src/common/waiter.cpp

   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, &lttng_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 }