1 /*
2 * include/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
26 __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
27 {
28 if (unlikely(atomic_xchg(count, 0) != 1))
29 fail_fn(count);
30 }
31
32 /**
33 * __mutex_fastpath_lock_retval - try to take the lock by moving the count
34 * from 1 to a 0 value
35 * @count: pointer of type atomic_t
36 * @fail_fn: function to call if the original value was not 1
37 *
38 * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
39 * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
40 * or anything the slow path function returns
41 */
42 static inline int
43 __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
44 {
45 if (unlikely(atomic_xchg(count, 0) != 1))
46 return fail_fn(count);
47 return 0;
48 }
49
50 /**
51 * __mutex_fastpath_unlock - try to promote the mutex from 0 to 1
52 * @count: pointer of type atomic_t
53 * @fail_fn: function to call if the original value was not 0
54 *
55 * try to promote the mutex from 0 to 1. if it wasn't 0, call <function>
56 * In the failure case, this function is allowed to either set the value to
57 * 1, or to set it to a value lower than one.
58 * If the implementation sets it to a value of lower than one, the
59 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
60 * to return 0 otherwise.
61 */
62 static inline void
63 __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
64 {
65 if (unlikely(atomic_xchg(count, 1) != 0))
66 fail_fn(count);
67 }
68
69 #define __mutex_slowpath_needs_to_unlock() 0
70
71 /**
72 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
73 *
74 * @count: pointer of type atomic_t
75 * @fail_fn: spinlock based trylock implementation
76 *
77 * Change the count from 1 to a value lower than 1, and return 0 (failure)
78 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
79 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
80 * Additionally, if the value was < 0 originally, this function must not leave
81 * it to 0 on failure.
82 *
83 * If the architecture has no effective trylock variant, it should call the
84 * <fail_fn> spinlock-based trylock variant unconditionally.
85 */
86 static inline int
87 __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
88 {
89 int prev = atomic_xchg(count, 0);
90
91 if (unlikely(prev < 0)) {
92 /*
93 * The lock was marked contended so we must restore that
94 * state. If while doing so we get back a prev value of 1
95 * then we just own it.
96 *
97 * [ In the rare case of the mutex going to 1, to 0, to -1
98 * and then back to 0 in this few-instructions window,
99 * this has the potential to trigger the slowpath for the
100 * owner's unlock path needlessly, but that's not a problem
101 * in practice. ]
102 */
103 prev = atomic_xchg(count, prev);
104 if (prev < 0)
105 prev = 0;
106 }
107
108 return prev;
109 }
110
111 #endif
112
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