| 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 | namespace { |
| 18 | /* Number of busy-loop attempts before waiting on futex. */ |
| 19 | constexpr auto wait_attempt_count = 1000; |
| 20 | |
| 21 | enum waiter_state { |
| 22 | /* WAITER_WAITING is compared directly (futex compares it). */ |
| 23 | WAITER_WAITING = 0, |
| 24 | /* non-zero are used as masks. */ |
| 25 | WAITER_WOKEN_UP = (1 << 0), |
| 26 | WAITER_RUNNING = (1 << 1), |
| 27 | WAITER_TEARDOWN = (1 << 2), |
| 28 | }; |
| 29 | } /* namespace */ |
| 30 | |
| 31 | lttng::synchro::waiter::waiter() |
| 32 | { |
| 33 | arm(); |
| 34 | } |
| 35 | |
| 36 | void lttng::synchro::waiter::arm() noexcept |
| 37 | { |
| 38 | cds_wfs_node_init(&_wait_queue_node); |
| 39 | uatomic_set(&_state, WAITER_WAITING); |
| 40 | cmm_smp_mb(); |
| 41 | } |
| 42 | |
| 43 | /* |
| 44 | * User must arm "waiter" before passing its memory to waker thread. |
| 45 | */ |
| 46 | void lttng::synchro::waiter::wait() |
| 47 | { |
| 48 | DBG("Beginning of waiter \"wait\" period"); |
| 49 | |
| 50 | /* Load and test condition before read state. */ |
| 51 | cmm_smp_rmb(); |
| 52 | for (unsigned int i = 0; i < wait_attempt_count; i++) { |
| 53 | if (uatomic_read(&_state) != WAITER_WAITING) { |
| 54 | goto skip_futex_wait; |
| 55 | } |
| 56 | |
| 57 | caa_cpu_relax(); |
| 58 | } |
| 59 | |
| 60 | while (uatomic_read(&_state) == WAITER_WAITING) { |
| 61 | if (!futex_noasync(&_state, FUTEX_WAIT, WAITER_WAITING, nullptr, nullptr, 0)) { |
| 62 | /* |
| 63 | * Prior queued wakeups queued by unrelated code |
| 64 | * using the same address can cause futex wait to |
| 65 | * return 0 even through the futex value is still |
| 66 | * WAITER_WAITING (spurious wakeups). Check |
| 67 | * the value again in user-space to validate |
| 68 | * whether it really differs from WAITER_WAITING. |
| 69 | */ |
| 70 | continue; |
| 71 | } |
| 72 | |
| 73 | switch (errno) { |
| 74 | case EAGAIN: |
| 75 | /* Value already changed. */ |
| 76 | goto skip_futex_wait; |
| 77 | case EINTR: |
| 78 | /* Retry if interrupted by signal. */ |
| 79 | break; /* Get out of switch. Check again. */ |
| 80 | default: |
| 81 | /* Unexpected error. */ |
| 82 | PERROR("futex_noasync"); |
| 83 | abort(); |
| 84 | } |
| 85 | } |
| 86 | skip_futex_wait: |
| 87 | |
| 88 | /* Tell waker thread than we are running. */ |
| 89 | uatomic_or(&_state, WAITER_RUNNING); |
| 90 | |
| 91 | /* |
| 92 | * Wait until waker thread lets us know it's ok to tear down |
| 93 | * memory allocated for struct lttng_waiter. |
| 94 | */ |
| 95 | for (unsigned int i = 0; i < wait_attempt_count; i++) { |
| 96 | if (uatomic_read(&_state) & WAITER_TEARDOWN) { |
| 97 | break; |
| 98 | } |
| 99 | |
| 100 | caa_cpu_relax(); |
| 101 | } |
| 102 | |
| 103 | while (!(uatomic_read(&_state) & WAITER_TEARDOWN)) { |
| 104 | poll(nullptr, 0, 10); |
| 105 | } |
| 106 | |
| 107 | LTTNG_ASSERT(uatomic_read(&_state) & WAITER_TEARDOWN); |
| 108 | DBG("End of waiter \"wait\" period"); |
| 109 | } |
| 110 | |
| 111 | lttng::synchro::waker lttng::synchro::waiter::get_waker() |
| 112 | { |
| 113 | return lttng::synchro::waker(_state); |
| 114 | } |
| 115 | |
| 116 | /* |
| 117 | * Note: lttng_waiter_wake needs waiter to stay allocated throughout its |
| 118 | * execution. In this scheme, the waiter owns the node memory, and we only allow |
| 119 | * it to free this memory when it sees the WAITER_TEARDOWN flag. |
| 120 | */ |
| 121 | void lttng::synchro::waker::wake() |
| 122 | { |
| 123 | cmm_smp_mb(); |
| 124 | |
| 125 | LTTNG_ASSERT(uatomic_read(&_state.get()) == WAITER_WAITING); |
| 126 | |
| 127 | uatomic_set(&_state.get(), WAITER_WOKEN_UP); |
| 128 | if (!(uatomic_read(&_state.get()) & WAITER_RUNNING)) { |
| 129 | if (futex_noasync(&_state.get(), FUTEX_WAKE, 1, nullptr, nullptr, 0) < 0) { |
| 130 | PERROR("futex_noasync"); |
| 131 | abort(); |
| 132 | } |
| 133 | } |
| 134 | |
| 135 | /* Allow teardown of struct urcu_wait memory. */ |
| 136 | uatomic_or(&_state.get(), WAITER_TEARDOWN); |
| 137 | } |
| 138 | |
| 139 | lttng::synchro::wait_queue::wait_queue() |
| 140 | { |
| 141 | cds_wfs_init(&_stack); |
| 142 | } |
| 143 | |
| 144 | void lttng::synchro::wait_queue::add(waiter& waiter) noexcept |
| 145 | { |
| 146 | (void) cds_wfs_push(&_stack, &waiter._wait_queue_node); |
| 147 | } |
| 148 | |
| 149 | void lttng::synchro::wait_queue::wake_all() |
| 150 | { |
| 151 | /* Move all waiters from the queue to our local stack. */ |
| 152 | auto *waiters = __cds_wfs_pop_all(&_stack); |
| 153 | |
| 154 | /* Wake all waiters in our stack head. */ |
| 155 | cds_wfs_node *iter, *iter_n; |
| 156 | cds_wfs_for_each_blocking_safe (waiters, iter, iter_n) { |
| 157 | auto& waiter = *lttng::utils::container_of( |
| 158 | iter, <tng::synchro::waiter::_wait_queue_node); |
| 159 | |
| 160 | /* Don't wake already running threads. */ |
| 161 | if (waiter._state & WAITER_RUNNING) { |
| 162 | continue; |
| 163 | } |
| 164 | |
| 165 | waiter.get_waker().wake(); |
| 166 | } |
| 167 | } |