source: trunk/packages/xen-common/xen-common/xen/include/asm-ia64/linux/seqlock.h @ 34

#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 consitent 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/config.h>
#include <linux/spinlock.h>
#include <linux/preempt.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 { 0, SPIN_LOCK_UNLOCKED }
#define seqlock_init(x) do { *(x) = (seqlock_t) SEQLOCK_UNLOCKED; } while (0)


/* 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 inline unsigned read_seqbegin(const seqlock_t *sl)
{
        unsigned ret = sl->sequence;
        smp_rmb();
        return ret;
}

/* Test if reader processed invalid data.
 * If initial values is odd, 
 *      then writer had already started when section was entered
 * If sequence value changed
 *      then writer changed data while in section
 *    
 * Using xor saves one conditional branch.
 */
static inline int read_seqretry(const seqlock_t *sl, unsigned iv)
{
        smp_rmb();
        return (iv & 1) | (sl->sequence ^ iv);
}


/*
 * 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)

/* Start of read using pointer to a sequence counter only.  */
static inline unsigned read_seqcount_begin(const seqcount_t *s)
{
        unsigned ret = s->sequence;
        smp_rmb();
        return ret;
}

/* Test if reader processed invalid data.
 * Equivalent to: iv is odd or sequence number has changed.
 *                (iv & 1) || (*s != iv)
 * Using xor saves one conditional branch.
 */
static inline int read_seqcount_retry(const seqcount_t *s, unsigned iv)
{
        smp_rmb();
        return (iv & 1) | (s->sequence ^ iv);
}


/*
 * 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++;
}

/*
 * 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 */