1/*
2 * Copyright 2010-2015 Samy Al Bahra.
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 *    notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 *    notice, this list of conditions and the following disclaimer in the
12 *    documentation and/or other materials provided with the distribution.
13 *
14 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
15 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
16 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
17 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
18 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
19 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
20 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
21 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
22 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
23 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
24 * SUCH DAMAGE.
25 */
26
27#ifndef CK_SPINLOCK_DEC_H
28#define CK_SPINLOCK_DEC_H
29
30#include <ck_backoff.h>
31#include <ck_cc.h>
32#include <ck_elide.h>
33#include <ck_pr.h>
34#include <ck_stdbool.h>
35
36#ifndef CK_F_SPINLOCK_DEC
37#define CK_F_SPINLOCK_DEC
38/*
39 * This is similar to the CACAS lock but makes use of an atomic decrement
40 * operation to check if the lock value was decremented to 0 from 1. The
41 * idea is that a decrement operation is cheaper than a compare-and-swap.
42 */
43struct ck_spinlock_dec {
44	unsigned int value;
45};
46typedef struct ck_spinlock_dec ck_spinlock_dec_t;
47
48#define CK_SPINLOCK_DEC_INITIALIZER	{1}
49
50CK_CC_INLINE static void
51ck_spinlock_dec_init(struct ck_spinlock_dec *lock)
52{
53
54	lock->value = 1;
55	ck_pr_barrier();
56	return;
57}
58
59CK_CC_INLINE static bool
60ck_spinlock_dec_trylock(struct ck_spinlock_dec *lock)
61{
62	unsigned int value;
63
64	value = ck_pr_fas_uint(&lock->value, 0);
65	ck_pr_fence_lock();
66	return value == 1;
67}
68
69CK_CC_INLINE static bool
70ck_spinlock_dec_locked(struct ck_spinlock_dec *lock)
71{
72	bool r;
73
74	r = ck_pr_load_uint(&lock->value) != 1;
75	ck_pr_fence_acquire();
76	return r;
77}
78
79CK_CC_INLINE static void
80ck_spinlock_dec_lock(struct ck_spinlock_dec *lock)
81{
82	bool r;
83
84	for (;;) {
85		/*
86		 * Only one thread is guaranteed to decrement lock to 0.
87		 * Overflow must be protected against. No more than
88		 * UINT_MAX lock requests can happen while the lock is held.
89		 */
90		ck_pr_dec_uint_zero(&lock->value, &r);
91		if (r == true)
92			break;
93
94		/* Load value without generating write cycles. */
95		while (ck_pr_load_uint(&lock->value) != 1)
96			ck_pr_stall();
97	}
98
99	ck_pr_fence_lock();
100	return;
101}
102
103CK_CC_INLINE static void
104ck_spinlock_dec_lock_eb(struct ck_spinlock_dec *lock)
105{
106	ck_backoff_t backoff = CK_BACKOFF_INITIALIZER;
107	bool r;
108
109	for (;;) {
110		ck_pr_dec_uint_zero(&lock->value, &r);
111		if (r == true)
112			break;
113
114		while (ck_pr_load_uint(&lock->value) != 1)
115			ck_backoff_eb(&backoff);
116	}
117
118	ck_pr_fence_lock();
119	return;
120}
121
122CK_CC_INLINE static void
123ck_spinlock_dec_unlock(struct ck_spinlock_dec *lock)
124{
125
126	ck_pr_fence_unlock();
127
128	/*
129	 * Unconditionally set lock value to 1 so someone can decrement lock
130	 * to 0.
131	 */
132	ck_pr_store_uint(&lock->value, 1);
133	return;
134}
135
136CK_ELIDE_PROTOTYPE(ck_spinlock_dec, ck_spinlock_dec_t,
137    ck_spinlock_dec_locked, ck_spinlock_dec_lock,
138    ck_spinlock_dec_locked, ck_spinlock_dec_unlock)
139
140CK_ELIDE_TRYLOCK_PROTOTYPE(ck_spinlock_dec, ck_spinlock_dec_t,
141    ck_spinlock_dec_locked, ck_spinlock_dec_trylock)
142
143#endif /* CK_F_SPINLOCK_DEC */
144#endif /* CK_SPINLOCK_DEC_H */
145