seqlock: Fold seqcount_LOCKNAME_init() definition

[linux-2.6-microblaze.git] / include / linux / seqlock.h

/* 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 <linux/ww_mutex.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_LOCKTYPE_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_LOCKTYPE_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
 */

#ifdef CONFIG_LOCKDEP
#define __SEQ_LOCK(expr)        expr
#else
#define __SEQ_LOCK(expr)
#endif

#define SEQCOUNT_LOCKTYPE_ZERO(seq_name, assoc_lock) {                  \
        .seqcount               = SEQCNT_ZERO(seq_name.seqcount),       \
        __SEQ_LOCK(.lock        = (assoc_lock))                         \
}

/**
 * SEQCNT_SPINLOCK_ZERO - static initializer for seqcount_spinlock_t
 * @name:       Name of the seqcount_spinlock_t instance
 * @lock:       Pointer to the associated spinlock
 */
#define SEQCNT_SPINLOCK_ZERO(name, lock)                                \
        SEQCOUNT_LOCKTYPE_ZERO(name, lock)

/**
 * SEQCNT_RAW_SPINLOCK_ZERO - static initializer for seqcount_raw_spinlock_t
 * @name:       Name of the seqcount_raw_spinlock_t instance
 * @lock:       Pointer to the associated raw_spinlock
 */
#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock)                            \
        SEQCOUNT_LOCKTYPE_ZERO(name, lock)

/**
 * SEQCNT_RWLOCK_ZERO - static initializer for seqcount_rwlock_t
 * @name:       Name of the seqcount_rwlock_t instance
 * @lock:       Pointer to the associated rwlock
 */
#define SEQCNT_RWLOCK_ZERO(name, lock)                                  \
        SEQCOUNT_LOCKTYPE_ZERO(name, lock)

/**
 * SEQCNT_MUTEX_ZERO - static initializer for seqcount_mutex_t
 * @name:       Name of the seqcount_mutex_t instance
 * @lock:       Pointer to the associated mutex
 */
#define SEQCNT_MUTEX_ZERO(name, lock)                                   \
        SEQCOUNT_LOCKTYPE_ZERO(name, lock)

/**
 * SEQCNT_WW_MUTEX_ZERO - static initializer for seqcount_ww_mutex_t
 * @name:       Name of the seqcount_ww_mutex_t instance
 * @lock:       Pointer to the associated ww_mutex
 */
#define SEQCNT_WW_MUTEX_ZERO(name, lock)                                \
        SEQCOUNT_LOCKTYPE_ZERO(name, lock)

/**
 * typedef seqcount_LOCKNAME_t - sequence counter with LOCKTYPR associated
 * @seqcount:   The real sequence counter
 * @lock:       Pointer to the associated spinlock
 *
 * A plain sequence counter with external writer synchronization by a
 * spinlock. The spinlock is associated to the sequence count in the
 * static initializer or init function. This enables lockdep to validate
 * that the write side critical section is properly serialized.
 */

/**
 * seqcount_LOCKNAME_init() - runtime initializer for seqcount_LOCKNAME_t
 * @s:          Pointer to the seqcount_LOCKNAME_t instance
 * @lock:       Pointer to the associated LOCKTYPE
 */

/*
 * SEQCOUNT_LOCKTYPE() - Instantiate seqcount_LOCKNAME_t and helpers
 * @locktype:           actual typename
 * @lockname:           name
 * @preemptible:        preemptibility of above locktype
 * @lockmember:         argument for lockdep_assert_held()
 */
#define SEQCOUNT_LOCKTYPE(locktype, lockname, preemptible, lockmember)  \
typedef struct seqcount_##lockname {                                    \
        seqcount_t              seqcount;                               \
        __SEQ_LOCK(locktype     *lock);                                 \
} seqcount_##lockname##_t;                                              \
                                                                        \
static __always_inline void                                             \
seqcount_##lockname##_init(seqcount_##lockname##_t *s, locktype *lock)  \
{                                                                       \
        seqcount_init(&s->seqcount);                                    \
        __SEQ_LOCK(s->lock = lock);                                     \
}                                                                       \
                                                                        \
static __always_inline seqcount_t *                                     \
__seqcount_##lockname##_ptr(seqcount_##lockname##_t *s)                 \
{                                                                       \
        return &s->seqcount;                                            \
}                                                                       \
                                                                        \
static __always_inline bool                                             \
__seqcount_##lockname##_preemptible(seqcount_##lockname##_t *s)         \
{                                                                       \
        return preemptible;                                             \
}                                                                       \
                                                                        \
static __always_inline void                                             \
__seqcount_##lockname##_assert(seqcount_##lockname##_t *s)              \
{                                                                       \
        __SEQ_LOCK(lockdep_assert_held(lockmember));                    \
}

/*
 * __seqprop() for seqcount_t
 */

static inline seqcount_t *__seqcount_ptr(seqcount_t *s)
{
        return s;
}

static inline bool __seqcount_preemptible(seqcount_t *s)
{
        return false;
}

static inline void __seqcount_assert(seqcount_t *s)
{
        lockdep_assert_preemption_disabled();
}

SEQCOUNT_LOCKTYPE(raw_spinlock_t,       raw_spinlock,   false,  s->lock)
SEQCOUNT_LOCKTYPE(spinlock_t,           spinlock,       false,  s->lock)
SEQCOUNT_LOCKTYPE(rwlock_t,             rwlock,         false,  s->lock)
SEQCOUNT_LOCKTYPE(struct mutex,         mutex,          true,   s->lock)
SEQCOUNT_LOCKTYPE(struct ww_mutex,      ww_mutex,       true,   &s->lock->base)

#define __seqprop_case(s, lockname, prop)                               \
        seqcount_##lockname##_t: __seqcount_##lockname##_##prop((void *)(s))

#define __seqprop(s, prop) _Generic(*(s),                               \
        seqcount_t:             __seqcount_##prop((void *)(s)),         \
        __seqprop_case((s),     raw_spinlock,   prop),                  \
        __seqprop_case((s),     spinlock,       prop),                  \
        __seqprop_case((s),     rwlock,         prop),                  \
        __seqprop_case((s),     mutex,          prop),                  \
        __seqprop_case((s),     ww_mutex,       prop))

#define __to_seqcount_t(s)                              __seqprop(s, ptr)
#define __associated_lock_exists_and_is_preemptible(s)  __seqprop(s, preemptible)
#define __assert_write_section_is_protected(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_locktype_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)                                        \
        __read_seqcount_t_begin(__to_seqcount_t(s))

static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
{
        unsigned ret;

repeat:
        ret = READ_ONCE(s->sequence);
        if (unlikely(ret & 1)) {
                cpu_relax();
                goto repeat;
        }
        kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
        return ret;
}

/**
 * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
 * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
 *
 * Return: count to be passed to read_seqcount_retry()
 */
#define raw_read_seqcount_begin(s)                                      \
        raw_read_seqcount_t_begin(__to_seqcount_t(s))

static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
{
        unsigned ret = __read_seqcount_t_begin(s);
        smp_rmb();
        return ret;
}

/**
 * read_seqcount_begin() - begin a seqcount_t read critical section
 * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
 *
 * Return: count to be passed to read_seqcount_retry()
 */
#define read_seqcount_begin(s)                                          \
        read_seqcount_t_begin(__to_seqcount_t(s))

static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
{
        seqcount_lockdep_reader_access(s);
        return raw_read_seqcount_t_begin(s);
}

/**
 * raw_read_seqcount() - read the raw seqcount_t counter value
 * @s: Pointer to seqcount_t or any of the seqcount_locktype_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)                                            \
        raw_read_seqcount_t(__to_seqcount_t(s))

static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
{
        unsigned ret = READ_ONCE(s->sequence);
        smp_rmb();
        kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
        return ret;
}

/**
 * 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_locktype_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)                                           \
        raw_seqcount_t_begin(__to_seqcount_t(s))

static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
{
        /*
         * If the counter is odd, let read_seqcount_retry() fail
         * by decrementing the counter.
         */
        return raw_read_seqcount_t(s) & ~1;
}

/**
 * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
 * @s: Pointer to seqcount_t or any of the seqcount_locktype_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)                                 \
        __read_seqcount_t_retry(__to_seqcount_t(s), start)

static inline int __read_seqcount_t_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_locktype_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)                                   \
        read_seqcount_t_retry(__to_seqcount_t(s), start)

static inline int read_seqcount_t_retry(const seqcount_t *s, unsigned start)
{
        smp_rmb();
        return __read_seqcount_t_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_locktype_t variants
 */
#define raw_write_seqcount_begin(s)                                     \
do {                                                                    \
        if (__associated_lock_exists_and_is_preemptible(s))             \
                preempt_disable();                                      \
                                                                        \
        raw_write_seqcount_t_begin(__to_seqcount_t(s));                 \
} while (0)

static inline void raw_write_seqcount_t_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_locktype_t variants
 */
#define raw_write_seqcount_end(s)                                       \
do {                                                                    \
        raw_write_seqcount_t_end(__to_seqcount_t(s));                   \
                                                                        \
        if (__associated_lock_exists_and_is_preemptible(s))             \
                preempt_enable();                                       \
} while (0)

static inline void raw_write_seqcount_t_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_locktype_t variants
 * @subclass: lockdep nesting level
 *
 * See Documentation/locking/lockdep-design.rst
 */
#define write_seqcount_begin_nested(s, subclass)                        \
do {                                                                    \
        __assert_write_section_is_protected(s);                         \
                                                                        \
        if (__associated_lock_exists_and_is_preemptible(s))             \
                preempt_disable();                                      \
                                                                        \
        write_seqcount_t_begin_nested(__to_seqcount_t(s), subclass);    \
} while (0)

static inline void write_seqcount_t_begin_nested(seqcount_t *s, int subclass)
{
        raw_write_seqcount_t_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_locktype_t variants
 *
 * write_seqcount_begin opens a write side critical section of the given
 * seqcount_t.
 *
 * Context: seqcount_t write side critical sections must be serialized and
 * non-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 {                                                                    \
        __assert_write_section_is_protected(s);                         \
                                                                        \
        if (__associated_lock_exists_and_is_preemptible(s))             \
                preempt_disable();                                      \
                                                                        \
        write_seqcount_t_begin(__to_seqcount_t(s));                     \
} while (0)

static inline void write_seqcount_t_begin(seqcount_t *s)
{
        write_seqcount_t_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_locktype_t variants
 *
 * The write section must've been opened with write_seqcount_begin().
 */
#define write_seqcount_end(s)                                           \
do {                                                                    \
        write_seqcount_t_end(__to_seqcount_t(s));                       \
                                                                        \
        if (__associated_lock_exists_and_is_preemptible(s))             \
                preempt_enable();                                       \
} while (0)

static inline void write_seqcount_t_end(seqcount_t *s)
{
        seqcount_release(&s->dep_map, _RET_IP_);
        raw_write_seqcount_t_end(s);
}

/**
 * raw_write_seqcount_barrier() - do a seqcount_t write barrier
 * @s: Pointer to seqcount_t or any of the seqcount_locktype_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)                                   \
        raw_write_seqcount_t_barrier(__to_seqcount_t(s))

static inline void raw_write_seqcount_t_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_locktype_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)                                    \
        write_seqcount_t_invalidate(__to_seqcount_t(s))

static inline void write_seqcount_t_invalidate(seqcount_t *s)
{
        smp_wmb();
        kcsan_nestable_atomic_begin();
        s->sequence+=2;
        kcsan_nestable_atomic_end();
}

/**
 * raw_read_seqcount_latch() - pick even/odd seqcount_t latch data copy
 * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
 *
 * Use seqcount_t latching to switch between two storage places protected
 * by a sequence counter. Doing so allows having interruptible, preemptible,
 * seqcount_t write side critical sections.
 *
 * Check raw_write_seqcount_latch() for more details and a full reader and
 * 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 value must then be
 * checked with read_seqcount_retry().
 */
#define raw_read_seqcount_latch(s)                                      \
        raw_read_seqcount_t_latch(__to_seqcount_t(s))

static inline int raw_read_seqcount_t_latch(seqcount_t *s)
{
        /* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
        int seq = READ_ONCE(s->sequence); /* ^^^ */
        return seq;
}

/**
 * raw_write_seqcount_latch() - redirect readers to even/odd copy
 * @s: Pointer to seqcount_t or any of the seqcount_locktype_t variants
 *
 * 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_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++;
 *              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++;
 *              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], ...);
 *
 *              // read_seqcount_retry() includes needed smp_rmb()
 *              } while (read_seqcount_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.
 *
 * NOTE:
 *
 *      When data is a dynamic data structure; one should use regular RCU
 *      patterns to manage the lifetimes of the objects within.
 */
#define raw_write_seqcount_latch(s)                                     \
        raw_write_seqcount_t_latch(__to_seqcount_t(s))

static inline void raw_write_seqcount_t_latch(seqcount_t *s)
{
       smp_wmb();      /* prior stores before incrementing "sequence" */
       s->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 {
        struct seqcount seqcount;
        spinlock_t lock;
} seqlock_t;

#define __SEQLOCK_UNLOCKED(lockname)                                    \
        {                                                               \
                .seqcount = SEQCNT_ZERO(lockname),                      \
                .lock = __SPIN_LOCK_UNLOCKED(lockname)                  \
        }

/**
 * seqlock_init() - dynamic initializer for seqlock_t
 * @sl: Pointer to the seqlock_t instance
 */
#define seqlock_init(sl)                                                \
        do {                                                            \
                seqcount_init(&(sl)->seqcount);                         \
                spin_lock_init(&(sl)->lock);                            \
        } while (0)

/**
 * DEFINE_SEQLOCK() - 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);
}

/**
 * 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);
        write_seqcount_t_begin(&sl->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)
{
        write_seqcount_t_end(&sl->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);
        write_seqcount_t_begin(&sl->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)
{
        write_seqcount_t_end(&sl->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);
        write_seqcount_t_begin(&sl->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)
{
        write_seqcount_t_end(&sl->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);
        write_seqcount_t_begin(&sl->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)
{
        write_seqcount_t_end(&sl->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 */