:        The ringbuffer to re-reserve and extend data in.
  * @r:         The record structure to allocate buffers for.
  * @caller_id: The caller ID of the caller (reserving writer).
  * @max_size:  Fail if the extended size would be greater than this.
  *
  * This is the public function available to writers to re-reserve and extend
  * data.
  *
  * The writer specifies the text size to extend (not the new total size) by
  * setting the @text_buf_size field of @r. To ensure proper initialization
  * of @r, prb_rec_init_wr() should be used.
  *
  * This function will fail if @caller_id does not match the caller ID of the
  * newest record. In that case the caller must reserve new data using
  * prb_reserve().
  *
  * Context: Any context. Disables local interrupts on success.
  * Return: true if text data could be extended, otherwise false.
  *
  * On success:
  *
  *   - @r->text_buf points to the beginning of the entire text buffer.
  *
  *   - @r->text_buf_size is set to the new total size of the buffer.
  *
  *   - @r->info is not touched so that @r->info->text_len could be used
  *     to append the text.
  *
  *   - prb_record_text_space() can be used on @e to query the new
  *     actually used space.
  *
  * Important: All @r->info fields will already be set with the current values
  *            for the record. I.e. @r->info->text_len will be less than
  *            @text_buf_size. Writers can use @r->info->text_len to know
  *            where concatenation begins and writers should update
  *            @r->info->text_len after concatenating.
  */
 bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
              struct printk_record *r, u32 caller_id, unsigned int max_size)
 {
     struct prb_desc_ring *desc_ring = &rb->desc_ring;
     struct printk_info *info;
     unsigned int data_size;
     struct prb_desc *d;
     unsigned long id;
 
     local_irq_save(e->irqflags);
 
     /* Transition the newest descriptor back to the reserved state. */
     d = desc_reopen_last(desc_ring, caller_id, &id);
     if (!d) {
         local_irq_restore(e->irqflags);
         goto fail_reopen;
     }
 
     /* Now the writer has exclusive access: LMM(prb_reserve_in_last:A) */
 
     info = to_info(desc_ring, id);
 
     /*
      * Set the @e fields here so that prb_commit() can be used if
      * anything fails from now on.
      */
     e->rb = rb;
     e->id = id;
 
     /*
      * desc_reopen_last() checked the caller_id, but there was no
      * exclusive access at that point. The descriptor may have
      * changed since then.
      */
     if (caller_id != info->caller_id)
         goto fail;
 
     if (BLK_DATALESS(&d->text_blk_lpos)) {
         if (WARN_ON_ONCE(info->text_len != 0)) {
             pr_warn_once("wrong text_len value (%hu, expecting 0)\n",
                      info->text_len);
             info->text_len = 0;
         }
 
         if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
             goto fail;
 
         if (r->text_buf_size > max_size)
             goto fail;
 
         r->text_buf = data_alloc(rb, r->text_buf_size,
                      &d->text_blk_lpos, id);
     } else {
         if (!get_data(&rb->text_data_ring, &d->text_blk_lpos, &data_size))
             goto fail;
 
         /*
          * Increase the buffer size to include the original size. If
          * the meta data (@text_len) is not sane, use the full data
          * block size.
          */
         if (WARN_ON_ONCE(info->text_len > data_size)) {
             pr_warn_once("wrong text_len value (%hu, expecting <=%u)\n",
                      info->text_len, data_size);
             info->text_len = data_size;
         }
         r->text_buf_size += info->text_len;
 
         if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
             goto fail;
 
         if (r->text_buf_size > max_size)
             goto fail;
 
         r->text_buf = data_realloc(rb, r->text_buf_size,
                        &d->text_blk_lpos, id);
     }
     if (r->text_buf_size && !r->text_buf)
         goto fail;
 
     r->info = info;
 
     e->text_space = space_used(&rb->text_data_ring, &d->text_blk_lpos);
 
     return true;
 fail:
     prb_commit(e);
     /* prb_commit() re-enabled interrupts. */
 fail_reopen:
     /* Make it clear to the caller that the re-reserve failed. */
     memset(r, 0, sizeof(*r));
     return false;
 }
 
 /*
  * Attempt to finalize a specified descriptor. If this fails, the descriptor
  * is either already final or it will finalize itself when the writer commits.
  */
 static void desc_make_final(struct prb_desc_ring *desc_ring, unsigned long id)
 {
     unsigned long prev_state_val = DESC_SV(id, desc_committed);
     struct prb_desc *d = to_desc(desc_ring, id);
 
     atomic_long_cmpxchg_relaxed(&d->state_var, prev_state_val,
             DESC_SV(id, desc_finalized)); /* LMM(desc_make_final:A) */
 
     /* Best effort to remember the last finalized @id. */
     atomic_long_set(&desc_ring->last_finalized_id, id);
 }
 
 /**
  * prb_reserve() - Reserve space in the ringbuffer.
  *
  * @e:  The entry structure to setup.
  * @rb: The ringbuffer to reserve data in.
  * @r:  The record structure to allocate buffers for.
  *
  * This is the public function available to writers to reserve data.
  *
  * The writer specifies the text size to reserve by setting the
  * @text_buf_size field of @r. To ensure proper initialization of @r,
  * prb_rec_init_wr() should be used.
  *
  * Context: Any context. Disables local interrupts on success.
  * Return: true if at least text data could be allocated, otherwise false.
  *
  * On success, the fields @info and @text_buf of @r will be set by this
  * function and should be filled in by the writer before committing. Also
  * on success, prb_record_text_space() can be used on @e to query the actual
  * space used for the text data block.
  *
  * Important: @info->text_len needs to be set correctly by the writer in
  *            order for data to be readable and/or extended. Its value
  *            is initialized to 0.
  */
 bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
          struct printk_record *r)
 {
     struct prb_desc_ring *desc_ring = &rb->desc_ring;
     struct printk_info *info;
     struct prb_desc *d;
     unsigned long id;
     u64 seq;
 
     if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
         goto fail;
 
     /*
      * Descriptors in the reserved state act as blockers to all further
      * reservations once the desc_ring has fully wrapped. Disable
      * interrupts during the reserve/commit window in order to minimize
      * the likelihood of this happening.
      */
     local_irq_save(e->irqflags);
 
     if (!desc_reserve(rb, &id)) {
         /* Descriptor reservation failures are tracked. */
         atomic_long_inc(&rb->fail);
         local_irq_restore(e->irqflags);
         goto fail;
     }
 
     d = to_desc(desc_ring, id);
     info = to_info(desc_ring, id);
 
     /*
      * All @info fields (except @seq) are cleared and must be filled in
      * by the writer. Save @seq before clearing because it is used to
      * determine the new sequence number.
      */
     seq = info->seq;
     memset(info, 0, sizeof(*info));
 
     /*
      * Set the @e fields here so that prb_commit() can be used if
      * text data allocation fails.
      */
     e->rb = rb;
     e->id = id;
 
     /*
      * Initialize the sequence number if it has "never been set".
      * Otherwise just increment it by a full wrap.
      *
      * @seq is considered "never been set" if it has a value of 0,
      * _except_ for @infos[0], which was specially setup by the ringbuffer
      * initializer and therefore is always considered as set.
      *
      * See the "Bootstrap" comment block in printk_ringbuffer.h for
      * details about how the initializer bootstraps the descriptors.
      */
     if (seq == 0 && DESC_INDEX(desc_ring, id) != 0)
         info->seq = DESC_INDEX(desc_ring, id);
     else
         info->seq = seq + DESCS_COUNT(desc_ring);
 
     /*
      * New data is about to be reserved. Once that happens, previous
      * descriptors are no longer able to be extended. Finalize the
      * previous descriptor now so that it can be made available to
      * readers. (For seq==0 there is no previous descriptor.)
      */
     if (info->seq > 0)
         desc_make_final(desc_ring, DESC_ID(id - 1));
 
     r->text_buf = data_alloc(rb, r->text_buf_size, &d->text_blk_lpos, id);
     /* If text data allocation fails, a data-less record is committed. */
     if (r->text_buf_size && !r->text_buf) {
         prb_commit(e);
         /* prb_commit() re-enabled interrupts. */
         goto fail;
     }
 
     r->info = info;
 
     /* Record full text space used by record. */
     e->text_space = space_used(&rb->text_data_ring, &d->text_blk_lpos);
 
     return true;
 fail:
     /* Make it clear to the caller that the reserve failed. */
     memset(r, 0, sizeof(*r));
     return false;
 }
 
 /* Commit the data (possibly finalizing it) and restore interrupts. */
 static void _prb_commit(struct prb_reserved_entry *e, unsigned long state_val)
 {
     struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
     struct prb_desc *d = to_desc(desc_ring, e->id);
     unsigned long prev_state_val = DESC_SV(e->id, desc_reserved);
 
     /* Now the writer has finished all writing: LMM(_prb_commit:A) */
 
     /*
      * Set the descriptor as committed. See "ABA Issues" about why
      * cmpxchg() instead of set() is used.
      *
      * 1  Guarantee all record data is stored before the descriptor state
      *    is stored as committed. A write memory barrier is sufficient
      *    for this. This pairs with desc_read:B and desc_reopen_last:A.
      *
      * 2. Guarantee the descriptor state is stored as committed before
      *    re-checking the head ID in order to possibly finalize this
      *    descriptor. This pairs with desc_reserve:D.
      *
      *    Memory barrier involvement:
      *
      *    If prb_commit:A reads from desc_reserve:D, then
      *    desc_make_final:A reads from _prb_commit:B.
      *
      *    Relies on:
      *
      *    MB _prb_commit:B to prb_commit:A
      *       matching
      *    MB desc_reserve:D to desc_make_final:A
      */
     if (!atomic_long_try_cmpxchg(&d->state_var, &prev_state_val,
             DESC_SV(e->id, state_val))) { /* LMM(_prb_commit:B) */
         WARN_ON_ONCE(1);
     }
 
     /* Restore interrupts, the reserve/commit window is finished. */
     local_irq_restore(e->irqflags);
 }
 
 /**
  * prb_commit() - Commit (previously reserved) data to the ringbuffer.
  *
  * @e: The entry containing the reserved data information.
  *
  * This is the public function available to writers to commit data.
  *
  * Note that the data is not yet available to readers until it is finalized.
  * Finalizing happens automatically when space for the next record is
  * reserved.
  *
  * See prb_final_commit() for a version of this function that finalizes
  * immediately.
  *
  * Context: Any context. Enables local interrupts.
  */
 void prb_commit(struct prb_reserved_entry *e)
 {
     struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
     unsigned long head_id;
 
     _prb_commit(e, desc_committed);
 
     /*
      * If this descriptor is no longer the head (i.e. a new record has
      * been allocated), extending the data for this record is no longer
      * allowed and therefore it must be finalized.
      */
     head_id = atomic_long_read(&desc_ring->head_id); /* LMM(prb_commit:A) */
     if (head_id != e->id)
         desc_make_final(desc_ring, e->id);
 }
 
 /**
  * prb_final_commit() - Commit and finalize (previously reserved) data to
  *                      the ringbuffer.
  *
  * @e: The entry containing the reserved data information.
  *
  * This is the public function available to writers to commit+finalize data.
  *
  * By finalizing, the data is made immediately available to readers.
  *
  * This function should only be used if there are no intentions of extending
  * this data using prb_reserve_in_last().
  *
  * Context: Any context. Enables local interrupts.
  */
 void prb_final_commit(struct prb_reserved_entry *e)
 {
     struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
 
     _prb_commit(e, desc_finalized);
 
     /* Best effort to remember the last finalized @id. */
     atomic_long_set(&desc_ring->last_finalized_id, e->id);
 }
 
 /*
  * Count the number of lines in provided text. All text has at least 1 line
  * (even if @text_size is 0). Each '\n' processed is counted as an additional
  * line.
  */
 static unsigned int count_lines(const char *text, unsigned int text_size)
 {
     unsigned int next_size = text_size;
     unsigned int line_count = 1;
     const char *next = text;
 
     while (next_size) {
         next = memchr(next, '\n', next_size);
         if (!next)
             break;
         line_count++;
         next++;
         next_size = text_size - (next - text);
     }
 
     return line_count;
 }
 
 /*
  * Given @blk_lpos, copy an expected @len of data into the provided buffer.
  * If @line_count is provided, count the number of lines in the data.
  *
  * This function (used by readers) performs strict validation on the data
  * size to possibly detect bugs in the writer code. A WARN_ON_ONCE() is
  * triggered if an internal error is detected.
  */
 static bool copy_data(struct prb_data_ring *data_ring,
               struct prb_data_blk_lpos *blk_lpos, u16 len, char *buf,
               unsigned int buf_size, unsigned int *line_count)
 {
     unsigned int data_size;
     const char *data;
 
     /* Caller might not want any data. */
     if ((!buf || !buf_size) && !line_count)
         return true;
 
     data = get_data(data_ring, blk_lpos, &data_size);
     if (!data)
         return false;
 
     /*
      * Actual cannot be less than expected. It can be more than expected
      * because of the trailing alignment padding.
      *
      * Note that invalid @len values can occur because the caller loads
      * the value during an allowed data race.
      */
     if (data_size < (unsigned int)len)
         return false;
 
     /* Caller interested in the line count? */
     if (line_count)
         *line_count = count_lines(data, len);
 
     /* Caller interested in the data content? */
     if (!buf || !buf_size)
         return true;
 
     data_size = min_t(u16, buf_size, len);
 
     memcpy(&buf[0], data, data_size); /* LMM(copy_data:A) */
     return true;
 }
 
 /*
  * This is an extended version of desc_read(). It gets a copy of a specified
  * descriptor. However, it also verifies that the record is finalized and has
  * the sequence number @seq. On success, 0 is returned.
  *
  * Error return values:
  * -EINVAL: A finalized record with sequence number @seq does not exist.
  * -ENOENT: A finalized record with sequence number @seq exists, but its data
  *          is not available. This is a valid record, so readers should
  *          continue with the next record.
  */
 static int desc_read_finalized_seq(struct prb_desc_ring *desc_ring,
                    unsigned long id, u64 seq,
                    struct prb_desc *desc_out)
 {
     struct prb_data_blk_lpos *blk_lpos = &desc_out->text_blk_lpos;
     enum desc_state d_state;
     u64 s;
 
     d_state = desc_read(desc_ring, id, desc_out, &s, NULL);
 
     /*
      * An unexpected @id (desc_miss) or @seq mismatch means the record
      * does not exist. A descriptor in the reserved or committed state
      * means the record does not yet exist for the reader.
      */
     if (d_state == desc_miss ||
         d_state == desc_reserved ||
         d_state == desc_committed ||
         s != seq) {
         return -EINVAL;
     }
 
     /*
      * A descriptor in the reusable state may no longer have its data
      * available; report it as existing but with lost data. Or the record
      * may actually be a record with lost data.
      */
     if (d_state == desc_reusable ||
         (blk_lpos->begin == FAILED_LPOS && blk_lpos->next == FAILED_LPOS)) {
         return -ENOENT;
     }
 
     return 0;
 }
 
 /*
  * Copy the ringbuffer data from the record with @seq to the provided
  * @r buffer. On success, 0 is returned.
  *
  * See desc_read_finalized_seq() for error return values.
  */
 static int prb_read(struct printk_ringbuffer *rb, u64 seq,
             struct printk_record *r, unsigned int *line_count)
 {
     struct prb_desc_ring *desc_ring = &rb->desc_ring;
     struct printk_info *info = to_info(desc_ring, seq);
     struct prb_desc *rdesc = to_desc(desc_ring, seq);
     atomic_long_t *state_var = &rdesc->state_var;
     struct prb_desc desc;
     unsigned long id;
     int err;
 
     /* Extract the ID, used to specify the descriptor to read. */
     id = DESC_ID(atomic_long_read(state_var));
 
     /* Get a local copy of the correct descriptor (if available). */
     err = desc_read_finalized_seq(desc_ring, id, seq, &desc);
 
     /*
      * If @r is NULL, the caller is only interested in the availability
      * of the record.
      */
     if (err || !r)
         return err;
 
     /* If requested, copy meta data. */
     if (r->info)
         memcpy(r->info, info, sizeof(*(r->info)));
 
     /* Copy text data. If it fails, this is a data-less record. */
     if (!copy_data(&rb->text_data_ring, &desc.text_blk_lpos, info->text_len,
                r->text_buf, r->text_buf_size, line_count)) {
         return -ENOENT;
     }
 
     /* Ensure the record is still finalized and has the same @seq. */
     return desc_read_finalized_seq(desc_ring, id, seq, &desc);
 }
 
 /* Get the sequence number of the tail descriptor. */
 static u64 prb_first_seq(struct printk_ringbuffer *rb)
 {
     struct prb_desc_ring *desc_ring = &rb->desc_ring;
     enum desc_state d_state;
     struct prb_desc desc;
     unsigned long id;
     u64 seq;
 
     for (;;) {
         id = atomic_long_read(&rb->desc_ring.tail_id); /* LMM(prb_first_seq:A) */
 
         d_state = desc_read(desc_ring, id, &desc, &seq, NULL); /* LMM(prb_first_seq:B) */
 
         /*
          * This loop will not be infinite because the tail is
          * _always_ in the finalized or reusable state.
          */
         if (d_state == desc_finalized || d_state == desc_reusable)
             break;
 
         /*
          * Guarantee the last state load from desc_read() is before
          * reloading @tail_id in order to see a new tail in the case
          * that the descriptor has been recycled. This pairs with
          * desc_reserve:D.
          *
          * Memory barrier involvement:
          *
          * If prb_first_seq:B reads from desc_reserve:F, then
          * prb_first_seq:A reads from desc_push_tail:B.
          *
          * Relies on:
          *
          * MB from desc_push_tail:B to desc_reserve:F
          *    matching
          * RMB prb_first_seq:B to prb_first_seq:A
          */
         smp_rmb(); /* LMM(prb_first_seq:C) */
     }
 
     return seq;
 }
 
 /*
  * Non-blocking read of a record. Updates @seq to the last finalized record
  * (which may have no data available).
  *
  * See the description of prb_read_valid() and prb_read_valid_info()
  * for details.
  */
 static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
                 struct printk_record *r, unsigned int *line_count)
 {
     u64 tail_seq;
     int err;
 
     while ((err = prb_read(rb, *seq, r, line_count))) {
         tail_seq = prb_first_seq(rb);
 
         if (*seq < tail_seq) {
             /*
              * Behind the tail. Catch up and try again. This
              * can happen for -ENOENT and -EINVAL cases.
              */
             *seq = tail_seq;
 
         } else if (err == -ENOENT) {
             /* Record exists, but no data available. Skip. */
             (*seq)++;
 
         } else {
             /* Non-existent/non-finalized record. Must stop. */
             return false;
         }
     }
 
     return true;
 }
 
 /**
  * prb_read_valid() - Non-blocking read of a requested record or (if gone)
  *                    the next available record.
  *
  * @rb:  The ringbuffer to read from.
  * @seq: The sequence number of the record to read.
  * @r:   A record data buffer to store the read record to.
  *
  * This is the public function available to readers to read a record.
  *
  * The reader provides the @info and @text_buf buffers of @r to be
  * filled in. Any of the buffer pointers can be set to NULL if the reader
  * is not interested in that data. To ensure proper initialization of @r,
  * prb_rec_init_rd() should be used.
  *
  * Context: Any context.
  * Return: true if a record was read, otherwise false.
  *
  * On success, the reader must check r->info.seq to see which record was
  * actually read. This allows the reader to detect dropped records.
  *
  * Failure means @seq refers to a not yet written record.
  */
 bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq,
             struct printk_record *r)
 {
     return _prb_read_valid(rb, &seq, r, NULL);
 }
 
 /**
  * prb_read_valid_info() - Non-blocking read of meta data for a requested
  *                         record or (if gone) the next available record.
  *
  * @rb:         The ringbuffer to read from.
  * @seq:        The sequence number of the record to read.
  * @info:       A buffer to store the read record meta data to.
  * @line_count: A buffer to store the number of lines in the record text.
  *
  * This is the public function available to readers to read only the
  * meta data of a record.
  *
  * The reader provides the @info, @line_count buffers to be filled in.
  * Either of the buffer pointers can be set to NULL if the reader is not
  * interested in that data.
  *
  * Context: Any context.
  * Return: true if a record's meta data was read, otherwise false.
  *
  * On success, the reader must check info->seq to see which record meta data
  * was actually read. This allows the reader to detect dropped records.
  *
  * Failure means @seq refers to a not yet written record.
  */
 bool prb_read_valid_info(struct printk_ringbuffer *rb, u64 seq,
              struct printk_info *info, unsigned int *line_count)
 {
     struct printk_record r;
 
     prb_rec_init_rd(&r, info, NULL, 0);
 
     return _prb_read_valid(rb, &seq, &r, line_count);
 }
 
 /**
  * prb_first_valid_seq() - Get the sequence number of the oldest available
  *                         record.
  *
  * @rb: The ringbuffer to get the sequence number from.
  *
  * This is the public function available to readers to see what the
  * first/oldest valid sequence number is.
  *
  * This provides readers a starting point to begin iterating the ringbuffer.
  *
  * Context: Any context.
  * Return: The sequence number of the first/oldest record or, if the
  *         ringbuffer is empty, 0 is returned.
  */
 u64 prb_first_valid_seq(struct printk_ringbuffer *rb)
 {
     u64 seq = 0;
 
     if (!_prb_read_valid(rb, &seq, NULL, NULL))
         return 0;
 
     return seq;
 }
 
 /**
  * prb_next_seq() - Get the sequence number after the last available record.
  *
  * @rb:  The ringbuffer to get the sequence number from.
  *
  * This is the public function available to readers to see what the next
  * newest sequence number available to readers will be.
  *
  * This provides readers a sequence number to jump to if all currently
  * available records should be skipped.
  *
  * Context: Any context.
  * Return: The sequence number of the next newest (not yet available) record
  *         for readers.
  */
 u64 prb_next_seq(struct printk_ringbuffer *rb)
 {
     struct prb_desc_ring *desc_ring = &rb->desc_ring;
     enum desc_state d_state;
     unsigned long id;
     u64 seq;
 
     /* Check if the cached @id still points to a valid @seq. */
     id = atomic_long_read(&desc_ring->last_finalized_id);
     d_state = desc_read(desc_ring, id, NULL, &seq, NULL);
 
     if (d_state == desc_finalized || d_state == desc_reusable) {
         /*
          * Begin searching after the last finalized record.
          *
          * On 0, the search must begin at 0 because of hack#2
          * of the bootstrapping phase it is not known if a
          * record at index 0 exists.
          */
         if (seq != 0)
             seq++;
     } else {
         /*
          * The information about the last finalized sequence number
          * has gone. It should happen only when there is a flood of
          * new messages and the ringbuffer is rapidly recycled.
          * Give up and start from the beginning.
          */
         seq = 0;
     }
 
     /*
      * The information about the last finalized @seq might be inaccurate.
      * Search forward to find the current one.
      */
     while (_prb_read_valid(rb, &seq, NULL, NULL))
         seq++;
 
     return seq;
 }
 
 /**
  * prb_init() - Initialize a ringbuffer to use provided external buffers.
  *
  * @rb:       The ringbuffer to initialize.
  * @text_buf: The data buffer for text data.
  * @textbits: The size of @text_buf as a power-of-2 value.
  * @descs:    The descriptor buffer for ringbuffer records.
  * @descbits: The count of @descs items as a power-of-2 value.
  * @infos:    The printk_info buffer for ringbuffer records.
  *
  * This is the public function available to writers to setup a ringbuffer
  * during runtime using provided buffers.
  *
  * This must match the initialization of DEFINE_PRINTKRB().
  *
  * Context: Any context.
  */
 void prb_init(struct printk_ringbuffer *rb,
           char *text_buf, unsigned int textbits,
           struct prb_desc *descs, unsigned int descbits,
           struct printk_info *infos)
 {
     memset(descs, 0, _DESCS_COUNT(descbits) * sizeof(descs[0]));
     memset(infos, 0, _DESCS_COUNT(descbits) * sizeof(infos[0]));
 
     rb->desc_ring.count_bits = descbits;
     rb->desc_ring.descs = descs;
     rb->desc_ring.infos = infos;
     atomic_long_set(&rb->desc_ring.head_id, DESC0_ID(descbits));
     atomic_long_set(&rb->desc_ring.tail_id, DESC0_ID(descbits));
     atomic_long_set(&rb->desc_ring.last_finalized_id, DESC0_ID(descbits));
 
     rb->text_data_ring.size_bits = textbits;
     rb->text_data_ring.data = text_buf;
     atomic_long_set(&rb->text_data_ring.head_lpos, BLK0_LPOS(textbits));
     atomic_long_set(&rb->text_data_ring.tail_lpos, BLK0_LPOS(textbits));
 
     atomic_long_set(&rb->fail, 0);
 
     atomic_long_set(&(descs[_DESCS_COUNT(descbits) - 1].state_var), DESC0_SV(descbits));
     descs[_DESCS_COUNT(descbits) - 1].text_blk_lpos.begin = FAILED_LPOS;
     descs[_DESCS_COUNT(descbits) - 1].text_blk_lpos.next = FAILED_LPOS;
 
     infos[0].seq = -(u64)_DESCS_COUNT(descbits);
     infos[_DESCS_COUNT(descbits) - 1].seq = 0;
 }
 
 /**
  * prb_record_text_space() - Query the full actual used ringbuffer space for
  *                           the text data of a reserved entry.
  *
  * @e: The successfully reserved entry to query.
  *
  * This is the public function available to writers to see how much actual
  * space is used in the ringbuffer to store the text data of the specified
  * entry.
  *
  * This function is only valid if @e has been successfully reserved using
  * prb_reserve().
  *
  * Context: Any context.
  * Return: The size in bytes used by the text data of the associated record.
  */
 unsigned int prb_record_text_space(struct prb_reserved_entry *e)
 {
     return e->text_space;
 }

This page was automatically generated by the 2.1.0 LXR engine. The LXR team