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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};
21
22#ifndef arch_mcs_spin_lock_contended
23/*
24 * Using smp_load_acquire() provides a memory barrier that ensures
25 * subsequent operations happen after the lock is acquired.
26 */
27#define arch_mcs_spin_lock_contended(l) \
28do { \
29 while (!(smp_load_acquire(l))) \
30 arch_mutex_cpu_relax(); \
31} while (0)
32#endif
33
34#ifndef arch_mcs_spin_unlock_contended
35/*
36 * smp_store_release() provides a memory barrier to ensure all
37 * operations in the critical section has been completed before
38 * unlocking.
39 */
40#define arch_mcs_spin_unlock_contended(l) \
41 smp_store_release((l), 1)
42#endif
43
44/*
45 * Note: the smp_load_acquire/smp_store_release pair is not
46 * sufficient to form a full memory barrier across
47 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
48 * For applications that need a full barrier across multiple cpus
49 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
50 * used after mcs_lock.
51 */
52
53/*
54 * In order to acquire the lock, the caller should declare a local node and
55 * pass a reference of the node to this function in addition to the lock.
56 * If the lock has already been acquired, then this will proceed to spin
57 * on this node->locked until the previous lock holder sets the node->locked
58 * in mcs_spin_unlock().
59 *
60 * We don't inline mcs_spin_lock() so that perf can correctly account for the
61 * time spent in this lock function.
62 */
63static inline
64void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
65{
66 struct mcs_spinlock *prev;
67
68 /* Init node */
69 node->locked = 0;
70 node->next = NULL;
71
72 prev = xchg(lock, node);
73 if (likely(prev == NULL)) {
74 /*
75 * Lock acquired, don't need to set node->locked to 1. Threads
76 * only spin on its own node->locked value for lock acquisition.
77 * However, since this thread can immediately acquire the lock
78 * and does not proceed to spin on its own node->locked, this
79 * value won't be used. If a debug mode is needed to
80 * audit lock status, then set node->locked value here.
81 */
82 return;
83 }
84 ACCESS_ONCE(prev->next) = node;
85
86 /* Wait until the lock holder passes the lock down. */
87 arch_mcs_spin_lock_contended(&node->locked);
88}
89
90/*
91 * Releases the lock. The caller should pass in the corresponding node that
92 * was used to acquire the lock.
93 */
94static inline
95void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
96{
97 struct mcs_spinlock *next = ACCESS_ONCE(node->next);
98
99 if (likely(!next)) {
100 /*
101 * Release the lock by setting it to NULL
102 */
103 if (likely(cmpxchg(lock, node, NULL) == node))
104 return;
105 /* Wait until the next pointer is set */
106 while (!(next = ACCESS_ONCE(node->next)))
107 arch_mutex_cpu_relax();
108 }
109
110 /* Pass lock to next waiter. */
111 arch_mcs_spin_unlock_contended(&next->locked);
112}
113
114/*
115 * Cancellable version of the MCS lock above.
116 *
117 * Intended for adaptive spinning of sleeping locks:
118 * mutex_lock()/rwsem_down_{read,write}() etc.
119 */
120
121struct optimistic_spin_queue {
122 struct optimistic_spin_queue *next, *prev;
123 int locked; /* 1 if lock acquired */
124};
125
126extern bool osq_lock(struct optimistic_spin_queue **lock);
127extern void osq_unlock(struct optimistic_spin_queue **lock);
128
129#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_load_acquire() provides a memory barrier that ensures
27 * subsequent operations happen after the lock is acquired.
28 */
29#define arch_mcs_spin_lock_contended(l) \
30do { \
31 while (!(smp_load_acquire(l))) \
32 cpu_relax(); \
33} while (0)
34#endif
35
36#ifndef arch_mcs_spin_unlock_contended
37/*
38 * smp_store_release() provides a memory barrier to ensure all
39 * operations in the critical section has been completed before
40 * unlocking.
41 */
42#define arch_mcs_spin_unlock_contended(l) \
43 smp_store_release((l), 1)
44#endif
45
46/*
47 * Note: the smp_load_acquire/smp_store_release pair is not
48 * sufficient to form a full memory barrier across
49 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
50 * For applications that need a full barrier across multiple cpus
51 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
52 * used after mcs_lock.
53 */
54
55/*
56 * In order to acquire the lock, the caller should declare a local node and
57 * pass a reference of the node to this function in addition to the lock.
58 * If the lock has already been acquired, then this will proceed to spin
59 * on this node->locked until the previous lock holder sets the node->locked
60 * in mcs_spin_unlock().
61 */
62static inline
63void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
64{
65 struct mcs_spinlock *prev;
66
67 /* Init node */
68 node->locked = 0;
69 node->next = NULL;
70
71 /*
72 * We rely on the full barrier with global transitivity implied by the
73 * below xchg() to order the initialization stores above against any
74 * observation of @node. And to provide the ACQUIRE ordering associated
75 * with a LOCK primitive.
76 */
77 prev = xchg(lock, node);
78 if (likely(prev == NULL)) {
79 /*
80 * Lock acquired, don't need to set node->locked to 1. Threads
81 * only spin on its own node->locked value for lock acquisition.
82 * However, since this thread can immediately acquire the lock
83 * and does not proceed to spin on its own node->locked, this
84 * value won't be used. If a debug mode is needed to
85 * audit lock status, then set node->locked value here.
86 */
87 return;
88 }
89 WRITE_ONCE(prev->next, node);
90
91 /* Wait until the lock holder passes the lock down. */
92 arch_mcs_spin_lock_contended(&node->locked);
93}
94
95/*
96 * Releases the lock. The caller should pass in the corresponding node that
97 * was used to acquire the lock.
98 */
99static inline
100void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
101{
102 struct mcs_spinlock *next = READ_ONCE(node->next);
103
104 if (likely(!next)) {
105 /*
106 * Release the lock by setting it to NULL
107 */
108 if (likely(cmpxchg_release(lock, node, NULL) == node))
109 return;
110 /* Wait until the next pointer is set */
111 while (!(next = READ_ONCE(node->next)))
112 cpu_relax();
113 }
114
115 /* Pass lock to next waiter. */
116 arch_mcs_spin_unlock_contended(&next->locked);
117}
118
119#endif /* __LINUX_MCS_SPINLOCK_H */