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
__mutex_fastpath_lock(atomic_t * count,void (* fail_fn)(atomic_t *))26 __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
27 {
28 if (unlikely(atomic_xchg(count, 0) != 1))
29 /*
30 * We failed to acquire the lock, so mark it contended
31 * to ensure that any waiting tasks are woken up by the
32 * unlock slow path.
33 */
34 if (likely(atomic_xchg(count, -1) != 1))
35 fail_fn(count);
36 }
37
38 /**
39 * __mutex_fastpath_lock_retval - try to take the lock by moving the count
40 * from 1 to a 0 value
41 * @count: pointer of type atomic_t
42 * @fail_fn: function to call if the original value was not 1
43 *
44 * Change the count from 1 to a value lower than 1, and call <fail_fn> if it
45 * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
46 * or anything the slow path function returns
47 */
48 static inline int
__mutex_fastpath_lock_retval(atomic_t * count,int (* fail_fn)(atomic_t *))49 __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
50 {
51 if (unlikely(atomic_xchg(count, 0) != 1))
52 if (likely(atomic_xchg(count, -1) != 1))
53 return fail_fn(count);
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 <function>
63 * In the failure case, this function is allowed to either set the value to
64 * 1, or to set it to a value lower than one.
65 * If the implementation sets it to a value of lower than one, the
66 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
67 * to return 0 otherwise.
68 */
69 static inline void
__mutex_fastpath_unlock(atomic_t * count,void (* fail_fn)(atomic_t *))70 __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
71 {
72 if (unlikely(atomic_xchg(count, 1) != 0))
73 fail_fn(count);
74 }
75
76 #define __mutex_slowpath_needs_to_unlock() 0
77
78 /**
79 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
80 *
81 * @count: pointer of type atomic_t
82 * @fail_fn: spinlock based trylock implementation
83 *
84 * Change the count from 1 to a value lower than 1, and return 0 (failure)
85 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
86 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
87 * Additionally, if the value was < 0 originally, this function must not leave
88 * it to 0 on failure.
89 *
90 * If the architecture has no effective trylock variant, it should call the
91 * <fail_fn> spinlock-based trylock variant unconditionally.
92 */
93 static inline int
__mutex_fastpath_trylock(atomic_t * count,int (* fail_fn)(atomic_t *))94 __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
95 {
96 int prev = atomic_xchg(count, 0);
97
98 if (unlikely(prev < 0)) {
99 /*
100 * The lock was marked contended so we must restore that
101 * state. If while doing so we get back a prev value of 1
102 * then we just own it.
103 *
104 * [ In the rare case of the mutex going to 1, to 0, to -1
105 * and then back to 0 in this few-instructions window,
106 * this has the potential to trigger the slowpath for the
107 * owner's unlock path needlessly, but that's not a problem
108 * in practice. ]
109 */
110 prev = atomic_xchg(count, prev);
111 if (prev < 0)
112 prev = 0;
113 }
114
115 return prev;
116 }
117
118 #endif
119