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 /* SPDX-License-Identifier: GPL-2.0 */
 #ifndef __LINUX_SEQLOCK_H
 #define __LINUX_SEQLOCK_H
 
 /*
  * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
  * lockless readers (read-only retry loops), and no writer starvation.
  *
  * See Documentation/locking/seqlock.rst
  *
  * Copyrights:
  * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
  * - Sequence counters with associated locks, (C) 2020 Linutronix GmbH
  */
 
 #include <linux/compiler.h>
 #include <linux/kcsan-checks.h>
 #include <linux/lockdep.h>
 #include <linux/mutex.h>
 #include <linux/preempt.h>
 #include <linux/spinlock.h>
 
 #include <asm/processor.h>
 
 /*
  * The seqlock seqcount_t interface does not prescribe a precise sequence of
  * read begin/retry/end. For readers, typically there is a call to
  * read_seqcount_begin() and read_seqcount_retry(), however, there are more
  * esoteric cases which do not follow this pattern.
  *
  * As a consequence, we take the following best-effort approach for raw usage
  * via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
  * pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
  * atomics; if there is a matching read_seqcount_retry() call, no following
  * memory operations are considered atomic. Usage of the seqlock_t interface
  * is not affected.
  */
 #define KCSAN_SEQLOCK_REGION_MAX 1000
 
 /*
  * Sequence counters (seqcount_t)
  *
  * This is the raw counting mechanism, without any writer protection.
  *
  * Write side critical sections must be serialized and non-preemptible.
  *
  * If readers can be invoked from hardirq or softirq contexts,
  * interrupts or bottom halves must also be respectively disabled before
  * entering the write section.
  *
  * This mechanism can't be used if the protected data contains pointers,
  * as the writer can invalidate a pointer that a reader is following.
  *
  * If the write serialization mechanism is one of the common kernel
  * locking primitives, use a sequence counter with associated lock
  * (seqcount_LOCKNAME_t) instead.
  *
  * If it's desired to automatically handle the sequence counter writer
  * serialization and non-preemptibility requirements, use a sequential
  * lock (seqlock_t) instead.
  *
  * See Documentation/locking/seqlock.rst
  */
 typedef struct seqcount {
     unsigned sequence;
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
     struct lockdep_map dep_map;
 #endif
 } seqcount_t;
 
 static inline void __seqcount_init(seqcount_t *s, const char *name,
                       struct lock_class_key *key)
 {
     /*
      * Make sure we are not reinitializing a held lock:
      */
     lockdep_init_map(&s->dep_map, name, key, 0);
     s->sequence = 0;
 }
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
 # define SEQCOUNT_DEP_MAP_INIT(lockname)                \
         .dep_map = { .name = #lockname }
 
 /**
  * seqcount_init() - runtime initializer for seqcount_t
  * @s: Pointer to the seqcount_t instance
  */
 # define seqcount_init(s)                       \
     do {                                \
         static struct lock_class_key __key;         \
         __seqcount_init((s), #s, &__key);           \
     } while (0)
 
 static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
 {
     seqcount_t *l = (seqcount_t *)s;
     unsigned long flags;
 
     local_irq_save(flags);
     seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
     seqcount_release(&l->dep_map, _RET_IP_);
     local_irq_restore(flags);
 }
 
 #else
 # define SEQCOUNT_DEP_MAP_INIT(lockname)
 # define seqcount_init(s) __seqcount_init(s, NULL, NULL)
 # define seqcount_lockdep_reader_access(x)
 #endif
 
 /**
  * SEQCNT_ZERO() - static initializer for seqcount_t
  * @name: Name of the seqcount_t instance
  */
 #define SEQCNT_ZERO(name) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(name) }
 
 /*
  * Sequence counters with associated locks (seqcount_LOCKNAME_t)
  *
  * A sequence counter which associates the lock used for writer
  * serialization at initialization time. This enables lockdep to validate
  * that the write side critical section is properly serialized.
  *
  * For associated locks which do not implicitly disable preemption,
  * preemption protection is enforced in the write side function.
  *
  * Lockdep is never used in any for the raw write variants.
  *
  * See Documentation/locking/seqlock.rst
  */
 
 /*
  * For PREEMPT_RT, seqcount_LOCKNAME_t write side critical sections cannot
  * disable preemption. It can lead to higher latencies, and the write side
  * sections will not be able to acquire locks which become sleeping locks
  * (e.g. spinlock_t).
  *
  * To remain preemptible while avoiding a possible livelock caused by the
  * reader preempting the writer, use a different technique: let the reader
  * detect if a seqcount_LOCKNAME_t writer is in progress. If that is the
  * case, acquire then release the associated LOCKNAME writer serialization
  * lock. This will allow any possibly-preempted writer to make progress
  * until the end of its writer serialization lock critical section.
  *
  * This lock-unlock technique must be implemented for all of PREEMPT_RT
  * sleeping locks.  See Documentation/locking/locktypes.rst
  */
 #if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT)
 #define __SEQ_LOCK(expr)    expr
 #else
 #define __SEQ_LOCK(expr)
 #endif
 
 /*
  * typedef seqcount_LOCKNAME_t - sequence counter with LOCKNAME associated
  * @seqcount:   The real sequence counter
  * @lock:   Pointer to the associated lock
  *
  * A plain sequence counter with external writer synchronization by
  * LOCKNAME @lock. The lock is associated to the sequence counter in the
  * static initializer or init function. This enables lockdep to validate
  * that the write side critical section is properly serialized.
  *
  * LOCKNAME:    raw_spinlock, spinlock, rwlock or mutex
  */
 
 /*
  * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
  * @s:      Pointer to the seqcount_LOCKNAME_t instance
  * @lock:   Pointer to the associated lock
  */
 
 #define seqcount_LOCKNAME_init(s, _lock, lockname)          \
     do {                                \
         seqcount_##lockname##_t *____s = (s);           \
         seqcount_init(&____s->seqcount);            \
         __SEQ_LOCK(____s->lock = (_lock));          \
     } while (0)
 
 #define seqcount_raw_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, raw_spinlock)
 #define seqcount_spinlock_init(s, lock)     seqcount_LOCKNAME_init(s, lock, spinlock)
 #define seqcount_rwlock_init(s, lock)       seqcount_LOCKNAME_init(s, lock, rwlock)
 #define seqcount_mutex_init(s, lock)        seqcount_LOCKNAME_init(s, lock, mutex)
 
 /*
  * SEQCOUNT_LOCKNAME()  - Instantiate seqcount_LOCKNAME_t and helpers
  * seqprop_LOCKNAME_*() - Property accessors for seqcount_LOCKNAME_t
  *
  * @lockname:       "LOCKNAME" part of seqcount_LOCKNAME_t
  * @locktype:       LOCKNAME canonical C data type
  * @preemptible:    preemptibility of above locktype
  * @lockmember:     argument for lockdep_assert_held()
  * @lockbase:       associated lock release function (prefix only)
  * @lock_acquire:   associated lock acquisition function (full call)
  */
 #define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember, lockbase, lock_acquire) \
 typedef struct seqcount_##lockname {                    \
     seqcount_t      seqcount;               \
     __SEQ_LOCK(locktype *lock);                 \
 } seqcount_##lockname##_t;                      \
                                     \
 static __always_inline seqcount_t *                 \
 __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s)          \
 {                                   \
     return &s->seqcount;                        \
 }                                   \
                                     \
 static __always_inline unsigned                     \
 __seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s)   \
 {                                   \
     unsigned seq = READ_ONCE(s->seqcount.sequence);         \
                                     \
     if (!IS_ENABLED(CONFIG_PREEMPT_RT))             \
         return seq;                     \
                                     \
     if (preemptible && unlikely(seq & 1)) {             \
         __SEQ_LOCK(lock_acquire);               \
         __SEQ_LOCK(lockbase##_unlock(s->lock));         \
                                     \
         /*                          \
          * Re-read the sequence counter since the (possibly \
          * preempted) writer made progress.         \
          */                         \
         seq = READ_ONCE(s->seqcount.sequence);          \
     }                               \
                                     \
     return seq;                         \
 }                                   \
                                     \
 static __always_inline bool                     \
 __seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s)    \
 {                                   \
     if (!IS_ENABLED(CONFIG_PREEMPT_RT))             \
         return preemptible;                 \
                                     \
     /* PREEMPT_RT relies on the above LOCK+UNLOCK */        \
     return false;                           \
 }                                   \
                                     \
 static __always_inline void                     \
 __seqprop_##lockname##_assert(const seqcount_##lockname##_t *s)     \
 {                                   \
     __SEQ_LOCK(lockdep_assert_held(lockmember));            \
 }
 
 /*
  * __seqprop() for seqcount_t
  */
 
 static inline seqcount_t *__seqprop_ptr(seqcount_t *s)
 {
     return s;
 }
 
 static inline unsigned __seqprop_sequence(const seqcount_t *s)
 {
     return READ_ONCE(s->sequence);
 }
 
 static inline bool __seqprop_preemptible(const seqcount_t *s)
 {
     return false;
 }
 
 static inline void __seqprop_assert(const seqcount_t *s)
 {
     lockdep_assert_preemption_disabled();
 }
 
 #define __SEQ_RT    IS_ENABLED(CONFIG_PREEMPT_RT)
 
 SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t,  false,    s->lock,        raw_spin, raw_spin_lock(s->lock))
 SEQCOUNT_LOCKNAME(spinlock,     spinlock_t,      __SEQ_RT, s->lock,        spin,     spin_lock(s->lock))
 SEQCOUNT_LOCKNAME(rwlock,       rwlock_t,        __SEQ_RT, s->lock,        read,     read_lock(s->lock))
 SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     s->lock,        mutex,    mutex_lock(s->lock))
 
 /*
  * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
  * @name:   Name of the seqcount_LOCKNAME_t instance
  * @lock:   Pointer to the associated LOCKNAME
  */
 
 #define SEQCOUNT_LOCKNAME_ZERO(seq_name, assoc_lock) {          \
     .seqcount       = SEQCNT_ZERO(seq_name.seqcount),   \
     __SEQ_LOCK(.lock    = (assoc_lock))             \
 }
 
 #define SEQCNT_RAW_SPINLOCK_ZERO(name, lock)    SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_SPINLOCK_ZERO(name, lock)    SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_RWLOCK_ZERO(name, lock)      SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_MUTEX_ZERO(name, lock)       SEQCOUNT_LOCKNAME_ZERO(name, lock)
 #define SEQCNT_WW_MUTEX_ZERO(name, lock)    SEQCOUNT_LOCKNAME_ZERO(name, lock)
 
 #define __seqprop_case(s, lockname, prop)               \
     seqcount_##lockname##_t: __seqprop_##lockname##_##prop((void *)(s))
 
 #define __seqprop(s, prop) _Generic(*(s),               \
     seqcount_t:     __seqprop_##prop((void *)(s)),      \
     __seqprop_case((s), raw_spinlock,   prop),          \
     __seqprop_case((s), spinlock,   prop),          \
     __seqprop_case((s), rwlock,     prop),          \
     __seqprop_case((s), mutex,      prop))
 
 #define seqprop_ptr(s)          __seqprop(s, ptr)
 #define seqprop_sequence(s)     __seqprop(s, sequence)
 #define seqprop_preemptible(s)      __seqprop(s, preemptible)
 #define seqprop_assert(s)       __seqprop(s, assert)
 
 /**
  * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
  * provided before actually loading any of the variables that are to be
  * protected in this critical section.
  *
  * Use carefully, only in critical code, and comment how the barrier is
  * provided.
  *
  * Return: count to be passed to read_seqcount_retry()
  */
 #define __read_seqcount_begin(s)                    \
 ({                                  \
     unsigned __seq;                         \
                                     \
     while ((__seq = seqprop_sequence(s)) & 1)           \
         cpu_relax();                        \
                                     \
     kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);            \
     __seq;                              \
 })
 
 /**
  * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_read_seqcount_begin(s)                  \
 ({                                  \
     unsigned _seq = __read_seqcount_begin(s);           \
                                     \
     smp_rmb();                          \
     _seq;                               \
 })
 
 /**
  * read_seqcount_begin() - begin a seqcount_t read critical section
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * Return: count to be passed to read_seqcount_retry()
  */
 #define read_seqcount_begin(s)                      \
 ({                                  \
     seqcount_lockdep_reader_access(seqprop_ptr(s));         \
     raw_read_seqcount_begin(s);                 \
 })
 
 /**
  * raw_read_seqcount() - read the raw seqcount_t counter value
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * raw_read_seqcount opens a read critical section of the given
  * seqcount_t, without any lockdep checking, and without checking or
  * masking the sequence counter LSB. Calling code is responsible for
  * handling that.
  *
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_read_seqcount(s)                        \
 ({                                  \
     unsigned __seq = seqprop_sequence(s);               \
                                     \
     smp_rmb();                          \
     kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);            \
     __seq;                              \
 })
 
 /**
  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
  *                        lockdep and w/o counter stabilization
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * raw_seqcount_begin opens a read critical section of the given
  * seqcount_t. Unlike read_seqcount_begin(), this function will not wait
  * for the count to stabilize. If a writer is active when it begins, it
  * will fail the read_seqcount_retry() at the end of the read critical
  * section instead of stabilizing at the beginning of it.
  *
  * Use this only in special kernel hot paths where the read section is
  * small and has a high probability of success through other external
  * means. It will save a single branching instruction.
  *
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_seqcount_begin(s)                       \
 ({                                  \
     /*                              \
      * If the counter is odd, let read_seqcount_retry() fail    \
      * by decrementing the counter.                 \
      */                             \
     raw_read_seqcount(s) & ~1;                  \
 })
 
 /**
  * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  * @start: count, from read_seqcount_begin()
  *
  * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
  * provided before actually loading any of the variables that are to be
  * protected in this critical section.
  *
  * Use carefully, only in critical code, and comment how the barrier is
  * provided.
  *
  * Return: true if a read section retry is required, else false
  */
 #define __read_seqcount_retry(s, start)                 \
     do___read_seqcount_retry(seqprop_ptr(s), start)
 
 static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
 {
     kcsan_atomic_next(0);
     return unlikely(READ_ONCE(s->sequence) != start);
 }
 
 /**
  * read_seqcount_retry() - end a seqcount_t read critical section
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  * @start: count, from read_seqcount_begin()
  *
  * read_seqcount_retry closes the read critical section of given
  * seqcount_t.  If the critical section was invalid, it must be ignored
  * (and typically retried).
  *
  * Return: true if a read section retry is required, else false
  */
 #define read_seqcount_retry(s, start)                   \
     do_read_seqcount_retry(seqprop_ptr(s), start)
 
 static inline int do_read_seqcount_retry(const seqcount_t *s, unsigned start)
 {
     smp_rmb();
     return do___read_seqcount_retry(s, start);
 }
 
 /**
  * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * Context: check write_seqcount_begin()
  */
 #define raw_write_seqcount_begin(s)                 \
 do {                                    \
     if (seqprop_preemptible(s))                 \
         preempt_disable();                  \
                                     \
     do_raw_write_seqcount_begin(seqprop_ptr(s));            \
 } while (0)
 
 static inline void do_raw_write_seqcount_begin(seqcount_t *s)
 {
     kcsan_nestable_atomic_begin();
     s->sequence++;
     smp_wmb();
 }
 
 /**
  * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * Context: check write_seqcount_end()
  */
 #define raw_write_seqcount_end(s)                   \
 do {                                    \
     do_raw_write_seqcount_end(seqprop_ptr(s));          \
                                     \
     if (seqprop_preemptible(s))                 \
         preempt_enable();                   \
 } while (0)
 
 static inline void do_raw_write_seqcount_end(seqcount_t *s)
 {
     smp_wmb();
     s->sequence++;
     kcsan_nestable_atomic_end();
 }
 
 /**
  * write_seqcount_begin_nested() - start a seqcount_t write section with
  *                                 custom lockdep nesting level
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  * @subclass: lockdep nesting level
  *
  * See Documentation/locking/lockdep-design.rst
  * Context: check write_seqcount_begin()
  */
 #define write_seqcount_begin_nested(s, subclass)            \
 do {                                    \
     seqprop_assert(s);                      \
                                     \
     if (seqprop_preemptible(s))                 \
         preempt_disable();                  \
                                     \
     do_write_seqcount_begin_nested(seqprop_ptr(s), subclass);   \
 } while (0)
 
 static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass)
 {
     do_raw_write_seqcount_begin(s);
     seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
 }
 
 /**
  * write_seqcount_begin() - start a seqcount_t write side critical section
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * Context: sequence counter write side sections must be serialized and
  * non-preemptible. Preemption will be automatically disabled if and
  * only if the seqcount write serialization lock is associated, and
  * preemptible.  If readers can be invoked from hardirq or softirq
  * context, interrupts or bottom halves must be respectively disabled.
  */
 #define write_seqcount_begin(s)                     \
 do {                                    \
     seqprop_assert(s);                      \
                                     \
     if (seqprop_preemptible(s))                 \
         preempt_disable();                  \
                                     \
     do_write_seqcount_begin(seqprop_ptr(s));            \
 } while (0)
 
 static inline void do_write_seqcount_begin(seqcount_t *s)
 {
     do_write_seqcount_begin_nested(s, 0);
 }
 
 /**
  * write_seqcount_end() - end a seqcount_t write side critical section
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * Context: Preemption will be automatically re-enabled if and only if
  * the seqcount write serialization lock is associated, and preemptible.
  */
 #define write_seqcount_end(s)                       \
 do {                                    \
     do_write_seqcount_end(seqprop_ptr(s));              \
                                     \
     if (seqprop_preemptible(s))                 \
         preempt_enable();                   \
 } while (0)
 
 static inline void do_write_seqcount_end(seqcount_t *s)
 {
     seqcount_release(&s->dep_map, _RET_IP_);
     do_raw_write_seqcount_end(s);
 }
 
 /**
  * raw_write_seqcount_barrier() - do a seqcount_t write barrier
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * This can be used to provide an ordering guarantee instead of the usual
  * consistency guarantee. It is one wmb cheaper, because it can collapse
  * the two back-to-back wmb()s.
  *
  * Note that writes surrounding the barrier should be declared atomic (e.g.
  * via WRITE_ONCE): a) to ensure the writes become visible to other threads
  * atomically, avoiding compiler optimizations; b) to document which writes are
  * meant to propagate to the reader critical section. This is necessary because
  * neither writes before and after the barrier are enclosed in a seq-writer
  * critical section that would ensure readers are aware of ongoing writes::
  *
  *  seqcount_t seq;
  *  bool X = true, Y = false;
  *
  *  void read(void)
  *  {
  *      bool x, y;
  *
  *      do {
  *          int s = read_seqcount_begin(&seq);
  *
  *          x = X; y = Y;
  *
  *      } while (read_seqcount_retry(&seq, s));
  *
  *      BUG_ON(!x && !y);
  *      }
  *
  *      void write(void)
  *      {
  *      WRITE_ONCE(Y, true);
  *
  *      raw_write_seqcount_barrier(seq);
  *
  *      WRITE_ONCE(X, false);
  *      }
  */
 #define raw_write_seqcount_barrier(s)                   \
     do_raw_write_seqcount_barrier(seqprop_ptr(s))
 
 static inline void do_raw_write_seqcount_barrier(seqcount_t *s)
 {
     kcsan_nestable_atomic_begin();
     s->sequence++;
     smp_wmb();
     s->sequence++;
     kcsan_nestable_atomic_end();
 }
 
 /**
  * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
  *                               side operations
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
  *
  * After write_seqcount_invalidate, no seqcount_t read side operations
  * will complete successfully and see data older than this.
  */
 #define write_seqcount_invalidate(s)                    \
     do_write_seqcount_invalidate(seqprop_ptr(s))
 
 static inline void do_write_seqcount_invalidate(seqcount_t *s)
 {
     smp_wmb();
     kcsan_nestable_atomic_begin();
     s->sequence+=2;
     kcsan_nestable_atomic_end();
 }
 
 /*
  * Latch sequence counters (seqcount_latch_t)
  *
  * A sequence counter variant where the counter even/odd value is used to
  * switch between two copies of protected data. This allows the read path,
  * typically NMIs, to safely interrupt the write side critical section.
  *
  * As the write sections are fully preemptible, no special handling for
  * PREEMPT_RT is needed.
  */
 typedef struct {
     seqcount_t seqcount;
 } seqcount_latch_t;
 
 /**
  * SEQCNT_LATCH_ZERO() - static initializer for seqcount_latch_t
  * @seq_name: Name of the seqcount_latch_t instance
  */
 #define SEQCNT_LATCH_ZERO(seq_name) {                   \
     .seqcount       = SEQCNT_ZERO(seq_name.seqcount),   \
 }
 
 /**
  * seqcount_latch_init() - runtime initializer for seqcount_latch_t
  * @s: Pointer to the seqcount_latch_t instance
  */
 #define seqcount_latch_init(s) seqcount_init(&(s)->seqcount)
 
 /**
  * raw_read_seqcount_latch() - pick even/odd latch data copy
  * @s: Pointer to seqcount_latch_t
  *
  * See raw_write_seqcount_latch() for details and a full reader/writer
  * usage example.
  *
  * Return: sequence counter raw value. Use the lowest bit as an index for
  * picking which data copy to read. The full counter must then be checked
  * with read_seqcount_latch_retry().
  */
 static inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
 {
     /*
      * Pairs with the first smp_wmb() in raw_write_seqcount_latch().
      * Due to the dependent load, a full smp_rmb() is not needed.
      */
     return READ_ONCE(s->seqcount.sequence);
 }
 
 /**
  * read_seqcount_latch_retry() - end a seqcount_latch_t read section
  * @s:      Pointer to seqcount_latch_t
  * @start:  count, from raw_read_seqcount_latch()
  *
  * Return: true if a read section retry is required, else false
  */
 static inline int
 read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
 {
     return read_seqcount_retry(&s->seqcount, start);
 }
 
 /**
  * raw_write_seqcount_latch() - redirect latch readers to even/odd copy
  * @s: Pointer to seqcount_latch_t
  *
  * The latch technique is a multiversion concurrency control method that allows
  * queries during non-atomic modifications. If you can guarantee queries never
  * interrupt the modification -- e.g. the concurrency is strictly between CPUs
  * -- you most likely do not need this.
  *
  * Where the traditional RCU/lockless data structures rely on atomic
  * modifications to ensure queries observe either the old or the new state the
  * latch allows the same for non-atomic updates. The trade-off is doubling the
  * cost of storage; we have to maintain two copies of the entire data
  * structure.
  *
  * Very simply put: we first modify one copy and then the other. This ensures
  * there is always one copy in a stable state, ready to give us an answer.
  *
  * The basic form is a data structure like::
  *
  *  struct latch_struct {
  *      seqcount_latch_t    seq;
  *      struct data_struct  data[2];
  *  };
  *
  * Where a modification, which is assumed to be externally serialized, does the
  * following::
  *
  *  void latch_modify(struct latch_struct *latch, ...)
  *  {
  *      smp_wmb();  // Ensure that the last data[1] update is visible
  *      latch->seq.sequence++;
  *      smp_wmb();  // Ensure that the seqcount update is visible
  *
  *      modify(latch->data[0], ...);
  *
  *      smp_wmb();  // Ensure that the data[0] update is visible
  *      latch->seq.sequence++;
  *      smp_wmb();  // Ensure that the seqcount update is visible
  *
  *      modify(latch->data[1], ...);
  *  }
  *
  * The query will have a form like::
  *
  *  struct entry *latch_query(struct latch_struct *latch, ...)
  *  {
  *      struct entry *entry;
  *      unsigned seq, idx;
  *
  *      do {
  *          seq = raw_read_seqcount_latch(&latch->seq);
  *
  *          idx = seq & 0x01;
  *          entry = data_query(latch->data[idx], ...);
  *
  *      // This includes needed smp_rmb()
  *      } while (read_seqcount_latch_retry(&latch->seq, seq));
  *
  *      return entry;
  *  }
  *
  * So during the modification, queries are first redirected to data[1]. Then we
  * modify data[0]. When that is complete, we redirect queries back to data[0]
  * and we can modify data[1].
  *
  * NOTE:
  *
  *  The non-requirement for atomic modifications does _NOT_ include
  *  the publishing of new entries in the case where data is a dynamic
  *  data structure.
  *
  *  An iteration might start in data[0] and get suspended long enough
  *  to miss an entire modification sequence, once it resumes it might
  *  observe the new entry.
  *
  * NOTE2:
  *
  *  When data is a dynamic data structure; one should use regular RCU
  *  patterns to manage the lifetimes of the objects within.
  */
 static inline void raw_write_seqcount_latch(seqcount_latch_t *s)
 {
     smp_wmb();  /* prior stores before incrementing "sequence" */
     s->seqcount.sequence++;
     smp_wmb();      /* increment "sequence" before following stores */
 }
 
 /*
  * Sequential locks (seqlock_t)
  *
  * Sequence counters with an embedded spinlock for writer serialization
  * and non-preemptibility.
  *
  * For more info, see:
  *    - Comments on top of seqcount_t
  *    - Documentation/locking/seqlock.rst
  */
 typedef struct {
     /*
      * Make sure that readers don't starve writers on PREEMPT_RT: use
      * seqcount_spinlock_t instead of seqcount_t. Check __SEQ_LOCK().
      */
     seqcount_spinlock_t seqcount;
     spinlock_t lock;
 } seqlock_t;
 
 #define __SEQLOCK_UNLOCKED(lockname)                    \
     {                               \
         .seqcount = SEQCNT_SPINLOCK_ZERO(lockname, &(lockname).lock), \
         .lock = __SPIN_LOCK_UNLOCKED(lockname)          \
     }
 
 /**
  * seqlock_init() - dynamic initializer for seqlock_t
  * @sl: Pointer to the seqlock_t instance
  */
 #define seqlock_init(sl)                        \
     do {                                \
         spin_lock_init(&(sl)->lock);                \
         seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock);   \
     } while (0)
 
 /**
  * DEFINE_SEQLOCK(sl) - Define a statically allocated seqlock_t
  * @sl: Name of the seqlock_t instance
  */
 #define DEFINE_SEQLOCK(sl) \
         seqlock_t sl = __SEQLOCK_UNLOCKED(sl)
 
 /**
  * read_seqbegin() - start a seqlock_t read side critical section
  * @sl: Pointer to seqlock_t
  *
  * Return: count, to be passed to read_seqretry()
  */
 static inline unsigned read_seqbegin(const seqlock_t *sl)
 {
     unsigned ret = read_seqcount_begin(&sl->seqcount);
 
     kcsan_atomic_next(0);  /* non-raw usage, assume closing read_seqretry() */
     kcsan_flat_atomic_begin();
     return ret;
 }
 
 /**
  * read_seqretry() - end a seqlock_t read side section
  * @sl: Pointer to seqlock_t
  * @start: count, from read_seqbegin()
  *
  * read_seqretry closes the read side critical section of given seqlock_t.
  * If the critical section was invalid, it must be ignored (and typically
  * retried).
  *
  * Return: true if a read section retry is required, else false
  */
 static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
 {
     /*
      * Assume not nested: read_seqretry() may be called multiple times when
      * completing read critical section.
      */
     kcsan_flat_atomic_end();
 
     return read_seqcount_retry(&sl->seqcount, start);
 }
 
 /*
  * For all seqlock_t write side functions, use the the internal
  * do_write_seqcount_begin() instead of generic write_seqcount_begin().
  * This way, no redundant lockdep_assert_held() checks are added.
  */
 
 /**
  * write_seqlock() - start a seqlock_t write side critical section
  * @sl: Pointer to seqlock_t
  *
  * write_seqlock opens a write side critical section for the given
  * seqlock_t.  It also implicitly acquires the spinlock_t embedded inside
  * that sequential lock. All seqlock_t write side sections are thus
  * automatically serialized and non-preemptible.
  *
  * Context: if the seqlock_t read section, or other write side critical
  * sections, can be invoked from hardirq or softirq contexts, use the
  * _irqsave or _bh variants of this function instead.
  */
 static inline void write_seqlock(seqlock_t *sl)
 {
     spin_lock(&sl->lock);
     do_write_seqcount_begin(&sl->seqcount.seqcount);
 }
 
 /**
  * write_sequnlock() - end a seqlock_t write side critical section
  * @sl: Pointer to seqlock_t
  *
  * write_sequnlock closes the (serialized and non-preemptible) write side
  * critical section of given seqlock_t.
  */
 static inline void write_sequnlock(seqlock_t *sl)
 {
     do_write_seqcount_end(&sl->seqcount.seqcount);
     spin_unlock(&sl->lock);
 }
 
 /**
  * write_seqlock_bh() - start a softirqs-disabled seqlock_t write section
  * @sl: Pointer to seqlock_t
  *
  * _bh variant of write_seqlock(). Use only if the read side section, or
  * other write side sections, can be invoked from softirq contexts.
  */
 static inline void write_seqlock_bh(seqlock_t *sl)
 {
     spin_lock_bh(&sl->lock);
     do_write_seqcount_begin(&sl->seqcount.seqcount);
 }
 
 /**
  * write_sequnlock_bh() - end a softirqs-disabled seqlock_t write section
  * @sl: Pointer to seqlock_t
  *
  * write_sequnlock_bh closes the serialized, non-preemptible, and
  * softirqs-disabled, seqlock_t write side critical section opened with
  * write_seqlock_bh().
  */
 static inline void write_sequnlock_bh(seqlock_t *sl)
 {
     do_write_seqcount_end(&sl->seqcount.seqcount);
     spin_unlock_bh(&sl->lock);
 }
 
 /**
  * write_seqlock_irq() - start a non-interruptible seqlock_t write section
  * @sl: Pointer to seqlock_t
  *
  * _irq variant of write_seqlock(). Use only if the read side section, or
  * other write sections, can be invoked from hardirq contexts.
  */
 static inline void write_seqlock_irq(seqlock_t *sl)
 {
     spin_lock_irq(&sl->lock);
     do_write_seqcount_begin(&sl->seqcount.seqcount);
 }
 
 /**
  * write_sequnlock_irq() - end a non-interruptible seqlock_t write section
  * @sl: Pointer to seqlock_t
  *
  * write_sequnlock_irq closes the serialized and non-interruptible
  * seqlock_t write side section opened with write_seqlock_irq().
  */
 static inline void write_sequnlock_irq(seqlock_t *sl)
 {
     do_write_seqcount_end(&sl->seqcount.seqcount);
     spin_unlock_irq(&sl->lock);
 }
 
 static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
 {
     unsigned long flags;
 
     spin_lock_irqsave(&sl->lock, flags);
     do_write_seqcount_begin(&sl->seqcount.seqcount);
     return flags;
 }
 
 /**
  * write_seqlock_irqsave() - start a non-interruptible seqlock_t write
  *                           section
  * @lock:  Pointer to seqlock_t
  * @flags: Stack-allocated storage for saving caller's local interrupt
  *         state, to be passed to write_sequnlock_irqrestore().
  *
  * _irqsave variant of write_seqlock(). Use it only if the read side
  * section, or other write sections, can be invoked from hardirq context.
  */
 #define write_seqlock_irqsave(lock, flags)              \
     do { flags = __write_seqlock_irqsave(lock); } while (0)
 
 /**
  * write_sequnlock_irqrestore() - end non-interruptible seqlock_t write
  *                                section
  * @sl:    Pointer to seqlock_t
  * @flags: Caller's saved interrupt state, from write_seqlock_irqsave()
  *
  * write_sequnlock_irqrestore closes the serialized and non-interruptible
  * seqlock_t write section previously opened with write_seqlock_irqsave().
  */
 static inline void
 write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
 {
     do_write_seqcount_end(&sl->seqcount.seqcount);
     spin_unlock_irqrestore(&sl->lock, flags);
 }
 
 /**
  * read_seqlock_excl() - begin a seqlock_t locking reader section
  * @sl: Pointer to seqlock_t
  *
  * read_seqlock_excl opens a seqlock_t locking reader critical section.  A
  * locking reader exclusively locks out *both* other writers *and* other
  * locking readers, but it does not update the embedded sequence number.
  *
  * Locking readers act like a normal spin_lock()/spin_unlock().
  *
  * Context: if the seqlock_t write section, *or other read sections*, can
  * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
  * variant of this function instead.
  *
  * The opened read section must be closed with read_sequnlock_excl().
  */
 static inline void read_seqlock_excl(seqlock_t *sl)
 {
     spin_lock(&sl->lock);
 }
 
 /**
  * read_sequnlock_excl() - end a seqlock_t locking reader critical section
  * @sl: Pointer to seqlock_t
  */
 static inline void read_sequnlock_excl(seqlock_t *sl)
 {
     spin_unlock(&sl->lock);
 }
 
 /**
  * read_seqlock_excl_bh() - start a seqlock_t locking reader section with
  *              softirqs disabled
  * @sl: Pointer to seqlock_t
  *
  * _bh variant of read_seqlock_excl(). Use this variant only if the
  * seqlock_t write side section, *or other read sections*, can be invoked
  * from softirq contexts.
  */
 static inline void read_seqlock_excl_bh(seqlock_t *sl)
 {
     spin_lock_bh(&sl->lock);
 }
 
 /**
  * read_sequnlock_excl_bh() - stop a seqlock_t softirq-disabled locking
  *                reader section
  * @sl: Pointer to seqlock_t
  */
 static inline void read_sequnlock_excl_bh(seqlock_t *sl)
 {
     spin_unlock_bh(&sl->lock);
 }
 
 /**
  * read_seqlock_excl_irq() - start a non-interruptible seqlock_t locking
  *               reader section
  * @sl: Pointer to seqlock_t
  *
  * _irq variant of read_seqlock_excl(). Use this only if the seqlock_t
  * write side section, *or other read sections*, can be invoked from a
  * hardirq context.
  */
 static inline void read_seqlock_excl_irq(seqlock_t *sl)
 {
     spin_lock_irq(&sl->lock);
 }
 
 /**
  * read_sequnlock_excl_irq() - end an interrupts-disabled seqlock_t
  *                             locking reader section
  * @sl: Pointer to seqlock_t
  */
 static inline void read_sequnlock_excl_irq(seqlock_t *sl)
 {
     spin_unlock_irq(&sl->lock);
 }
 
 static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
 {
     unsigned long flags;
 
     spin_lock_irqsave(&sl->lock, flags);
     return flags;
 }
 
 /**
  * read_seqlock_excl_irqsave() - start a non-interruptible seqlock_t
  *               locking reader section
  * @lock:  Pointer to seqlock_t
  * @flags: Stack-allocated storage for saving caller's local interrupt
  *         state, to be passed to read_sequnlock_excl_irqrestore().
  *
  * _irqsave variant of read_seqlock_excl(). Use this only if the seqlock_t
  * write side section, *or other read sections*, can be invoked from a
  * hardirq context.
  */
 #define read_seqlock_excl_irqsave(lock, flags)              \
     do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
 
 /**
  * read_sequnlock_excl_irqrestore() - end non-interruptible seqlock_t
  *                    locking reader section
  * @sl:    Pointer to seqlock_t
  * @flags: Caller saved interrupt state, from read_seqlock_excl_irqsave()
  */
 static inline void
 read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
 {
     spin_unlock_irqrestore(&sl->lock, flags);
 }
 
 /**
  * read_seqbegin_or_lock() - begin a seqlock_t lockless or locking reader
  * @lock: Pointer to seqlock_t
  * @seq : Marker and return parameter. If the passed value is even, the
  * reader will become a *lockless* seqlock_t reader as in read_seqbegin().
  * If the passed value is odd, the reader will become a *locking* reader
  * as in read_seqlock_excl().  In the first call to this function, the
  * caller *must* initialize and pass an even value to @seq; this way, a
  * lockless read can be optimistically tried first.
  *
  * read_seqbegin_or_lock is an API designed to optimistically try a normal
  * lockless seqlock_t read section first.  If an odd counter is found, the
  * lockless read trial has failed, and the next read iteration transforms
  * itself into a full seqlock_t locking reader.
  *
  * This is typically used to avoid seqlock_t lockless readers starvation
  * (too much retry loops) in the case of a sharp spike in write side
  * activity.
  *
  * Context: if the seqlock_t write section, *or other read sections*, can
  * be invoked from hardirq or softirq contexts, use the _irqsave or _bh
  * variant of this function instead.
  *
  * Check Documentation/locking/seqlock.rst for template example code.
  *
  * Return: the encountered sequence counter value, through the @seq
  * parameter, which is overloaded as a return parameter. This returned
  * value must be checked with need_seqretry(). If the read section need to
  * be retried, this returned value must also be passed as the @seq
  * parameter of the next read_seqbegin_or_lock() iteration.
  */
 static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
 {
     if (!(*seq & 1))    /* Even */
         *seq = read_seqbegin(lock);
     else            /* Odd */
         read_seqlock_excl(lock);
 }
 
 /**
  * need_seqretry() - validate seqlock_t "locking or lockless" read section
  * @lock: Pointer to seqlock_t
  * @seq: sequence count, from read_seqbegin_or_lock()
  *
  * Return: true if a read section retry is required, false otherwise
  */
 static inline int need_seqretry(seqlock_t *lock, int seq)
 {
     return !(seq & 1) && read_seqretry(lock, seq);
 }
 
 /**
  * done_seqretry() - end seqlock_t "locking or lockless" reader section
  * @lock: Pointer to seqlock_t
  * @seq: count, from read_seqbegin_or_lock()
  *
  * done_seqretry finishes the seqlock_t read side critical section started
  * with read_seqbegin_or_lock() and validated by need_seqretry().
  */
 static inline void done_seqretry(seqlock_t *lock, int seq)
 {
     if (seq & 1)
         read_sequnlock_excl(lock);
 }
 
 /**
  * read_seqbegin_or_lock_irqsave() - begin a seqlock_t lockless reader, or
  *                                   a non-interruptible locking reader
  * @lock: Pointer to seqlock_t
  * @seq:  Marker and return parameter. Check read_seqbegin_or_lock().
  *
  * This is the _irqsave variant of read_seqbegin_or_lock(). Use it only if
  * the seqlock_t write section, *or other read sections*, can be invoked
  * from hardirq context.
  *
  * Note: Interrupts will be disabled only for "locking reader" mode.
  *
  * Return:
  *
  *   1. The saved local interrupts state in case of a locking reader, to
  *      be passed to done_seqretry_irqrestore().
  *
  *   2. The encountered sequence counter value, returned through @seq
  *      overloaded as a return parameter. Check read_seqbegin_or_lock().
  */
 static inline unsigned long
 read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
 {
     unsigned long flags = 0;
 
     if (!(*seq & 1))    /* Even */
         *seq = read_seqbegin(lock);
     else            /* Odd */
         read_seqlock_excl_irqsave(lock, flags);
 
     return flags;
 }
 
 /**
  * done_seqretry_irqrestore() - end a seqlock_t lockless reader, or a
  *              non-interruptible locking reader section
  * @lock:  Pointer to seqlock_t
  * @seq:   Count, from read_seqbegin_or_lock_irqsave()
  * @flags: Caller's saved local interrupt state in case of a locking
  *     reader, also from read_seqbegin_or_lock_irqsave()
  *
  * This is the _irqrestore variant of done_seqretry(). The read section
  * must've been opened with read_seqbegin_or_lock_irqsave(), and validated
  * by need_seqretry().
  */
 static inline void
 done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
 {
     if (seq & 1)
         read_sequnlock_excl_irqrestore(lock, flags);
 }
 #endif /* __LINUX_SEQLOCK_H */

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