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ea753d81e8
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: Paolo Bonzini <pbonzini@redhat.com>
73 lines
1.5 KiB
C
73 lines
1.5 KiB
C
/*
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* Seqlock implementation for QEMU
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*
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* Copyright Red Hat, Inc. 2013
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*
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* Author:
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* Paolo Bonzini <pbonzini@redhat.com>
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*
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* This work is licensed under the terms of the GNU GPL, version 2 or later.
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* See the COPYING file in the top-level directory.
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*
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*/
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#ifndef QEMU_SEQLOCK_H
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#define QEMU_SEQLOCK_H 1
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#include <qemu/atomic.h>
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#include <qemu/thread.h>
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typedef struct QemuSeqLock QemuSeqLock;
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struct QemuSeqLock {
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QemuMutex *mutex;
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unsigned sequence;
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};
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static inline void seqlock_init(QemuSeqLock *sl, QemuMutex *mutex)
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{
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sl->mutex = mutex;
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sl->sequence = 0;
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}
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/* Lock out other writers and update the count. */
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static inline void seqlock_write_lock(QemuSeqLock *sl)
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{
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if (sl->mutex) {
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qemu_mutex_lock(sl->mutex);
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}
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++sl->sequence;
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/* Write sequence before updating other fields. */
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smp_wmb();
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}
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static inline void seqlock_write_unlock(QemuSeqLock *sl)
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{
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/* Write other fields before finalizing sequence. */
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smp_wmb();
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++sl->sequence;
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if (sl->mutex) {
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qemu_mutex_unlock(sl->mutex);
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}
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}
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static inline unsigned seqlock_read_begin(QemuSeqLock *sl)
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{
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/* Always fail if a write is in progress. */
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unsigned ret = sl->sequence & ~1;
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/* Read sequence before reading other fields. */
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smp_rmb();
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return ret;
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}
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static int seqlock_read_retry(const QemuSeqLock *sl, unsigned start)
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{
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/* Read other fields before reading final sequence. */
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smp_rmb();
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return unlikely(sl->sequence != start);
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}
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#endif
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