Merge tag 'gpio-v3.12-2' of git://git.kernel.org/pub/scm/linux/kernel/git/linusw...

[mirror_ubuntu-artful-kernel.git] / include / linux / seqlock.h

#ifndef __LINUX_SEQLOCK_H
#define __LINUX_SEQLOCK_H
/*
 * Reader/writer consistent mechanism without starving writers. This type of
 * lock for data where the reader wants a consistent set of information
 * and is willing to retry if the information changes. There are two types
 * of readers:
 * 1. Sequence readers which never block a writer but they may have to retry
 *    if a writer is in progress by detecting change in sequence number.
 *    Writers do not wait for a sequence reader.
 * 2. Locking readers which will wait if a writer or another locking reader
 *    is in progress. A locking reader in progress will also block a writer
 *    from going forward. Unlike the regular rwlock, the read lock here is
 *    exclusive so that only one locking reader can get it.
 *
 * This is not as cache friendly as brlock. Also, this may not work well
 * for data that contains pointers, because any writer could
 * invalidate a pointer that a reader was following.
 *
 * Expected non-blocking reader usage:
 *      do {
 *          seq = read_seqbegin(&foo);
 *      ...
 *      } while (read_seqretry(&foo, seq));
 *
 *
 * On non-SMP the spin locks disappear but the writer still needs
 * to increment the sequence variables because an interrupt routine could
 * change the state of the data.
 *
 * Based on x86_64 vsyscall gettimeofday 
 * by Keith Owens and Andrea Arcangeli
 */

#include <linux/spinlock.h>
#include <linux/preempt.h>
#include <asm/processor.h>

/*
 * Version using sequence counter only.
 * This can be used when code has its own mutex protecting the
 * updating starting before the write_seqcountbeqin() and ending
 * after the write_seqcount_end().
 */
typedef struct seqcount {
        unsigned sequence;
} seqcount_t;

#define SEQCNT_ZERO { 0 }
#define seqcount_init(x)        do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)

/**
 * __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 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.
 */
static inline unsigned __read_seqcount_begin(const seqcount_t *s)
{
        unsigned ret;

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

/**
 * 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 opens a read critical section of the given seqcount.
 * Validity of the critical section is tested by checking read_seqcount_retry
 * function.
 */
static inline unsigned read_seqcount_begin(const seqcount_t *s)
{
        unsigned ret = __read_seqcount_begin(s);
        smp_rmb();
        return ret;
}

/**
 * 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 opens a read critical section of the given seqcount.
 * Validity of the critical section is tested by checking read_seqcount_retry
 * function.
 *
 * 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.
 */
static inline unsigned raw_seqcount_begin(const seqcount_t *s)
{
        unsigned ret = ACCESS_ONCE(s->sequence);
        smp_rmb();
        return ret & ~1;
}

/**
 * __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 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.
 */
static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
{
        return unlikely(s->sequence != 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 closes a read critical section of the given seqcount.
 * If the critical section was invalid, it must be ignored (and typically
 * retried).
 */
static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
{
        smp_rmb();
        return __read_seqcount_retry(s, start);
}


/*
 * Sequence counter only version assumes that callers are using their
 * own mutexing.
 */
static inline void write_seqcount_begin(seqcount_t *s)
{
        s->sequence++;
        smp_wmb();
}

static inline void write_seqcount_end(seqcount_t *s)
{
        smp_wmb();
        s->sequence++;
}

/**
 * write_seqcount_barrier - invalidate in-progress read-side seq operations
 * @s: pointer to seqcount_t
 *
 * After write_seqcount_barrier, no read-side seq operations will complete
 * successfully and see data older than this.
 */
static inline void write_seqcount_barrier(seqcount_t *s)
{
        smp_wmb();
        s->sequence+=2;
}

typedef struct {
        struct seqcount seqcount;
        spinlock_t lock;
} seqlock_t;

/*
 * These macros triggered gcc-3.x compile-time problems.  We think these are
 * OK now.  Be cautious.
 */
#define __SEQLOCK_UNLOCKED(lockname)                    \
        {                                               \
                .seqcount = SEQCNT_ZERO,                \
                .lock = __SPIN_LOCK_UNLOCKED(lockname)  \
        }

#define seqlock_init(x)                                 \
        do {                                            \
                seqcount_init(&(x)->seqcount);          \
                spin_lock_init(&(x)->lock);             \
        } while (0)

#define DEFINE_SEQLOCK(x) \
                seqlock_t x = __SEQLOCK_UNLOCKED(x)

/*
 * Read side functions for starting and finalizing a read side section.
 */
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
        return read_seqcount_begin(&sl->seqcount);
}

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.
 */
static inline void write_seqlock(seqlock_t *sl)
{
        spin_lock(&sl->lock);
        write_seqcount_begin(&sl->seqcount);
}

static inline void write_sequnlock(seqlock_t *sl)
{
        write_seqcount_end(&sl->seqcount);
        spin_unlock(&sl->lock);
}

static inline void write_seqlock_bh(seqlock_t *sl)
{
        spin_lock_bh(&sl->lock);
        write_seqcount_begin(&sl->seqcount);
}

static inline void write_sequnlock_bh(seqlock_t *sl)
{
        write_seqcount_end(&sl->seqcount);
        spin_unlock_bh(&sl->lock);
}

static inline void write_seqlock_irq(seqlock_t *sl)
{
        spin_lock_irq(&sl->lock);
        write_seqcount_begin(&sl->seqcount);
}

static inline void write_sequnlock_irq(seqlock_t *sl)
{
        write_seqcount_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_begin(&sl->seqcount);
        return flags;
}

#define write_seqlock_irqsave(lock, flags)                              \
        do { flags = __write_seqlock_irqsave(lock); } while (0)

static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
        write_seqcount_end(&sl->seqcount);
        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.
 */
static inline void read_seqlock_excl(seqlock_t *sl)
{
        spin_lock(&sl->lock);
}

static inline void read_sequnlock_excl(seqlock_t *sl)
{
        spin_unlock(&sl->lock);
}

static inline void read_seqlock_excl_bh(seqlock_t *sl)
{
        spin_lock_bh(&sl->lock);
}

static inline void read_sequnlock_excl_bh(seqlock_t *sl)
{
        spin_unlock_bh(&sl->lock);
}

static inline void read_seqlock_excl_irq(seqlock_t *sl)
{
        spin_lock_irq(&sl->lock);
}

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;
}

#define read_seqlock_excl_irqsave(lock, flags)                          \
        do { flags = __read_seqlock_excl_irqsave(lock); } while (0)

static inline void
read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
{
        spin_unlock_irqrestore(&sl->lock, flags);
}

#endif /* __LINUX_SEQLOCK_H */