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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 */ | |
20 | }; | |
21 | ||
e207552e WD |
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) \ | |
28 | do { \ | |
29 | while (!(smp_load_acquire(l))) \ | |
3a6bfbc9 | 30 | cpu_relax_lowlatency(); \ |
e207552e WD |
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 | ||
e7224674 TC |
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 | */ | |
5faeb8ad JL |
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(). | |
5faeb8ad | 59 | */ |
e7224674 TC |
60 | static inline |
61 | void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) | |
62 | { | |
63 | struct mcs_spinlock *prev; | |
64 | ||
65 | /* Init node */ | |
66 | node->locked = 0; | |
67 | node->next = NULL; | |
68 | ||
69 | prev = xchg(lock, node); | |
70 | if (likely(prev == NULL)) { | |
5faeb8ad JL |
71 | /* |
72 | * Lock acquired, don't need to set node->locked to 1. Threads | |
73 | * only spin on its own node->locked value for lock acquisition. | |
74 | * However, since this thread can immediately acquire the lock | |
75 | * and does not proceed to spin on its own node->locked, this | |
76 | * value won't be used. If a debug mode is needed to | |
77 | * audit lock status, then set node->locked value here. | |
78 | */ | |
e7224674 TC |
79 | return; |
80 | } | |
81 | ACCESS_ONCE(prev->next) = node; | |
e207552e WD |
82 | |
83 | /* Wait until the lock holder passes the lock down. */ | |
84 | arch_mcs_spin_lock_contended(&node->locked); | |
e7224674 TC |
85 | } |
86 | ||
5faeb8ad JL |
87 | /* |
88 | * Releases the lock. The caller should pass in the corresponding node that | |
89 | * was used to acquire the lock. | |
90 | */ | |
e7224674 TC |
91 | static inline |
92 | void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) | |
93 | { | |
94 | struct mcs_spinlock *next = ACCESS_ONCE(node->next); | |
95 | ||
96 | if (likely(!next)) { | |
97 | /* | |
98 | * Release the lock by setting it to NULL | |
99 | */ | |
5faeb8ad | 100 | if (likely(cmpxchg(lock, node, NULL) == node)) |
e7224674 TC |
101 | return; |
102 | /* Wait until the next pointer is set */ | |
103 | while (!(next = ACCESS_ONCE(node->next))) | |
3a6bfbc9 | 104 | cpu_relax_lowlatency(); |
e7224674 | 105 | } |
e207552e WD |
106 | |
107 | /* Pass lock to next waiter. */ | |
108 | arch_mcs_spin_unlock_contended(&next->locked); | |
e7224674 TC |
109 | } |
110 | ||
111 | #endif /* __LINUX_MCS_SPINLOCK_H */ |