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(&waiter->state, FUTEX_WAIT, WAITER_WAITING, NULL, NULL, 0)) {
  55                         /*
  56                          * Prior queued wakeups queued by unrelated code
  57                          * using the same address can cause futex wait to
  58                          * return 0 even through the futex value is still
  59                          * WAITER_WAITING (spurious wakeups). Check
  60                          * the value again in user-space to validate
  61                          * whether it really differs from WAITER_WAITING.
  62                          */
  63                         continue;
  64                 }
  65                 switch (errno) {
  66                 case EAGAIN:
  67                         /* Value already changed. */
  68                         goto skip_futex_wait;
  69                 case EINTR:
  70                         /* Retry if interrupted by signal. */
  71                         break; /* Get out of switch. Check again. */
  72                 default:
  73                         /* Unexpected error. */
  74                         PERROR("futex_noasync");
  75                         abort();
  76                 }
  77         }
  78 skip_futex_wait:
  79
  80         /* Tell waker thread than we are running. */
  81         uatomic_or(&waiter->state, WAITER_RUNNING);
  82
  83         /*
  84          * Wait until waker thread lets us know it's ok to tear down
  85          * memory allocated for struct lttng_waiter.
  86          */
  87         for (i = 0; i < WAIT_ATTEMPTS; i++) {
  88                 if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) {
  89                         break;
  90                 }
  91                 caa_cpu_relax();
  92         }
  93         while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) {
  94                 poll(NULL, 0, 10);
  95         }
  96         LTTNG_ASSERT(uatomic_read(&waiter->state) & WAITER_TEARDOWN);
  97         DBG("End of waiter wait period");
  98 }
  99
 100 /*
 101  * Note: lttng_waiter_wake needs waiter to stay allocated throughout its
 102  * execution. In this scheme, the waiter owns the node memory, and we only allow
 103  * it to free this memory when it sees the WAITER_TEARDOWN flag.
 104  */
 105 void lttng_waiter_wake_up(struct lttng_waiter *waiter)
 106 {
 107         cmm_smp_mb();
 108         LTTNG_ASSERT(uatomic_read(&waiter->state) == WAITER_WAITING);
 109         uatomic_set(&waiter->state, WAITER_WOKEN_UP);
 110         if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) {
 111                 if (futex_noasync(&waiter->state, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
 112                         PERROR("futex_noasync");
 113                         abort();
 114                 }
 115         }
 116         /* Allow teardown of struct urcu_wait memory. */
 117         uatomic_or(&waiter->state, WAITER_TEARDOWN);
 118 }