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 *
46 * Change the count from 1 to a value lower than 1. This function returns 0
47 * if the fastpath succeeds, or -1 otherwise.
48 */
__mutex_fastpath_lock_retval(atomic_t * count)49 static inline int __mutex_fastpath_lock_retval(atomic_t *count)
50 {
51 if (unlikely(atomic_dec_return(count) < 0))
52 return -1;
53 else
54 return 0;
55 }
56
57 /**
58 * __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
59 * @count: pointer of type atomic_t
60 * @fail_fn: function to call if the original value was not 0
61 *
62 * try to promote the mutex from 0 to 1. if it wasn't 0, call <fail_fn>.
63 * In the failure case, this function is allowed to either set the value
64 * to 1, or to set it to a value lower than 1.
65 *
66 * If the implementation sets it to a value of lower than 1, the
67 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
68 * to return 0 otherwise.
69 */
70 #define __mutex_fastpath_unlock(count, fail_fn) \
71 do { \
72 unsigned int dummy; \
73 \
74 typecheck(atomic_t *, count); \
75 typecheck_fn(void (*)(atomic_t *), fail_fn); \
76 \
77 asm volatile(LOCK_PREFIX " incl (%%eax)\n" \
78 " jg 1f\n" \
79 " call " #fail_fn "\n" \
80 "1:\n" \
81 : "=a" (dummy) \
82 : "a" (count) \
83 : "memory", "ecx", "edx"); \
84 } while (0)
85
86 #define __mutex_slowpath_needs_to_unlock() 1
87
88 /**
89 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
90 *
91 * @count: pointer of type atomic_t
92 * @fail_fn: fallback function
93 *
94 * Change the count from 1 to a value lower than 1, and return 0 (failure)
95 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
96 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
97 * Additionally, if the value was < 0 originally, this function must not leave
98 * it to 0 on failure.
99 */
__mutex_fastpath_trylock(atomic_t * count,int (* fail_fn)(atomic_t *))100 static inline int __mutex_fastpath_trylock(atomic_t *count,
101 int (*fail_fn)(atomic_t *))
102 {
103 /* cmpxchg because it never induces a false contention state. */
104 if (likely(atomic_cmpxchg(count, 1, 0) == 1))
105 return 1;
106
107 return 0;
108 }
109
110 #endif /* _ASM_X86_MUTEX_32_H */
111