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))
30 * We failed to acquire the lock, so mark it contended
31 * to ensure that any waiting tasks are woken up by the
34 if (likely(atomic_xchg(count
, -1) != 1))
39 * __mutex_fastpath_lock_retval - try to take the lock by moving the count
41 * @count: pointer of type atomic_t
43 * Change the count from 1 to a value lower than 1. This function returns 0
44 * if the fastpath succeeds, or -1 otherwise.
47 __mutex_fastpath_lock_retval(atomic_t
*count
)
49 if (unlikely(atomic_xchg(count
, 0) != 1))
50 if (likely(atomic_xchg(count
, -1) != 1))
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
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.
68 __mutex_fastpath_unlock(atomic_t
*count
, void (*fail_fn
)(atomic_t
*))
70 if (unlikely(atomic_xchg(count
, 1) != 0))
74 #define __mutex_slowpath_needs_to_unlock() 0
77 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
79 * @count: pointer of type atomic_t
80 * @fail_fn: spinlock based trylock implementation
82 * Change the count from 1 to a value lower than 1, and return 0 (failure)
83 * if it wasn't 1 originally, or return 1 (success) otherwise. This function
84 * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
85 * Additionally, if the value was < 0 originally, this function must not leave
88 * If the architecture has no effective trylock variant, it should call the
89 * <fail_fn> spinlock-based trylock variant unconditionally.
92 __mutex_fastpath_trylock(atomic_t
*count
, int (*fail_fn
)(atomic_t
*))
94 int prev
= atomic_xchg(count
, 0);
96 if (unlikely(prev
< 0)) {
98 * The lock was marked contended so we must restore that
99 * state. If while doing so we get back a prev value of 1
100 * then we just own it.
102 * [ In the rare case of the mutex going to 1, to 0, to -1
103 * and then back to 0 in this few-instructions window,
104 * this has the potential to trigger the slowpath for the
105 * owner's unlock path needlessly, but that's not a problem
108 prev
= atomic_xchg(count
, prev
);
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