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 */
25static inline void
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 */
44static inline int
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 */
67static inline void
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 */
92static inline int
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