提交 ea753d81 编写于 作者: P Paolo Bonzini

seqlock: introduce read-write seqlock

Seqlock implementation for QEMU. Usage idiom

reader:
    do {
        start = seqlock_read_begin(&sl);
        ...
    } while (seqlock_read_retry(&sl, start));

writer:
    seqlock_write_lock(&sl);
    ...
    seqlock_write_unlock(&sl);

initialization:
    seqlock_init(QemuSeqLock *sl, QemuMutex *mutex)

    mutex could be NULL if the caller will provide its own protection
    for concurrent write sides (typically using the BQL).
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
上级 c46860ea
/*
* Seqlock implementation for QEMU
*
* Copyright Red Hat, Inc. 2013
*
* Author:
* Paolo Bonzini <pbonzini@redhat.com>
*
* This work is licensed under the terms of the GNU GPL, version 2 or later.
* See the COPYING file in the top-level directory.
*
*/
#ifndef QEMU_SEQLOCK_H
#define QEMU_SEQLOCK_H 1
#include <qemu/atomic.h>
#include <qemu/thread.h>
typedef struct QemuSeqLock QemuSeqLock;
struct QemuSeqLock {
QemuMutex *mutex;
unsigned sequence;
};
static inline void seqlock_init(QemuSeqLock *sl, QemuMutex *mutex)
{
sl->mutex = mutex;
sl->sequence = 0;
}
/* Lock out other writers and update the count. */
static inline void seqlock_write_lock(QemuSeqLock *sl)
{
if (sl->mutex) {
qemu_mutex_lock(sl->mutex);
}
++sl->sequence;
/* Write sequence before updating other fields. */
smp_wmb();
}
static inline void seqlock_write_unlock(QemuSeqLock *sl)
{
/* Write other fields before finalizing sequence. */
smp_wmb();
++sl->sequence;
if (sl->mutex) {
qemu_mutex_unlock(sl->mutex);
}
}
static inline unsigned seqlock_read_begin(QemuSeqLock *sl)
{
/* Always fail if a write is in progress. */
unsigned ret = sl->sequence & ~1;
/* Read sequence before reading other fields. */
smp_rmb();
return ret;
}
static int seqlock_read_retry(const QemuSeqLock *sl, unsigned start)
{
/* Read other fields before reading final sequence. */
smp_rmb();
return unlikely(sl->sequence != start);
}
#endif
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