Commit | Line | Data |
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e7224674 TC |
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 | ||
ddf1d169 TC |
15 | #include <asm/mcs_spinlock.h> |
16 | ||
e7224674 TC |
17 | struct mcs_spinlock { |
18 | struct mcs_spinlock *next; | |
19 | int locked; /* 1 if lock acquired */ | |
a33fda35 | 20 | int count; /* nesting count, see qspinlock.c */ |
e7224674 TC |
21 | }; |
22 | ||
e207552e WD |
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))) \ | |
3a6bfbc9 | 31 | cpu_relax_lowlatency(); \ |
e207552e WD |
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 | ||
e7224674 TC |
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 | */ | |
5faeb8ad JL |
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(). | |
5faeb8ad | 60 | */ |
e7224674 TC |
61 | static inline |
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 | ||
920c720a PZ |
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); | |
e7224674 | 77 | if (likely(prev == NULL)) { |
5faeb8ad JL |
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 | */ | |
e7224674 TC |
86 | return; |
87 | } | |
4d3199e4 | 88 | WRITE_ONCE(prev->next, node); |
e207552e WD |
89 | |
90 | /* Wait until the lock holder passes the lock down. */ | |
91 | arch_mcs_spin_lock_contended(&node->locked); | |
e7224674 TC |
92 | } |
93 | ||
5faeb8ad JL |
94 | /* |
95 | * Releases the lock. The caller should pass in the corresponding node that | |
96 | * was used to acquire the lock. | |
97 | */ | |
e7224674 TC |
98 | static inline |
99 | void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) | |
100 | { | |
4d3199e4 | 101 | struct mcs_spinlock *next = READ_ONCE(node->next); |
e7224674 TC |
102 | |
103 | if (likely(!next)) { | |
104 | /* | |
105 | * Release the lock by setting it to NULL | |
106 | */ | |
3552a07a | 107 | if (likely(cmpxchg_release(lock, node, NULL) == node)) |
e7224674 TC |
108 | return; |
109 | /* Wait until the next pointer is set */ | |
4d3199e4 | 110 | while (!(next = READ_ONCE(node->next))) |
3a6bfbc9 | 111 | cpu_relax_lowlatency(); |
e7224674 | 112 | } |
e207552e WD |
113 | |
114 | /* Pass lock to next waiter. */ | |
115 | arch_mcs_spin_unlock_contended(&next->locked); | |
e7224674 TC |
116 | } |
117 | ||
118 | #endif /* __LINUX_MCS_SPINLOCK_H */ |