1 /*
2 * asm-generic/mutex-xchg.h
3 *
4 * Generic implementation of the mutex fastpath, based on xchg().
5 *
6 * NOTE: An xchg based implementation might be less optimal than an atomic
7 * decrement/increment based implementation. If your architecture
8 * has a reasonable atomic dec/inc then you should probably use
9 * asm-generic/mutex-dec.h instead, or you could open-code an
10 * optimized version in asm/mutex.h.
11 */
12 #ifndef _ASM_GENERIC_MUTEX_XCHG_H
13 #define _ASM_GENERIC_MUTEX_XCHG_H
14
15 /**
16 * __mutex_fastpath_lock - try to take the lock by moving the count
17 * from 1 to a 0 value
18 * @count: pointer of type atomic_t
19 * @fail_fn: function to call if the original value was not 1
20 *
21 * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
22 * wasn't 1 originally. This function MUST leave the value lower than 1
23 * even when the "1" assertion wasn't true.
24 */
25 static inline void
__mutex_fastpath_lock(atomic_t * count,fastcall void (* fail_fn)(atomic_t *))26 __mutex_fastpath_lock(atomic_t *count, fastcall void (*fail_fn)(atomic_t *))
27 {
28 if (unlikely(atomic_xchg(count, 0) != 1))
29 fail_fn(count);
30 else
31 smp_mb();
32 }
33
34 /**
35 * __mutex_fastpath_lock_retval - try to take the lock by moving the count
36 * from 1 to a 0 value
37 * @count: pointer of type atomic_t
38 * @fail_fn: function to call if the original value was not 1
39 *
40 * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
41 * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
42 * or anything the slow path function returns
43 */
44 static inline int
__mutex_fastpath_lock_retval(atomic_t * count,fastcall int (* fail_fn)(atomic_t *))45 __mutex_fastpath_lock_retval(atomic_t *count, fastcall int (*fail_fn)(atomic_t *))
46 {
47 if (unlikely(atomic_xchg(count, 0) != 1))
48 return fail_fn(count);
49 else {
50 smp_mb();
51 return 0;
52 }
53 }
54
55 /**
56 * __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
57 * @count: pointer of type atomic_t
58 * @fail_fn: function to call if the original value was not 0
59 *
60 * try to promote the mutex from 0 to 1. if it wasn't 0, call <function>
61 * In the failure case, this function is allowed to either set the value to
62 * 1, or to set it to a value lower than one.
63 * If the implementation sets it to a value of lower than one, the
64 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
65 * to return 0 otherwise.
66 */
67 static inline void
__mutex_fastpath_unlock(atomic_t * count,fastcall void (* fail_fn)(atomic_t *))68 __mutex_fastpath_unlock(atomic_t *count, fastcall void (*fail_fn)(atomic_t *))
69 {
70 smp_mb();
71 if (unlikely(atomic_xchg(count, 1) != 0))
72 fail_fn(count);
73 }
74
75 #define __mutex_slowpath_needs_to_unlock() 0
76
77 /**
78 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
79 *
80 * @count: pointer of type atomic_t
81 * @fail_fn: spinlock based trylock implementation
82 *
83 * Change the count from 1 to a value lower than 1, and return 0 (failure)
84 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
85 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
86 * Additionally, if the value was < 0 originally, this function must not leave
87 * it to 0 on failure.
88 *
89 * If the architecture has no effective trylock variant, it should call the
90 * <fail_fn> spinlock-based trylock variant unconditionally.
91 */
92 static inline int
__mutex_fastpath_trylock(atomic_t * count,int (* fail_fn)(atomic_t *))93 __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
94 {
95 int prev = atomic_xchg(count, 0);
96
97 if (unlikely(prev < 0)) {
98 /*
99 * The lock was marked contended so we must restore that
100 * state. If while doing so we get back a prev value of 1
101 * then we just own it.
102 *
103 * [ In the rare case of the mutex going to 1, to 0, to -1
104 * and then back to 0 in this few-instructions window,
105 * this has the potential to trigger the slowpath for the
106 * owner's unlock path needlessly, but that's not a problem
107 * in practice. ]
108 */
109 prev = atomic_xchg(count, prev);
110 if (prev < 0)
111 prev = 0;
112 }
113 smp_mb();
114
115 return prev;
116 }
117
118 #endif
119