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Ingo Molnar620a6fd2006-01-09 15:59:17 -08001/*
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 */
Nicolas Pitree358c1a2006-03-31 02:32:13 -080020static inline void
21__mutex_fastpath_lock(atomic_t *count, fastcall void (*fail_fn)(atomic_t *))
22{
23 if (unlikely(atomic_dec_return(count) < 0))
24 fail_fn(count);
25 else
26 smp_mb();
27}
Ingo Molnar620a6fd2006-01-09 15:59:17 -080028
29/**
30 * __mutex_fastpath_lock_retval - try to take the lock by moving the count
31 * from 1 to a 0 value
32 * @count: pointer of type atomic_t
33 * @fail_fn: function to call if the original value was not 1
34 *
35 * Change the count from 1 to a value lower than 1, and call <fail_fn> if
36 * it wasn't 1 originally. This function returns 0 if the fastpath succeeds,
37 * or anything the slow path function returns.
38 */
39static inline int
Nicolas Pitree358c1a2006-03-31 02:32:13 -080040__mutex_fastpath_lock_retval(atomic_t *count, fastcall int (*fail_fn)(atomic_t *))
Ingo Molnar620a6fd2006-01-09 15:59:17 -080041{
42 if (unlikely(atomic_dec_return(count) < 0))
43 return fail_fn(count);
44 else {
45 smp_mb();
46 return 0;
47 }
48}
49
50/**
51 * __mutex_fastpath_unlock - try to promote the count from 0 to 1
52 * @count: pointer of type atomic_t
53 * @fail_fn: function to call if the original value was not 0
54 *
55 * Try to promote the count from 0 to 1. If it wasn't 0, call <fail_fn>.
56 * In the failure case, this function is allowed to either set the value to
57 * 1, or to set it to a value lower than 1.
58 *
59 * If the implementation sets it to a value of lower than 1, then the
60 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
61 * to return 0 otherwise.
62 */
Nicolas Pitree358c1a2006-03-31 02:32:13 -080063static inline void
64__mutex_fastpath_unlock(atomic_t *count, fastcall void (*fail_fn)(atomic_t *))
65{
66 smp_mb();
67 if (unlikely(atomic_inc_return(count) <= 0))
68 fail_fn(count);
69}
Ingo Molnar620a6fd2006-01-09 15:59:17 -080070
71#define __mutex_slowpath_needs_to_unlock() 1
72
73/**
74 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
75 *
76 * @count: pointer of type atomic_t
77 * @fail_fn: fallback function
78 *
79 * Change the count from 1 to a value lower than 1, and return 0 (failure)
80 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
81 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
82 * Additionally, if the value was < 0 originally, this function must not leave
83 * it to 0 on failure.
84 *
85 * If the architecture has no effective trylock variant, it should call the
86 * <fail_fn> spinlock-based trylock variant unconditionally.
87 */
88static inline int
89__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
90{
91 /*
92 * We have two variants here. The cmpxchg based one is the best one
93 * because it never induce a false contention state. It is included
94 * here because architectures using the inc/dec algorithms over the
95 * xchg ones are much more likely to support cmpxchg natively.
96 *
97 * If not we fall back to the spinlock based variant - that is
98 * just as efficient (and simpler) as a 'destructive' probing of
99 * the mutex state would be.
100 */
101#ifdef __HAVE_ARCH_CMPXCHG
Linus Torvalds4cec8732006-01-11 15:50:47 -0800102 if (likely(atomic_cmpxchg(count, 1, 0) == 1)) {
Ingo Molnar620a6fd2006-01-09 15:59:17 -0800103 smp_mb();
104 return 1;
105 }
106 return 0;
107#else
108 return fail_fn(count);
109#endif
110}
111
112#endif