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 */
43 struct ck_spinlock_dec {
44 unsigned int value;
45 };
46 typedef struct ck_spinlock_dec ck_spinlock_dec_t;
47
48 #define CK_SPINLOCK_DEC_INITIALIZER {1}
49
50 CK_CC_INLINE static void
ck_spinlock_dec_init(struct ck_spinlock_dec * lock)51 ck_spinlock_dec_init(struct ck_spinlock_dec *lock)
52 {
53
54 lock->value = 1;
55 ck_pr_barrier();
56 return;
57 }
58
59 CK_CC_INLINE static bool
ck_spinlock_dec_trylock(struct ck_spinlock_dec * lock)60 ck_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
69 CK_CC_INLINE static bool
ck_spinlock_dec_locked(struct ck_spinlock_dec * lock)70 ck_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
79 CK_CC_INLINE static void
ck_spinlock_dec_lock(struct ck_spinlock_dec * lock)80 ck_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
103 CK_CC_INLINE static void
ck_spinlock_dec_lock_eb(struct ck_spinlock_dec * lock)104 ck_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
122 CK_CC_INLINE static void
ck_spinlock_dec_unlock(struct ck_spinlock_dec * lock)123 ck_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
136 CK_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
140 CK_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