| 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 */ |