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1 | #ifndef _URCU_WAIT_H |
2 | #define _URCU_WAIT_H | |
3 | ||
4 | /* | |
5 | * urcu-wait.h | |
6 | * | |
7 | * Userspace RCU library wait/wakeup management | |
8 | * | |
9 | * Copyright (c) 2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> | |
10 | * | |
11 | * This library is free software; you can redistribute it and/or | |
12 | * modify it under the terms of the GNU Lesser General Public | |
13 | * License as published by the Free Software Foundation; either | |
14 | * version 2.1 of the License, or (at your option) any later version. | |
15 | * | |
16 | * This library is distributed in the hope that it will be useful, | |
17 | * but WITHOUT ANY WARRANTY; without even the implied warranty of | |
18 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
19 | * Lesser General Public License for more details. | |
20 | * | |
21 | * You should have received a copy of the GNU Lesser General Public | |
22 | * License along with this library; if not, write to the Free Software | |
23 | * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA | |
24 | */ | |
25 | ||
26 | #include <urcu/uatomic.h> | |
27 | ||
28 | /* | |
29 | * Number of busy-loop attempts before waiting on futex for grace period | |
30 | * batching. | |
31 | */ | |
32 | #define URCU_WAIT_ATTEMPTS 1000 | |
33 | ||
34 | enum urcu_wait_state { | |
35 | /* URCU_WAIT_WAITING is compared directly (futex compares it). */ | |
36 | URCU_WAIT_WAITING = 0, | |
37 | /* non-zero are used as masks. */ | |
38 | URCU_WAIT_WAKEUP = (1 << 0), | |
39 | URCU_WAIT_AWAKENED = (1 << 1), | |
40 | URCU_WAIT_TEARDOWN = (1 << 2), | |
41 | }; | |
42 | ||
43 | struct urcu_wait { | |
44 | int32_t futex; | |
45 | }; | |
46 | ||
47 | static inline | |
48 | void urcu_wait_init(struct urcu_wait *wait) | |
49 | { | |
50 | wait->futex = URCU_WAIT_WAITING; | |
51 | } | |
52 | ||
53 | /* | |
54 | * Note: urcu_adaptative_wake_up needs "value" to stay allocated | |
55 | * throughout its execution. In this scheme, the waiter owns the futex | |
56 | * memory, and we only allow it to free this memory when it receives the | |
57 | * URCU_WAIT_TEARDOWN flag. | |
58 | */ | |
59 | static inline | |
60 | void urcu_adaptative_wake_up(struct urcu_wait *wait) | |
61 | { | |
62 | cmm_smp_mb(); | |
63 | assert(uatomic_read(&wait->futex) == URCU_WAIT_WAITING); | |
64 | uatomic_set(&wait->futex, URCU_WAIT_WAKEUP); | |
65 | if (!(uatomic_read(&wait->futex) & URCU_WAIT_AWAKENED)) | |
66 | futex_noasync(&wait->futex, FUTEX_WAKE, 1, NULL, NULL, 0); | |
67 | /* Allow teardown of struct urcu_wait memory. */ | |
68 | uatomic_or(&wait->futex, URCU_WAIT_TEARDOWN); | |
69 | } | |
70 | ||
71 | /* | |
72 | * Caller must initialize "value" to URCU_WAIT_WAITING before passing its | |
73 | * memory to waker thread. | |
74 | */ | |
75 | static void urcu_adaptative_busy_wait(struct urcu_wait *wait) | |
76 | { | |
77 | unsigned int i; | |
78 | ||
79 | /* Load and test condition before read futex */ | |
80 | cmm_smp_rmb(); | |
81 | for (i = 0; i < URCU_WAIT_ATTEMPTS; i++) { | |
82 | if (uatomic_read(&wait->futex) != URCU_WAIT_WAITING) | |
83 | goto skip_futex_wait; | |
84 | caa_cpu_relax(); | |
85 | } | |
86 | futex_noasync(&wait->futex, FUTEX_WAIT, | |
87 | URCU_WAIT_WAITING, NULL, NULL, 0); | |
88 | skip_futex_wait: | |
89 | ||
90 | /* Tell waker thread than we are awakened. */ | |
91 | uatomic_or(&wait->futex, URCU_WAIT_AWAKENED); | |
92 | ||
93 | /* | |
94 | * Wait until waker thread lets us know it's ok to tear down | |
95 | * memory allocated for struct urcu_wait. | |
96 | */ | |
97 | for (i = 0; i < URCU_WAIT_ATTEMPTS; i++) { | |
98 | if (uatomic_read(&wait->futex) & URCU_WAIT_TEARDOWN) | |
99 | break; | |
100 | caa_cpu_relax(); | |
101 | } | |
102 | while (!(uatomic_read(&wait->futex) & URCU_WAIT_TEARDOWN)) | |
103 | poll(NULL, 0, 10); | |
104 | assert(uatomic_read(&wait->futex) & URCU_WAIT_TEARDOWN); | |
105 | } | |
106 | ||
107 | #endif /* _URCU_WAIT_H */ |