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
18 struct 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) \
30 do { \
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 */
62 static inline
63 void 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 */
99 static inline
100 void 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 */
120
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