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_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) \
33 do { \
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 */
64 static inline
65 void 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 */
101 static inline
102 void 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 */