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Linux kernel mirror (for testing)
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linux
1/*
2 * asm-generic/mutex-dec.h
3 *
4 * Generic implementation of the mutex fastpath, based on atomic
5 * decrement/increment.
6 */
7#ifndef _ASM_GENERIC_MUTEX_DEC_H
8#define _ASM_GENERIC_MUTEX_DEC_H
9
10/**
11 * __mutex_fastpath_lock - try to take the lock by moving the count
12 * from 1 to a 0 value
13 * @count: pointer of type atomic_t
14 * @fail_fn: function to call if the original value was not 1
15 *
16 * Change the count from 1 to a value lower than 1, and call <fail_fn> if
17 * it wasn't 1 originally. This function MUST leave the value lower than
18 * 1 even when the "1" assertion wasn't true.
19 */
20#define __mutex_fastpath_lock(count, fail_fn) \
21do { \
22 if (unlikely(atomic_dec_return(count) < 0)) \
23 fail_fn(count); \
24 else \
25 smp_mb(); \
26} while (0)
27
28/**
29 * __mutex_fastpath_lock_retval - try to take the lock by moving the count
30 * from 1 to a 0 value
31 * @count: pointer of type atomic_t
32 * @fail_fn: function to call if the original value was not 1
33 *
34 * Change the count from 1 to a value lower than 1, and call <fail_fn> if
35 * it wasn't 1 originally. This function returns 0 if the fastpath succeeds,
36 * or anything the slow path function returns.
37 */
38static inline int
39__mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
40{
41 if (unlikely(atomic_dec_return(count) < 0))
42 return fail_fn(count);
43 else {
44 smp_mb();
45 return 0;
46 }
47}
48
49/**
50 * __mutex_fastpath_unlock - try to promote the count from 0 to 1
51 * @count: pointer of type atomic_t
52 * @fail_fn: function to call if the original value was not 0
53 *
54 * Try to promote the count from 0 to 1. If it wasn't 0, call <fail_fn>.
55 * In the failure case, this function is allowed to either set the value to
56 * 1, or to set it to a value lower than 1.
57 *
58 * If the implementation sets it to a value of lower than 1, then the
59 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
60 * to return 0 otherwise.
61 */
62#define __mutex_fastpath_unlock(count, fail_fn) \
63do { \
64 smp_mb(); \
65 if (unlikely(atomic_inc_return(count) <= 0)) \
66 fail_fn(count); \
67} while (0)
68
69#define __mutex_slowpath_needs_to_unlock() 1
70
71/**
72 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
73 *
74 * @count: pointer of type atomic_t
75 * @fail_fn: fallback function
76 *
77 * Change the count from 1 to a value lower than 1, and return 0 (failure)
78 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
79 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
80 * Additionally, if the value was < 0 originally, this function must not leave
81 * it to 0 on failure.
82 *
83 * If the architecture has no effective trylock variant, it should call the
84 * <fail_fn> spinlock-based trylock variant unconditionally.
85 */
86static inline int
87__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
88{
89 /*
90 * We have two variants here. The cmpxchg based one is the best one
91 * because it never induce a false contention state. It is included
92 * here because architectures using the inc/dec algorithms over the
93 * xchg ones are much more likely to support cmpxchg natively.
94 *
95 * If not we fall back to the spinlock based variant - that is
96 * just as efficient (and simpler) as a 'destructive' probing of
97 * the mutex state would be.
98 */
99#ifdef __HAVE_ARCH_CMPXCHG
100 if (likely(atomic_cmpxchg(count, 1, 0) == 1)) {
101 smp_mb();
102 return 1;
103 }
104 return 0;
105#else
106 return fail_fn(count);
107#endif
108}
109
110#endif