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/*
* 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
#include "qemu/atomic.h"
#include "qemu/thread.h"
#include "qemu/lockable.h"
typedef struct QemuSeqLock QemuSeqLock;
struct QemuSeqLock {
unsigned sequence;
};
static inline void seqlock_init(QemuSeqLock *sl)
{
sl->sequence = 0;
}
/* Lock out other writers and update the count. */
static inline void seqlock_write_begin(QemuSeqLock *sl)
{
qatomic_set(&sl->sequence, sl->sequence + 1);
/* Write sequence before updating other fields. */
smp_wmb();
}
static inline void seqlock_write_end(QemuSeqLock *sl)
{
/* Write other fields before finalizing sequence. */
smp_wmb();
qatomic_set(&sl->sequence, sl->sequence + 1);
}
/* Lock out other writers and update the count. */
static inline void seqlock_write_lock_impl(QemuSeqLock *sl, QemuLockable *lock)
{
qemu_lockable_lock(lock);
seqlock_write_begin(sl);
}
#define seqlock_write_lock(sl, lock) \
seqlock_write_lock_impl(sl, QEMU_MAKE_LOCKABLE(lock))
/* Update the count and release the lock. */
static inline void seqlock_write_unlock_impl(QemuSeqLock *sl, QemuLockable *lock)
{
seqlock_write_end(sl);
qemu_lockable_unlock(lock);
}
#define seqlock_write_unlock(sl, lock) \
seqlock_write_unlock_impl(sl, QEMU_MAKE_LOCKABLE(lock))
static inline unsigned seqlock_read_begin(const QemuSeqLock *sl)
{
/* Always fail if a write is in progress. */
unsigned ret = qatomic_read(&sl->sequence);
/* Read sequence before reading other fields. */
smp_rmb();
return ret & ~1;
}
static inline int seqlock_read_retry(const QemuSeqLock *sl, unsigned start)
{
/* Read other fields before reading final sequence. */
smp_rmb();
return unlikely(qatomic_read(&sl->sequence) != start);
}
#endif
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