seqlock.h

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#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.  Readers never
 * block but they may have to retry if a writer is in
 * progress. Writers do not wait for readers. 
 *
 * This is not as cache friendly as brlock. Also, this will not work
 * for data that contains pointers, because any writer could
 * invalidate a pointer that a reader was following.
 *
 * Expected 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>

typedef struct {
    unsigned sequence;
    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) \
         { 0, __SPIN_LOCK_UNLOCKED(lockname) }

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

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

/* 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);
    ++sl->sequence;
    smp_wmb();
}

static inline void write_sequnlock(seqlock_t *sl)
{
    smp_wmb();
    sl->sequence++;
    spin_unlock(&sl->lock);
}

static inline int write_tryseqlock(seqlock_t *sl)
{
    int ret = spin_trylock(&sl->lock);

    if (ret) {
        ++sl->sequence;
        smp_wmb();
    }
    return ret;
}

/* Start of read calculation -- fetch last complete writer token */
static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
{
    unsigned ret;

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

    return ret;
}

/*
 * Test if reader processed invalid data.
 *
 * If sequence value changed then writer changed data while in section.
 */
static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start)
{
    smp_rmb();

    return unlikely(sl->sequence != start);
}


/*
 * 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)

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;
}

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

static inline unsigned raw_seqcount_begin(const seqcount_t *s)
{
    unsigned ret = ACCESS_ONCE(s->sequence);
    smp_rmb();
    return ret & ~1;
}

static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
{
    return unlikely(s->sequence != start);
}

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++;
}

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

/*
 * Possible sw/hw IRQ protected versions of the interfaces.
 */
#define write_seqlock_irqsave(lock, flags)              \
    do { local_irq_save(flags); write_seqlock(lock); } while (0)
#define write_seqlock_irq(lock)                     \
    do { local_irq_disable();   write_seqlock(lock); } while (0)
#define write_seqlock_bh(lock)                      \
        do { local_bh_disable();    write_seqlock(lock); } while (0)

#define write_sequnlock_irqrestore(lock, flags)             \
    do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
#define write_sequnlock_irq(lock)                   \
    do { write_sequnlock(lock); local_irq_enable(); } while(0)
#define write_sequnlock_bh(lock)                    \
    do { write_sequnlock(lock); local_bh_enable(); } while(0)

#define read_seqbegin_irqsave(lock, flags)              \
    ({ local_irq_save(flags);   read_seqbegin(lock); })

#define read_seqretry_irqrestore(lock, iv, flags)           \
    ({                              \
        int ret = read_seqretry(lock, iv);          \
        local_irq_restore(flags);               \
        ret;                            \
    })

#endif /* __LINUX_SEQLOCK_H */