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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