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    Subject[PATCH v4 05/24] seqlock: Add kernel-doc for seqcount_t and seqlock_t APIs
    Date
    seqlock.h is now included by kernel's RST documentation, but a small
    number of the the exported seqlock.h functions are kernel-doc annotated.

    Add kernel-doc for all seqlock.h exported APIs.

    Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
    ---
    include/linux/seqlock.h | 425 ++++++++++++++++++++++++++++++++--------
    1 file changed, 348 insertions(+), 77 deletions(-)

    diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
    index 4c1456008d89..85fb3ac93ffb 100644
    --- a/include/linux/seqlock.h
    +++ b/include/linux/seqlock.h
    @@ -75,6 +75,10 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
    # 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; \
    @@ -98,13 +102,15 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
    # define seqcount_lockdep_reader_access(x)
    #endif

    -#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
    -
    +/**
    + * 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) }

    /**
    - * __read_seqcount_begin - begin a seq-read critical section (without barrier)
    - * @s: pointer to seqcount_t
    - * Returns: count to be passed to read_seqcount_retry
    + * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier
    + * @s: Pointer to seqcount_t
    *
    * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
    * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
    @@ -113,6 +119,8 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
    *
    * Use carefully, only in critical code, and comment how the barrier is
    * provided.
    + *
    + * Return: count to be passed to read_seqcount_retry()
    */
    static inline unsigned __read_seqcount_begin(const seqcount_t *s)
    {
    @@ -129,13 +137,10 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
    }

    /**
    - * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
    - * @s: pointer to seqcount_t
    - * Returns: count to be passed to read_seqcount_retry
    + * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
    + * @s: Pointer to seqcount_t
    *
    - * raw_read_seqcount_begin opens a read critical section of the given
    - * seqcount, but without any lockdep checking. Validity of the critical
    - * section is tested by checking read_seqcount_retry function.
    + * Return: count to be passed to read_seqcount_retry()
    */
    static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
    {
    @@ -145,13 +150,10 @@ static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
    }

    /**
    - * read_seqcount_begin - begin a seq-read critical section
    - * @s: pointer to seqcount_t
    - * Returns: count to be passed to read_seqcount_retry
    + * read_seqcount_begin() - begin a seqcount_t read critical section
    + * @s: Pointer to seqcount_t
    *
    - * read_seqcount_begin opens a read critical section of the given seqcount.
    - * Validity of the critical section is tested by checking read_seqcount_retry
    - * function.
    + * Return: count to be passed to read_seqcount_retry()
    */
    static inline unsigned read_seqcount_begin(const seqcount_t *s)
    {
    @@ -160,13 +162,15 @@ static inline unsigned read_seqcount_begin(const seqcount_t *s)
    }

    /**
    - * raw_read_seqcount - Read the raw seqcount
    - * @s: pointer to seqcount_t
    - * Returns: count to be passed to read_seqcount_retry
    + * raw_read_seqcount() - read the raw seqcount_t counter value
    + * @s: Pointer to seqcount_t
    *
    * raw_read_seqcount opens a read critical section of the given
    - * seqcount without any lockdep checking and without checking or
    - * masking the LSB. Calling code is responsible for handling that.
    + * 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()
    */
    static inline unsigned raw_read_seqcount(const seqcount_t *s)
    {
    @@ -177,18 +181,21 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
    }

    /**
    - * raw_seqcount_begin - begin a seq-read critical section
    - * @s: pointer to seqcount_t
    - * Returns: count to be passed to read_seqcount_retry
    + * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
    + * lockdep and w/o counter stabilization
    + * @s: Pointer to seqcount_t
    *
    - * raw_seqcount_begin opens a read critical section of the given seqcount.
    - * Validity of the critical section is tested by checking read_seqcount_retry
    - * function.
    + * 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.
    *
    - * Unlike read_seqcount_begin(), this function will not wait for the count
    - * to stabilize. If a writer is active when we begin, we will fail the
    - * read_seqcount_retry() instead of stabilizing at the beginning of the
    - * critical section.
    + * 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()
    */
    static inline unsigned raw_seqcount_begin(const seqcount_t *s)
    {
    @@ -199,10 +206,9 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
    }

    /**
    - * __read_seqcount_retry - end a seq-read critical section (without barrier)
    - * @s: pointer to seqcount_t
    - * @start: count, from read_seqcount_begin
    - * Returns: 1 if retry is required, else 0
    + * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
    + * @s: Pointer to seqcount_t
    + * @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
    @@ -211,6 +217,8 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
    *
    * Use carefully, only in critical code, and comment how the barrier is
    * provided.
    + *
    + * Return: true if a read section retry is required, else false
    */
    static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
    {
    @@ -219,14 +227,15 @@ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
    }

    /**
    - * read_seqcount_retry - end a seq-read critical section
    - * @s: pointer to seqcount_t
    - * @start: count, from read_seqcount_begin
    - * Returns: 1 if retry is required, else 0
    + * read_seqcount_retry() - end a seqcount_t read critical section
    + * @s: Pointer to seqcount_t
    + * @start: count, from read_seqcount_begin()
    *
    - * read_seqcount_retry closes a read critical section of the given seqcount.
    - * If the critical section was invalid, it must be ignored (and typically
    - * retried).
    + * 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
    */
    static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
    {
    @@ -234,6 +243,10 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
    return __read_seqcount_retry(s, start);
    }

    +/**
    + * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
    + * @s: Pointer to seqcount_t
    + */
    static inline void raw_write_seqcount_begin(seqcount_t *s)
    {
    kcsan_nestable_atomic_begin();
    @@ -241,6 +254,10 @@ static inline void raw_write_seqcount_begin(seqcount_t *s)
    smp_wmb();
    }

    +/**
    + * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
    + * @s: Pointer to seqcount_t
    + */
    static inline void raw_write_seqcount_end(seqcount_t *s)
    {
    smp_wmb();
    @@ -248,17 +265,42 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
    kcsan_nestable_atomic_end();
    }

    +/**
    + * write_seqcount_begin_nested() - start a seqcount_t write section with
    + * custom lockdep nesting level
    + * @s: Pointer to seqcount_t
    + * @subclass: lockdep nesting level
    + *
    + * See Documentation/locking/lockdep-design.rst
    + */
    static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
    {
    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
    + *
    + * 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.
    + */
    static inline void write_seqcount_begin(seqcount_t *s)
    {
    write_seqcount_begin_nested(s, 0);
    }

    +/**
    + * write_seqcount_end() - end a seqcount_t write side critical section
    + * @s: Pointer to seqcount_t
    + *
    + * The write section must've been opened with write_seqcount_begin().
    + */
    static inline void write_seqcount_end(seqcount_t *s)
    {
    seqcount_release(&s->dep_map, _RET_IP_);
    @@ -266,12 +308,12 @@ static inline void write_seqcount_end(seqcount_t *s)
    }

    /**
    - * raw_write_seqcount_barrier - do a seq write barrier
    - * @s: pointer to seqcount_t
    + * raw_write_seqcount_barrier() - do a seqcount_t write barrier
    + * @s: Pointer to seqcount_t
    *
    - * This can be used to provide an ordering guarantee instead of the
    - * usual consistency guarantee. It is one wmb cheaper, because we can
    - * collapse the two back-to-back wmb()s.
    + * 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
    @@ -316,11 +358,12 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
    }

    /**
    - * write_seqcount_invalidate - invalidate in-progress read-side seq operations
    - * @s: pointer to seqcount_t
    + * write_seqcount_invalidate() - invalidate in-progress seqcount_t read
    + * side operations
    + * @s: Pointer to seqcount_t
    *
    - * After write_seqcount_invalidate, no read-side seq operations will complete
    - * successfully and see data older than this.
    + * After write_seqcount_invalidate, no seqcount_t read side operations
    + * will complete successfully and see data older than this.
    */
    static inline void write_seqcount_invalidate(seqcount_t *s)
    {
    @@ -330,6 +373,21 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
    kcsan_nestable_atomic_end();
    }

    +/**
    + * raw_read_seqcount_latch() - pick even/odd seqcount_t latch data copy
    + * @s: Pointer to seqcount_t
    + *
    + * 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().
    + */
    static inline int raw_read_seqcount_latch(seqcount_t *s)
    {
    /* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
    @@ -338,8 +396,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
    }

    /**
    - * raw_write_seqcount_latch - redirect readers to even/odd copy
    - * @s: pointer to seqcount_t
    + * raw_write_seqcount_latch() - redirect readers to even/odd copy
    + * @s: Pointer to seqcount_t
    *
    * The latch technique is a multiversion concurrency control method that allows
    * queries during non-atomic modifications. If you can guarantee queries never
    @@ -446,17 +504,28 @@ typedef struct {
    .lock = __SPIN_LOCK_UNLOCKED(lockname) \
    }

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

    -#define DEFINE_SEQLOCK(x) \
    - seqlock_t x = __SEQLOCK_UNLOCKED(x)
    +/**
    + * 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 side functions for starting and finalizing a read side section.
    +/**
    + * 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)
    {
    @@ -467,6 +536,17 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
    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)
    {
    /*
    @@ -478,10 +558,18 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
    return read_seqcount_retry(&sl->seqcount, start);
    }

    -/*
    - * Lock out other writers and update the count.
    - * Acts like a normal spin_lock/unlock.
    - * Don't need preempt_disable() because that is in the spin_lock already.
    +/**
    + * 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)
    {
    @@ -489,30 +577,66 @@ static inline void write_seqlock(seqlock_t *sl)
    write_seqcount_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_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_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_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_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_end(&sl->seqcount);
    @@ -528,9 +652,28 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
    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)
    {
    @@ -538,36 +681,79 @@ write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
    spin_unlock_irqrestore(&sl->lock, flags);
    }

    -/*
    - * A locking reader exclusively locks out other writers and locking readers,
    - * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
    - * Don't need preempt_disable() because that is in the spin_lock already.
    +/**
    + * 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);
    @@ -581,9 +767,26 @@ static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
    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)
    {
    @@ -591,14 +794,35 @@ read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
    }

    /**
    - * read_seqbegin_or_lock - begin a sequence number check or locking block
    - * @lock: sequence lock
    - * @seq : sequence number to be checked
    - *
    - * First try it once optimistically without taking the lock. If that fails,
    - * take the lock. The sequence number is also used as a marker for deciding
    - * whether to be a reader (even) or writer (odd).
    - * N.B. seq must be initialized to an even number to begin with.
    + * 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)
    {
    @@ -608,17 +832,52 @@ static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
    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)
    {
    @@ -632,6 +891,18 @@ read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
    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)
    {
    --
    2.20.1
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