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1/*
2 * MCS lock defines
3 *
4 * This file contains the main data structure and API definitions of MCS lock.
5 *
6 * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
7 * with the desirable properties of being fair, and with each cpu trying
8 * to acquire the lock spinning on a local variable.
9 * It avoids expensive cache bouncings that common test-and-set spin-lock
10 * implementations incur.
11 */
12#ifndef __LINUX_MCS_SPINLOCK_H
13#define __LINUX_MCS_SPINLOCK_H
14
15#include <asm/mcs_spinlock.h>
16
17struct mcs_spinlock {
18 struct mcs_spinlock *next;
19 int locked; /* 1 if lock acquired */
20 int count; /* nesting count, see qspinlock.c */
21};
22
23#ifndef arch_mcs_spin_lock_contended
24/*
25 * Using smp_load_acquire() provides a memory barrier that ensures
26 * subsequent operations happen after the lock is acquired.
27 */
28#define arch_mcs_spin_lock_contended(l) \
29do { \
30 while (!(smp_load_acquire(l))) \
31 cpu_relax_lowlatency(); \
32} while (0)
33#endif
34
35#ifndef arch_mcs_spin_unlock_contended
36/*
37 * smp_store_release() provides a memory barrier to ensure all
38 * operations in the critical section has been completed before
39 * unlocking.
40 */
41#define arch_mcs_spin_unlock_contended(l) \
42 smp_store_release((l), 1)
43#endif
44
45/*
46 * Note: the smp_load_acquire/smp_store_release pair is not
47 * sufficient to form a full memory barrier across
48 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
49 * For applications that need a full barrier across multiple cpus
50 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
51 * used after mcs_lock.
52 */
53
54/*
55 * In order to acquire the lock, the caller should declare a local node and
56 * pass a reference of the node to this function in addition to the lock.
57 * If the lock has already been acquired, then this will proceed to spin
58 * on this node->locked until the previous lock holder sets the node->locked
59 * in mcs_spin_unlock().
60 */
61static inline
62void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
63{
64 struct mcs_spinlock *prev;
65
66 /* Init node */
67 node->locked = 0;
68 node->next = NULL;
69
70 /*
71 * We rely on the full barrier with global transitivity implied by the
72 * below xchg() to order the initialization stores above against any
73 * observation of @node. And to provide the ACQUIRE ordering associated
74 * with a LOCK primitive.
75 */
76 prev = xchg(lock, node);
77 if (likely(prev == NULL)) {
78 /*
79 * Lock acquired, don't need to set node->locked to 1. Threads
80 * only spin on its own node->locked value for lock acquisition.
81 * However, since this thread can immediately acquire the lock
82 * and does not proceed to spin on its own node->locked, this
83 * value won't be used. If a debug mode is needed to
84 * audit lock status, then set node->locked value here.
85 */
86 return;
87 }
88 WRITE_ONCE(prev->next, node);
89
90 /* Wait until the lock holder passes the lock down. */
91 arch_mcs_spin_lock_contended(&node->locked);
92}
93
94/*
95 * Releases the lock. The caller should pass in the corresponding node that
96 * was used to acquire the lock.
97 */
98static inline
99void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
100{
101 struct mcs_spinlock *next = READ_ONCE(node->next);
102
103 if (likely(!next)) {
104 /*
105 * Release the lock by setting it to NULL
106 */
107 if (likely(cmpxchg_release(lock, node, NULL) == node))
108 return;
109 /* Wait until the next pointer is set */
110 while (!(next = READ_ONCE(node->next)))
111 cpu_relax_lowlatency();
112 }
113
114 /* Pass lock to next waiter. */
115 arch_mcs_spin_unlock_contended(&next->locked);
116}
117
118#endif /* __LINUX_MCS_SPINLOCK_H */
1/* SPDX-License-Identifier: GPL-2.0 */
2/*
3 * MCS lock defines
4 *
5 * This file contains the main data structure and API definitions of MCS lock.
6 *
7 * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
8 * with the desirable properties of being fair, and with each cpu trying
9 * to acquire the lock spinning on a local variable.
10 * It avoids expensive cache bouncings that common test-and-set spin-lock
11 * implementations incur.
12 */
13#ifndef __LINUX_MCS_SPINLOCK_H
14#define __LINUX_MCS_SPINLOCK_H
15
16#include <asm/mcs_spinlock.h>
17
18struct mcs_spinlock {
19 struct mcs_spinlock *next;
20 int locked; /* 1 if lock acquired */
21 int count; /* nesting count, see qspinlock.c */
22};
23
24#ifndef arch_mcs_spin_lock_contended
25/*
26 * Using smp_cond_load_acquire() provides the acquire semantics
27 * required so that subsequent operations happen after the
28 * lock is acquired. Additionally, some architectures such as
29 * ARM64 would like to do spin-waiting instead of purely
30 * spinning, and smp_cond_load_acquire() provides that behavior.
31 */
32#define arch_mcs_spin_lock_contended(l) \
33do { \
34 smp_cond_load_acquire(l, VAL); \
35} while (0)
36#endif
37
38#ifndef arch_mcs_spin_unlock_contended
39/*
40 * smp_store_release() provides a memory barrier to ensure all
41 * operations in the critical section has been completed before
42 * unlocking.
43 */
44#define arch_mcs_spin_unlock_contended(l) \
45 smp_store_release((l), 1)
46#endif
47
48/*
49 * Note: the smp_load_acquire/smp_store_release pair is not
50 * sufficient to form a full memory barrier across
51 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
52 * For applications that need a full barrier across multiple cpus
53 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
54 * used after mcs_lock.
55 */
56
57/*
58 * In order to acquire the lock, the caller should declare a local node and
59 * pass a reference of the node to this function in addition to the lock.
60 * If the lock has already been acquired, then this will proceed to spin
61 * on this node->locked until the previous lock holder sets the node->locked
62 * in mcs_spin_unlock().
63 */
64static inline
65void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
66{
67 struct mcs_spinlock *prev;
68
69 /* Init node */
70 node->locked = 0;
71 node->next = NULL;
72
73 /*
74 * We rely on the full barrier with global transitivity implied by the
75 * below xchg() to order the initialization stores above against any
76 * observation of @node. And to provide the ACQUIRE ordering associated
77 * with a LOCK primitive.
78 */
79 prev = xchg(lock, node);
80 if (likely(prev == NULL)) {
81 /*
82 * Lock acquired, don't need to set node->locked to 1. Threads
83 * only spin on its own node->locked value for lock acquisition.
84 * However, since this thread can immediately acquire the lock
85 * and does not proceed to spin on its own node->locked, this
86 * value won't be used. If a debug mode is needed to
87 * audit lock status, then set node->locked value here.
88 */
89 return;
90 }
91 WRITE_ONCE(prev->next, node);
92
93 /* Wait until the lock holder passes the lock down. */
94 arch_mcs_spin_lock_contended(&node->locked);
95}
96
97/*
98 * Releases the lock. The caller should pass in the corresponding node that
99 * was used to acquire the lock.
100 */
101static inline
102void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
103{
104 struct mcs_spinlock *next = READ_ONCE(node->next);
105
106 if (likely(!next)) {
107 /*
108 * Release the lock by setting it to NULL
109 */
110 if (likely(cmpxchg_release(lock, node, NULL) == node))
111 return;
112 /* Wait until the next pointer is set */
113 while (!(next = READ_ONCE(node->next)))
114 cpu_relax();
115 }
116
117 /* Pass lock to next waiter. */
118 arch_mcs_spin_unlock_contended(&next->locked);
119}
120
121#endif /* __LINUX_MCS_SPINLOCK_H */