| 1 | /* |
| 2 | * Copyright (C) 2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> |
| 3 | * Copyright (C) 2017 Jérémie Galarneau <jeremie.galarneau@efficios.com> |
| 4 | * |
| 5 | * SPDX-License-Identifier: LGPL-2.1-only |
| 6 | * |
| 7 | */ |
| 8 | |
| 9 | #include "error.hpp" |
| 10 | #include "macros.hpp" |
| 11 | #include "waiter.hpp" |
| 12 | |
| 13 | #include <poll.h> |
| 14 | #include <urcu/futex.h> |
| 15 | #include <urcu/uatomic.h> |
| 16 | |
| 17 | /* |
| 18 | * Number of busy-loop attempts before waiting on futex. |
| 19 | */ |
| 20 | #define WAIT_ATTEMPTS 1000 |
| 21 | |
| 22 | enum waiter_state { |
| 23 | /* WAITER_WAITING is compared directly (futex compares it). */ |
| 24 | WAITER_WAITING = 0, |
| 25 | /* non-zero are used as masks. */ |
| 26 | WAITER_WOKEN_UP = (1 << 0), |
| 27 | WAITER_RUNNING = (1 << 1), |
| 28 | WAITER_TEARDOWN = (1 << 2), |
| 29 | }; |
| 30 | |
| 31 | void lttng_waiter_init(struct lttng_waiter *waiter) |
| 32 | { |
| 33 | cds_wfs_node_init(&waiter->wait_queue_node); |
| 34 | uatomic_set(&waiter->state, WAITER_WAITING); |
| 35 | cmm_smp_mb(); |
| 36 | } |
| 37 | |
| 38 | /* |
| 39 | * User must init "waiter" before passing its memory to waker thread. |
| 40 | */ |
| 41 | void lttng_waiter_wait(struct lttng_waiter *waiter) |
| 42 | { |
| 43 | unsigned int i; |
| 44 | |
| 45 | DBG("Beginning of waiter \"wait\" period"); |
| 46 | |
| 47 | /* Load and test condition before read state. */ |
| 48 | cmm_smp_rmb(); |
| 49 | for (i = 0; i < WAIT_ATTEMPTS; i++) { |
| 50 | if (uatomic_read(&waiter->state) != WAITER_WAITING) { |
| 51 | goto skip_futex_wait; |
| 52 | } |
| 53 | |
| 54 | caa_cpu_relax(); |
| 55 | } |
| 56 | |
| 57 | while (uatomic_read(&waiter->state) == WAITER_WAITING) { |
| 58 | if (!futex_noasync( |
| 59 | &waiter->state, FUTEX_WAIT, WAITER_WAITING, nullptr, nullptr, 0)) { |
| 60 | /* |
| 61 | * Prior queued wakeups queued by unrelated code |
| 62 | * using the same address can cause futex wait to |
| 63 | * return 0 even through the futex value is still |
| 64 | * WAITER_WAITING (spurious wakeups). Check |
| 65 | * the value again in user-space to validate |
| 66 | * whether it really differs from WAITER_WAITING. |
| 67 | */ |
| 68 | continue; |
| 69 | } |
| 70 | |
| 71 | switch (errno) { |
| 72 | case EAGAIN: |
| 73 | /* Value already changed. */ |
| 74 | goto skip_futex_wait; |
| 75 | case EINTR: |
| 76 | /* Retry if interrupted by signal. */ |
| 77 | break; /* Get out of switch. Check again. */ |
| 78 | default: |
| 79 | /* Unexpected error. */ |
| 80 | PERROR("futex_noasync"); |
| 81 | abort(); |
| 82 | } |
| 83 | } |
| 84 | skip_futex_wait: |
| 85 | |
| 86 | /* Tell waker thread than we are running. */ |
| 87 | uatomic_or(&waiter->state, WAITER_RUNNING); |
| 88 | |
| 89 | /* |
| 90 | * Wait until waker thread lets us know it's ok to tear down |
| 91 | * memory allocated for struct lttng_waiter. |
| 92 | */ |
| 93 | for (i = 0; i < WAIT_ATTEMPTS; i++) { |
| 94 | if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) { |
| 95 | break; |
| 96 | } |
| 97 | |
| 98 | caa_cpu_relax(); |
| 99 | } |
| 100 | |
| 101 | while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) { |
| 102 | poll(nullptr, 0, 10); |
| 103 | } |
| 104 | |
| 105 | LTTNG_ASSERT(uatomic_read(&waiter->state) & WAITER_TEARDOWN); |
| 106 | DBG("End of waiter \"wait\" period"); |
| 107 | } |
| 108 | |
| 109 | /* |
| 110 | * Note: lttng_waiter_wake needs waiter to stay allocated throughout its |
| 111 | * execution. In this scheme, the waiter owns the node memory, and we only allow |
| 112 | * it to free this memory when it sees the WAITER_TEARDOWN flag. |
| 113 | */ |
| 114 | void lttng_waiter_wake(struct lttng_waiter *waiter) |
| 115 | { |
| 116 | cmm_smp_mb(); |
| 117 | LTTNG_ASSERT(uatomic_read(&waiter->state) == WAITER_WAITING); |
| 118 | uatomic_set(&waiter->state, WAITER_WOKEN_UP); |
| 119 | if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) { |
| 120 | if (futex_noasync(&waiter->state, FUTEX_WAKE, 1, nullptr, nullptr, 0) < 0) { |
| 121 | PERROR("futex_noasync"); |
| 122 | abort(); |
| 123 | } |
| 124 | } |
| 125 | |
| 126 | /* Allow teardown of struct urcu_wait memory. */ |
| 127 | uatomic_or(&waiter->state, WAITER_TEARDOWN); |
| 128 | } |
| 129 | |
| 130 | void lttng_wait_queue_init(struct lttng_wait_queue *queue) |
| 131 | { |
| 132 | cds_wfs_init(&queue->stack); |
| 133 | } |
| 134 | |
| 135 | void lttng_wait_queue_add(struct lttng_wait_queue *queue, struct lttng_waiter *waiter) |
| 136 | { |
| 137 | (void) cds_wfs_push(&queue->stack, &waiter->wait_queue_node); |
| 138 | } |
| 139 | |
| 140 | void lttng_wait_queue_wake_all(struct lttng_wait_queue *queue) |
| 141 | { |
| 142 | cds_wfs_head *waiters; |
| 143 | cds_wfs_node *iter, *iter_n; |
| 144 | |
| 145 | /* Move all waiters from the queue to our local stack. */ |
| 146 | waiters = __cds_wfs_pop_all(&queue->stack); |
| 147 | |
| 148 | /* Wake all waiters in our stack head. */ |
| 149 | cds_wfs_for_each_blocking_safe (waiters, iter, iter_n) { |
| 150 | auto *waiter = lttng::utils::container_of(iter, <tng_waiter::wait_queue_node); |
| 151 | |
| 152 | /* Don't wake already running threads. */ |
| 153 | if (waiter->state & WAITER_RUNNING) { |
| 154 | continue; |
| 155 | } |
| 156 | |
| 157 | lttng_waiter_wake(waiter); |
| 158 | } |
| 159 | } |