1 /*
2 * Assembly implementation of the mutex fastpath, based on atomic
3 * decrement/increment.
4 *
5 * started by Ingo Molnar:
6 *
7 * Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
8 */
9 #ifndef _ASM_X86_MUTEX_32_H
10 #define _ASM_X86_MUTEX_32_H
11
12 #include <asm/alternative.h>
13
14 /**
15 * __mutex_fastpath_lock - try to take the lock by moving the count
16 * from 1 to a 0 value
17 * @count: pointer of type atomic_t
18 * @fn: function to call if the original value was not 1
19 *
20 * Change the count from 1 to a value lower than 1, and call <fn> if it
21 * wasn't 1 originally. This function MUST leave the value lower than 1
22 * even when the "1" assertion wasn't true.
23 */
24 #define __mutex_fastpath_lock(count, fail_fn) \
25 do { \
26 unsigned int dummy; \
27 \
28 typecheck(atomic_t *, count); \
29 typecheck_fn(void (*)(atomic_t *), fail_fn); \
30 \
31 asm volatile(LOCK_PREFIX " decl (%%eax)\n" \
32 " jns 1f \n" \
33 " call " #fail_fn "\n" \
34 "1:\n" \
35 : "=a" (dummy) \
36 : "a" (count) \
37 : "memory", "ecx", "edx"); \
38 } while (0)
39
40
41 /**
42 * __mutex_fastpath_lock_retval - try to take the lock by moving the count
43 * from 1 to a 0 value
44 * @count: pointer of type atomic_t
45 * @fail_fn: function to call if the original value was not 1
46 *
47 * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
48 * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
49 * or anything the slow path function returns
50 */
__mutex_fastpath_lock_retval(atomic_t * count,int (* fail_fn)(atomic_t *))51 static inline int __mutex_fastpath_lock_retval(atomic_t *count,
52 int (*fail_fn)(atomic_t *))
53 {
54 if (unlikely(atomic_dec_return(count) < 0))
55 return fail_fn(count);
56 else
57 return 0;
58 }
59
60 /**
61 * __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
62 * @count: pointer of type atomic_t
63 * @fail_fn: function to call if the original value was not 0
64 *
65 * try to promote the mutex from 0 to 1. if it wasn't 0, call <fail_fn>.
66 * In the failure case, this function is allowed to either set the value
67 * to 1, or to set it to a value lower than 1.
68 *
69 * If the implementation sets it to a value of lower than 1, the
70 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
71 * to return 0 otherwise.
72 */
73 #define __mutex_fastpath_unlock(count, fail_fn) \
74 do { \
75 unsigned int dummy; \
76 \
77 typecheck(atomic_t *, count); \
78 typecheck_fn(void (*)(atomic_t *), fail_fn); \
79 \
80 asm volatile(LOCK_PREFIX " incl (%%eax)\n" \
81 " jg 1f\n" \
82 " call " #fail_fn "\n" \
83 "1:\n" \
84 : "=a" (dummy) \
85 : "a" (count) \
86 : "memory", "ecx", "edx"); \
87 } while (0)
88
89 #define __mutex_slowpath_needs_to_unlock() 1
90
91 /**
92 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
93 *
94 * @count: pointer of type atomic_t
95 * @fail_fn: fallback function
96 *
97 * Change the count from 1 to a value lower than 1, and return 0 (failure)
98 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
99 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
100 * Additionally, if the value was < 0 originally, this function must not leave
101 * it to 0 on failure.
102 */
__mutex_fastpath_trylock(atomic_t * count,int (* fail_fn)(atomic_t *))103 static inline int __mutex_fastpath_trylock(atomic_t *count,
104 int (*fail_fn)(atomic_t *))
105 {
106 /*
107 * We have two variants here. The cmpxchg based one is the best one
108 * because it never induce a false contention state. It is included
109 * here because architectures using the inc/dec algorithms over the
110 * xchg ones are much more likely to support cmpxchg natively.
111 *
112 * If not we fall back to the spinlock based variant - that is
113 * just as efficient (and simpler) as a 'destructive' probing of
114 * the mutex state would be.
115 */
116 #ifdef __HAVE_ARCH_CMPXCHG
117 if (likely(atomic_cmpxchg(count, 1, 0) == 1))
118 return 1;
119 return 0;
120 #else
121 return fail_fn(count);
122 #endif
123 }
124
125 #endif /* _ASM_X86_MUTEX_32_H */
126