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
21 typedef struct QemuSeqLock QemuSeqLock;
22
23 struct QemuSeqLock {
24 unsigned sequence;
25 };
26
seqlock_init(QemuSeqLock * sl)27 static inline void seqlock_init(QemuSeqLock *sl)
28 {
29 sl->sequence = 0;
30 }
31
32 /* Lock out other writers and update the count. */
seqlock_write_begin(QemuSeqLock * sl)33 static inline void seqlock_write_begin(QemuSeqLock *sl)
34 {
35 qatomic_set(&sl->sequence, sl->sequence + 1);
36
37 /* Write sequence before updating other fields. */
38 smp_wmb();
39 }
40
seqlock_write_end(QemuSeqLock * sl)41 static inline void seqlock_write_end(QemuSeqLock *sl)
42 {
43 /* Write other fields before finalizing sequence. */
44 smp_wmb();
45
46 qatomic_set(&sl->sequence, sl->sequence + 1);
47 }
48
49 /* Lock out other writers and update the count. */
seqlock_write_lock_impl(QemuSeqLock * sl,QemuLockable * lock)50 static 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 /* Update the count and release the lock. */
seqlock_write_unlock_impl(QemuSeqLock * sl,QemuLockable * lock)59 static inline void seqlock_write_unlock_impl(QemuSeqLock *sl, QemuLockable *lock)
60 {
61 seqlock_write_end(sl);
62 qemu_lockable_unlock(lock);
63 }
64 #define seqlock_write_unlock(sl, lock) \
65 seqlock_write_unlock_impl(sl, QEMU_MAKE_LOCKABLE(lock))
66
67
seqlock_read_begin(const QemuSeqLock * sl)68 static inline unsigned seqlock_read_begin(const QemuSeqLock *sl)
69 {
70 /* Always fail if a write is in progress. */
71 unsigned ret = qatomic_read(&sl->sequence);
72
73 /* Read sequence before reading other fields. */
74 smp_rmb();
75 return ret & ~1;
76 }
77
seqlock_read_retry(const QemuSeqLock * sl,unsigned start)78 static 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(qatomic_read(&sl->sequence) != start);
83 }
84
85 #endif
86