src/common/waiter.c

   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 "waiter.h"
  10 #include <urcu/uatomic.h>
  11 #include <urcu/futex.h>
  12 #include <assert.h>
  13 #include "error.h"
  14 #include <poll.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 LTTNG_HIDDEN
  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 LTTNG_HIDDEN
  42 void lttng_waiter_wait(struct lttng_waiter *waiter)
  43 {
  44         unsigned int i;
  45
  46         DBG("Beginning of waiter wait period");
  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                 caa_cpu_relax();
  54         }
  55         while (uatomic_read(&waiter->state) == WAITER_WAITING) {
  56                 if (!futex_noasync(&waiter->state, FUTEX_WAIT, WAITER_WAITING, NULL, NULL, 0)) {
  57                         /*
  58                          * Prior queued wakeups queued by unrelated code
  59                          * using the same address can cause futex wait to
  60                          * return 0 even through the futex value is still
  61                          * WAITER_WAITING (spurious wakeups). Check
  62                          * the value again in user-space to validate
  63                          * whether it really differs from WAITER_WAITING.
  64                          */
  65                         continue;
  66                 }
  67                 switch (errno) {
  68                 case EAGAIN:
  69                         /* Value already changed. */
  70                         goto skip_futex_wait;
  71                 case EINTR:
  72                         /* Retry if interrupted by signal. */
  73                         break;  /* Get out of switch. Check again. */
  74                 default:
  75                         /* Unexpected error. */
  76                         PERROR("futex_noasync");
  77                         abort();
  78                 }
  79         }
  80 skip_futex_wait:
  81
  82         /* Tell waker thread than we are running. */
  83         uatomic_or(&waiter->state, WAITER_RUNNING);
  84
  85         /*
  86          * Wait until waker thread lets us know it's ok to tear down
  87          * memory allocated for struct lttng_waiter.
  88          */
  89         for (i = 0; i < WAIT_ATTEMPTS; i++) {
  90                 if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) {
  91                         break;
  92                 }
  93                 caa_cpu_relax();
  94         }
  95         while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) {
  96                 poll(NULL, 0, 10);
  97         }
  98         assert(uatomic_read(&waiter->state) & WAITER_TEARDOWN);
  99         DBG("End of waiter wait period");
 100 }
 101
 102 /*
 103  * Note: lttng_waiter_wake needs waiter to stay allocated throughout its
 104  * execution. In this scheme, the waiter owns the node memory, and we only allow
 105  * it to free this memory when it sees the WAITER_TEARDOWN flag.
 106  */
 107 LTTNG_HIDDEN
 108 void lttng_waiter_wake_up(struct lttng_waiter *waiter)
 109 {
 110         cmm_smp_mb();
 111         assert(uatomic_read(&waiter->state) == WAITER_WAITING);
 112         uatomic_set(&waiter->state, WAITER_WOKEN_UP);
 113         if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) {
 114                 if (futex_noasync(&waiter->state, FUTEX_WAKE, 1,
 115                                 NULL, NULL, 0) < 0) {
 116                         PERROR("futex_noasync");
 117                         abort();
 118                 }
 119         }
 120         /* Allow teardown of struct urcu_wait memory. */
 121         uatomic_or(&waiter->state, WAITER_TEARDOWN);
 122 }