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 "waiter.hpp"
  10 #include <urcu/uatomic.h>
  11 #include <urcu/futex.h>
  12 #include "error.hpp"
  13 #include <poll.h>
  14
  15 /*
  16  * Number of busy-loop attempts before waiting on futex.
  17  */
  18 #define WAIT_ATTEMPTS 1000
  19
  20 enum waiter_state {
  21         /* WAITER_WAITING is compared directly (futex compares it). */
  22         WAITER_WAITING =        0,
  23         /* non-zero are used as masks. */
  24         WAITER_WOKEN_UP =       (1 << 0),
  25         WAITER_RUNNING =        (1 << 1),
  26         WAITER_TEARDOWN =       (1 << 2),
  27 };
  28
  29 void lttng_waiter_init(struct lttng_waiter *waiter)
  30 {
  31         cds_wfs_node_init(&waiter->wait_queue_node);
  32         uatomic_set(&waiter->state, WAITER_WAITING);
  33         cmm_smp_mb();
  34 }
  35
  36 /*
  37  * User must init "waiter" before passing its memory to waker thread.
  38  */
  39 void lttng_waiter_wait(struct lttng_waiter *waiter)
  40 {
  41         unsigned int i;
  42
  43         DBG("Beginning of waiter wait period");
  44         /* Load and test condition before read state */
  45         cmm_smp_rmb();
  46         for (i = 0; i < WAIT_ATTEMPTS; i++) {
  47                 if (uatomic_read(&waiter->state) != WAITER_WAITING) {
  48                         goto skip_futex_wait;
  49                 }
  50                 caa_cpu_relax();
  51         }
  52         while (futex_noasync(&waiter->state, FUTEX_WAIT, WAITER_WAITING,
  53                         NULL, NULL, 0)) {
  54                 switch (errno) {
  55                 case EWOULDBLOCK:
  56                         /* Value already changed. */
  57                         goto skip_futex_wait;
  58                 case EINTR:
  59                         /* Retry if interrupted by signal. */
  60                         break;  /* Get out of switch. */
  61                 default:
  62                         /* Unexpected error. */
  63                         PERROR("futex_noasync");
  64                         abort();
  65                 }
  66         }
  67 skip_futex_wait:
  68
  69         /* Tell waker thread than we are running. */
  70         uatomic_or(&waiter->state, WAITER_RUNNING);
  71
  72         /*
  73          * Wait until waker thread lets us know it's ok to tear down
  74          * memory allocated for struct lttng_waiter.
  75          */
  76         for (i = 0; i < WAIT_ATTEMPTS; i++) {
  77                 if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) {
  78                         break;
  79                 }
  80                 caa_cpu_relax();
  81         }
  82         while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) {
  83                 poll(NULL, 0, 10);
  84         }
  85         LTTNG_ASSERT(uatomic_read(&waiter->state) & WAITER_TEARDOWN);
  86         DBG("End of waiter wait period");
  87 }
  88
  89 /*
  90  * Note: lttng_waiter_wake needs waiter to stay allocated throughout its
  91  * execution. In this scheme, the waiter owns the node memory, and we only allow
  92  * it to free this memory when it sees the WAITER_TEARDOWN flag.
  93  */
  94 void lttng_waiter_wake_up(struct lttng_waiter *waiter)
  95 {
  96         cmm_smp_mb();
  97         LTTNG_ASSERT(uatomic_read(&waiter->state) == WAITER_WAITING);
  98         uatomic_set(&waiter->state, WAITER_WOKEN_UP);
  99         if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) {
 100                 if (futex_noasync(&waiter->state, FUTEX_WAKE, 1,
 101                                 NULL, NULL, 0) < 0) {
 102                         PERROR("futex_noasync");
 103                         abort();
 104                 }
 105         }
 106         /* Allow teardown of struct urcu_wait memory. */
 107         uatomic_or(&waiter->state, WAITER_TEARDOWN);
 108 }