2 * include/asm-generic/mutex-xchg.h
4 * Generic implementation of the mutex fastpath, based on xchg().
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.
12 #ifndef _ASM_GENERIC_MUTEX_XCHG_H
13 #define _ASM_GENERIC_MUTEX_XCHG_H
16 * __mutex_fastpath_lock - try to take the lock by moving the count
18 * @count: pointer of type atomic_t
19 * @fail_fn: function to call if the original value was not 1
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.
26 __mutex_fastpath_lock(atomic_t
*count
, void (*fail_fn
)(atomic_t
*))
28 if (unlikely(atomic_xchg(count
, 0) != 1))
33 * __mutex_fastpath_lock_retval - try to take the lock by moving the count
35 * @count: pointer of type atomic_t
36 * @fail_fn: function to call if the original value was not 1
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
43 __mutex_fastpath_lock_retval(atomic_t
*count
, int (*fail_fn
)(atomic_t
*))
45 if (unlikely(atomic_xchg(count
, 0) != 1))
46 return fail_fn(count
);
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
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.
63 __mutex_fastpath_unlock(atomic_t
*count
, void (*fail_fn
)(atomic_t
*))
65 if (unlikely(atomic_xchg(count
, 1) != 0))
69 #define __mutex_slowpath_needs_to_unlock() 0
72 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
74 * @count: pointer of type atomic_t
75 * @fail_fn: spinlock based trylock implementation
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
83 * If the architecture has no effective trylock variant, it should call the
84 * <fail_fn> spinlock-based trylock variant unconditionally.
87 __mutex_fastpath_trylock(atomic_t
*count
, int (*fail_fn
)(atomic_t
*))
89 int prev
= atomic_xchg(count
, 0);
91 if (unlikely(prev
< 0)) {
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.
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
103 prev
= atomic_xchg(count
, prev
);
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