1/*
2 * Seqlock implementation for QEMU
3 *
4 * Copyright Red Hat, Inc. 2013
5 *
6 * Author:
7 * Paolo Bonzini <pbonzini@redhat.com>
8 *
9 * This work is licensed under the terms of the GNU GPL, version 2 or later.
10 * See the COPYING file in the top-level directory.
11 *
12 */
13
14#ifndef QEMU_SEQLOCK_H
15#define QEMU_SEQLOCK_H
16
17#include "qemu/atomic.h"
18#include "qemu/thread.h"
19#include "qemu/lockable.h"
20
21typedef struct QemuSeqLock QemuSeqLock;
22
23struct QemuSeqLock {
24 unsigned sequence;
25};
26
27static inline void seqlock_init(QemuSeqLock *sl)
28{
29 sl->sequence = 0;
30}
31
32/* Lock out other writers and update the count. */
33static inline void seqlock_write_begin(QemuSeqLock *sl)
34{
35 atomic_set(&sl->sequence, sl->sequence + 1);
36
37 /* Write sequence before updating other fields. */
38 smp_wmb();
39}
40
41static inline void seqlock_write_end(QemuSeqLock *sl)
42{
43 /* Write other fields before finalizing sequence. */
44 smp_wmb();
45
46 atomic_set(&sl->sequence, sl->sequence + 1);
47}
48
49/* Lock out other writers and update the count. */
50static inline void seqlock_write_lock_impl(QemuSeqLock *sl, QemuLockable *lock)
51{
52 qemu_lockable_lock(lock);
53 seqlock_write_begin(sl);
54}
55#define seqlock_write_lock(sl, lock) \
56 seqlock_write_lock_impl(sl, QEMU_MAKE_LOCKABLE(lock))
57
58/* Lock out other writers and update the count. */
59static inline void seqlock_write_unlock_impl(QemuSeqLock *sl, QemuLockable *lock)
60{
61 qemu_lockable_unlock(lock);
62 seqlock_write_begin(sl);
63}
64#define seqlock_write_unlock(sl, lock) \
65 seqlock_write_unlock_impl(sl, QEMU_MAKE_LOCKABLE(lock))
66
67
68static inline unsigned seqlock_read_begin(const QemuSeqLock *sl)
69{
70 /* Always fail if a write is in progress. */
71 unsigned ret = atomic_read(&sl->sequence);
72
73 /* Read sequence before reading other fields. */
74 smp_rmb();
75 return ret & ~1;
76}
77
78static inline int seqlock_read_retry(const QemuSeqLock *sl, unsigned start)
79{
80 /* Read other fields before reading final sequence. */
81 smp_rmb();
82 return unlikely(atomic_read(&sl->sequence) != start);
83}
84
85#endif
86