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620a6fd1 IM |
1 | /* |
2 | * asm-generic/mutex-xchg.h | |
3 | * | |
4 | * Generic implementation of the mutex fastpath, based on xchg(). | |
5 | * | |
6 | * NOTE: An xchg based implementation is less optimal than an atomic | |
7 | * decrement/increment based implementation. If your architecture | |
8 | * has a reasonable atomic dec/inc then you should probably use | |
9 | * asm-generic/mutex-dec.h instead, or you could open-code an | |
10 | * optimized version in asm/mutex.h. | |
11 | */ | |
12 | #ifndef _ASM_GENERIC_MUTEX_XCHG_H | |
13 | #define _ASM_GENERIC_MUTEX_XCHG_H | |
14 | ||
15 | /** | |
16 | * __mutex_fastpath_lock - try to take the lock by moving the count | |
17 | * from 1 to a 0 value | |
18 | * @count: pointer of type atomic_t | |
19 | * @fail_fn: function to call if the original value was not 1 | |
20 | * | |
21 | * Change the count from 1 to a value lower than 1, and call <fail_fn> if it | |
22 | * wasn't 1 originally. This function MUST leave the value lower than 1 | |
23 | * even when the "1" assertion wasn't true. | |
24 | */ | |
25 | #define __mutex_fastpath_lock(count, fail_fn) \ | |
26 | do { \ | |
27 | if (unlikely(atomic_xchg(count, 0) != 1)) \ | |
28 | fail_fn(count); \ | |
29 | else \ | |
30 | smp_mb(); \ | |
31 | } while (0) | |
32 | ||
33 | ||
34 | /** | |
35 | * __mutex_fastpath_lock_retval - try to take the lock by moving the count | |
36 | * from 1 to a 0 value | |
37 | * @count: pointer of type atomic_t | |
38 | * @fail_fn: function to call if the original value was not 1 | |
39 | * | |
40 | * Change the count from 1 to a value lower than 1, and call <fail_fn> if it | |
41 | * wasn't 1 originally. This function returns 0 if the fastpath succeeds, | |
42 | * or anything the slow path function returns | |
43 | */ | |
44 | static inline int | |
45 | __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *)) | |
46 | { | |
47 | if (unlikely(atomic_xchg(count, 0) != 1)) | |
48 | return fail_fn(count); | |
49 | else { | |
50 | smp_mb(); | |
51 | return 0; | |
52 | } | |
53 | } | |
54 | ||
55 | /** | |
56 | * __mutex_fastpath_unlock - try to promote the mutex from 0 to 1 | |
57 | * @count: pointer of type atomic_t | |
58 | * @fail_fn: function to call if the original value was not 0 | |
59 | * | |
60 | * try to promote the mutex from 0 to 1. if it wasn't 0, call <function> | |
61 | * In the failure case, this function is allowed to either set the value to | |
62 | * 1, or to set it to a value lower than one. | |
63 | * If the implementation sets it to a value of lower than one, the | |
64 | * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs | |
65 | * to return 0 otherwise. | |
66 | */ | |
67 | #define __mutex_fastpath_unlock(count, fail_fn) \ | |
68 | do { \ | |
69 | smp_mb(); \ | |
70 | if (unlikely(atomic_xchg(count, 1) != 0)) \ | |
71 | fail_fn(count); \ | |
72 | } while (0) | |
73 | ||
74 | #define __mutex_slowpath_needs_to_unlock() 0 | |
75 | ||
76 | /** | |
77 | * __mutex_fastpath_trylock - try to acquire the mutex, without waiting | |
78 | * | |
79 | * @count: pointer of type atomic_t | |
80 | * @fail_fn: spinlock based trylock implementation | |
81 | * | |
82 | * Change the count from 1 to a value lower than 1, and return 0 (failure) | |
83 | * if it wasn't 1 originally, or return 1 (success) otherwise. This function | |
84 | * MUST leave the value lower than 1 even when the "1" assertion wasn't true. | |
85 | * Additionally, if the value was < 0 originally, this function must not leave | |
86 | * it to 0 on failure. | |
87 | * | |
88 | * If the architecture has no effective trylock variant, it should call the | |
89 | * <fail_fn> spinlock-based trylock variant unconditionally. | |
90 | */ | |
91 | static inline int | |
92 | __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *)) | |
93 | { | |
94 | int prev = atomic_xchg(count, 0); | |
95 | ||
96 | if (unlikely(prev < 0)) { | |
97 | /* | |
98 | * The lock was marked contended so we must restore that | |
99 | * state. If while doing so we get back a prev value of 1 | |
100 | * then we just own it. | |
101 | * | |
102 | * [ In the rare case of the mutex going to 1, to 0, to -1 | |
103 | * and then back to 0 in this few-instructions window, | |
104 | * this has the potential to trigger the slowpath for the | |
105 | * owner's unlock path needlessly, but that's not a problem | |
106 | * in practice. ] | |
107 | */ | |
108 | prev = atomic_xchg(count, prev); | |
109 | if (prev < 0) | |
110 | prev = 0; | |
111 | } | |
112 | smp_mb(); | |
113 | ||
114 | return prev; | |
115 | } | |
116 | ||
117 | #endif |