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
17 struct 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) \
29 do { \
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 */
61 static inline
mcs_spin_lock(struct mcs_spinlock ** lock,struct mcs_spinlock * node)62 void 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 */
98 static inline
mcs_spin_unlock(struct mcs_spinlock ** lock,struct mcs_spinlock * node)99 void 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 */
119