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)			\
25do {								\
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 */
49static 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)			\
71do {								\
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 */
100static 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