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 "waiter.hpp"
  11
  12 #include <poll.h>
  13 #include <urcu/futex.h>
  14 #include <urcu/uatomic.h>
  15
  16 /*
  17  * Number of busy-loop attempts before waiting on futex.
  18  */
  19 #define WAIT_ATTEMPTS 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
  30 void lttng_waiter_init(struct lttng_waiter *waiter)
  31 {
  32         cds_wfs_node_init(&waiter->wait_queue_node);
  33         uatomic_set(&waiter->state, WAITER_WAITING);
  34         cmm_smp_mb();
  35 }
  36
  37 /*
  38  * User must init "waiter" before passing its memory to waker thread.
  39  */
  40 void lttng_waiter_wait(struct lttng_waiter *waiter)
  41 {
  42         unsigned int i;
  43
  44         DBG("Beginning of waiter wait period");
  45         /* Load and test condition before read state */
  46         cmm_smp_rmb();
  47         for (i = 0; i < WAIT_ATTEMPTS; i++) {
  48                 if (uatomic_read(&waiter->state) != WAITER_WAITING) {
  49                         goto skip_futex_wait;
  50                 }
  51                 caa_cpu_relax();
  52         }
  53         while (uatomic_read(&waiter->state) == WAITER_WAITING) {
  54                 if (!futex_noasync(
  55                             &waiter->state, FUTEX_WAIT, WAITER_WAITING, nullptr, nullptr, 0)) {
  56                         /*
  57                          * Prior queued wakeups queued by unrelated code
  58                          * using the same address can cause futex wait to
  59                          * return 0 even through the futex value is still
  60                          * WAITER_WAITING (spurious wakeups). Check
  61                          * the value again in user-space to validate
  62                          * whether it really differs from WAITER_WAITING.
  63                          */
  64                         continue;
  65                 }
  66                 switch (errno) {
  67                 case EAGAIN:
  68                         /* Value already changed. */
  69                         goto skip_futex_wait;
  70                 case EINTR:
  71                         /* Retry if interrupted by signal. */
  72                         break; /* Get out of switch. Check again. */
  73                 default:
  74                         /* Unexpected error. */
  75                         PERROR("futex_noasync");
  76                         abort();
  77                 }
  78         }
  79 skip_futex_wait:
  80
  81         /* Tell waker thread than we are running. */
  82         uatomic_or(&waiter->state, WAITER_RUNNING);
  83
  84         /*
  85          * Wait until waker thread lets us know it's ok to tear down
  86          * memory allocated for struct lttng_waiter.
  87          */
  88         for (i = 0; i < WAIT_ATTEMPTS; i++) {
  89                 if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) {
  90                         break;
  91                 }
  92                 caa_cpu_relax();
  93         }
  94         while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) {
  95                 poll(nullptr, 0, 10);
  96         }
  97         LTTNG_ASSERT(uatomic_read(&waiter->state) & WAITER_TEARDOWN);
  98         DBG("End of waiter wait period");
  99 }
 100
 101 /*
 102  * Note: lttng_waiter_wake needs waiter to stay allocated throughout its
 103  * execution. In this scheme, the waiter owns the node memory, and we only allow
 104  * it to free this memory when it sees the WAITER_TEARDOWN flag.
 105  */
 106 void lttng_waiter_wake_up(struct lttng_waiter *waiter)
 107 {
 108         cmm_smp_mb();
 109         LTTNG_ASSERT(uatomic_read(&waiter->state) == WAITER_WAITING);
 110         uatomic_set(&waiter->state, WAITER_WOKEN_UP);
 111         if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) {
 112                 if (futex_noasync(&waiter->state, FUTEX_WAKE, 1, nullptr, nullptr, 0) < 0) {
 113                         PERROR("futex_noasync");
 114                         abort();
 115                 }
 116         }
 117         /* Allow teardown of struct urcu_wait memory. */
 118         uatomic_or(&waiter->state, WAITER_TEARDOWN);
 119 }