| 1 | /* |
| 2 | * Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca> |
| 3 | * Mathieu Desnoyers <mathieu.desnoyers@efficios.com> |
| 4 | * |
| 5 | * This program is free software; you can redistribute it and/or modify |
| 6 | * it under the terms of the GNU General Public License, version 2 only, |
| 7 | * as published by the Free Software Foundation. |
| 8 | * |
| 9 | * This program is distributed in the hope that it will be useful, |
| 10 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 11 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 12 | * GNU General Public License for more details. |
| 13 | * |
| 14 | * You should have received a copy of the GNU General Public License along |
| 15 | * with this program; if not, write to the Free Software Foundation, Inc., |
| 16 | * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. |
| 17 | */ |
| 18 | |
| 19 | #define _LGPL_SOURCE |
| 20 | #include <limits.h> |
| 21 | #include <unistd.h> |
| 22 | #include <urcu.h> |
| 23 | #include <urcu/futex.h> |
| 24 | |
| 25 | #include <common/common.h> |
| 26 | |
| 27 | #include "futex.h" |
| 28 | |
| 29 | /* |
| 30 | * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the |
| 31 | * "nto1" added to all function signature. |
| 32 | * |
| 33 | * The code is adapted from the adaptative busy wait/wake_up scheme used in |
| 34 | * liburcu. |
| 35 | */ |
| 36 | |
| 37 | /* Number of busy-loop attempts before waiting on futex. */ |
| 38 | #define FUTEX_WAIT_ATTEMPTS 1000 |
| 39 | |
| 40 | enum futex_wait_state { |
| 41 | /* FUTEX_WAIT_WAITING is compared directly (futex() compares it). */ |
| 42 | FUTEX_WAIT_WAITING = 0, |
| 43 | /* non-zero are used as masks. */ |
| 44 | FUTEX_WAIT_WAKEUP = (1 << 0), |
| 45 | FUTEX_WAIT_RUNNING = (1 << 1), |
| 46 | FUTEX_WAIT_TEARDOWN = (1 << 2), |
| 47 | }; |
| 48 | |
| 49 | /* |
| 50 | * Update futex according to active or not. This scheme is used to wake every |
| 51 | * libust waiting on the shared memory map futex hence the INT_MAX used in the |
| 52 | * futex() call. If active, we set the value and wake everyone else we indicate |
| 53 | * that we are gone (cleanup() case). |
| 54 | */ |
| 55 | LTTNG_HIDDEN |
| 56 | void futex_wait_update(int32_t *futex, int active) |
| 57 | { |
| 58 | if (active) { |
| 59 | uatomic_set(futex, 1); |
| 60 | if (futex_async(futex, FUTEX_WAKE, |
| 61 | INT_MAX, NULL, NULL, 0) < 0) { |
| 62 | PERROR("futex_async"); |
| 63 | abort(); |
| 64 | } |
| 65 | } else { |
| 66 | uatomic_set(futex, 0); |
| 67 | } |
| 68 | |
| 69 | DBG("Futex wait update active %d", active); |
| 70 | } |
| 71 | |
| 72 | /* |
| 73 | * Prepare futex. |
| 74 | */ |
| 75 | LTTNG_HIDDEN |
| 76 | void futex_nto1_prepare(int32_t *futex) |
| 77 | { |
| 78 | uatomic_set(futex, FUTEX_WAIT_WAITING); |
| 79 | cmm_smp_mb(); |
| 80 | |
| 81 | DBG("Futex n to 1 prepare done"); |
| 82 | } |
| 83 | |
| 84 | /* |
| 85 | * Wait futex. |
| 86 | */ |
| 87 | LTTNG_HIDDEN |
| 88 | void futex_nto1_wait(int32_t *futex) |
| 89 | { |
| 90 | unsigned int i; |
| 91 | |
| 92 | /* Load and test condition before read state */ |
| 93 | cmm_smp_rmb(); |
| 94 | for (i = 0; i < FUTEX_WAIT_ATTEMPTS; i++) { |
| 95 | if (uatomic_read(futex) != FUTEX_WAIT_WAITING) |
| 96 | goto skip_futex_wait; |
| 97 | caa_cpu_relax(); |
| 98 | } |
| 99 | while (futex_noasync(futex, FUTEX_WAIT, FUTEX_WAIT_WAITING, |
| 100 | NULL, NULL, 0)) { |
| 101 | switch (errno) { |
| 102 | case EWOULDBLOCK: |
| 103 | /* Value already changed. */ |
| 104 | goto skip_futex_wait; |
| 105 | case EINTR: |
| 106 | /* Retry if interrupted by signal. */ |
| 107 | break; /* Get out of switch. */ |
| 108 | default: |
| 109 | /* Unexpected error. */ |
| 110 | PERROR("futex"); |
| 111 | abort(); |
| 112 | } |
| 113 | } |
| 114 | skip_futex_wait: |
| 115 | |
| 116 | /* Tell waker thread than we are running. */ |
| 117 | uatomic_or(futex, FUTEX_WAIT_RUNNING); |
| 118 | |
| 119 | /* |
| 120 | * Wait until waker thread lets us know it's ok to tear down |
| 121 | * memory allocated for the futex. |
| 122 | */ |
| 123 | for (i = 0; i < FUTEX_WAIT_ATTEMPTS; i++) { |
| 124 | if (uatomic_read(futex) & FUTEX_WAIT_TEARDOWN) |
| 125 | break; |
| 126 | caa_cpu_relax(); |
| 127 | } |
| 128 | while (!(uatomic_read(futex) & FUTEX_WAIT_TEARDOWN)) |
| 129 | poll(NULL, 0, 10); |
| 130 | assert(uatomic_read(futex) & FUTEX_WAIT_TEARDOWN); |
| 131 | DBG("Futex n to 1 wait done"); |
| 132 | } |
| 133 | |
| 134 | /* |
| 135 | * Wake 1 futex. |
| 136 | */ |
| 137 | LTTNG_HIDDEN |
| 138 | void futex_nto1_wake(int32_t *futex) |
| 139 | { |
| 140 | cmm_smp_mb(); |
| 141 | uatomic_set(futex, FUTEX_WAIT_WAKEUP); |
| 142 | if (!(uatomic_read(futex) & FUTEX_WAIT_RUNNING)) { |
| 143 | if (futex_noasync(futex, FUTEX_WAKE, 1, |
| 144 | NULL, NULL, 0) < 0) { |
| 145 | PERROR("futex_noasync"); |
| 146 | abort(); |
| 147 | } |
| 148 | } |
| 149 | /* Allow teardown of futex. */ |
| 150 | uatomic_or(futex, FUTEX_WAIT_TEARDOWN); |
| 151 | DBG("Futex n to 1 wake done"); |
| 152 | } |