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 "error.h"
  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 LTTNG_HIDDEN
  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 LTTNG_HIDDEN
  41 void lttng_waiter_wait(struct lttng_waiter *waiter)
  42 {
  43         unsigned int i;
  44
  45         DBG("Beginning of waiter wait period");
  46         /* Load and test condition before read state */
  47         cmm_smp_rmb();
  48         for (i = 0; i < WAIT_ATTEMPTS; i++) {
  49                 if (uatomic_read(&waiter->state) != WAITER_WAITING) {
  50                         goto skip_futex_wait;
  51                 }
  52                 caa_cpu_relax();
  53         }
  54         while (futex_noasync(&waiter->state, FUTEX_WAIT, WAITER_WAITING,
  55                         NULL, NULL, 0)) {
  56                 switch (errno) {
  57                 case EWOULDBLOCK:
  58                         /* Value already changed. */
  59                         goto skip_futex_wait;
  60                 case EINTR:
  61                         /* Retry if interrupted by signal. */
  62                         break;  /* Get out of switch. */
  63                 default:
  64                         /* Unexpected error. */
  65                         PERROR("futex_noasync");
  66                         abort();
  67                 }
  68         }
  69 skip_futex_wait:
  70
  71         /* Tell waker thread than we are running. */
  72         uatomic_or(&waiter->state, WAITER_RUNNING);
  73
  74         /*
  75          * Wait until waker thread lets us know it's ok to tear down
  76          * memory allocated for struct lttng_waiter.
  77          */
  78         for (i = 0; i < WAIT_ATTEMPTS; i++) {
  79                 if (uatomic_read(&waiter->state) & WAITER_TEARDOWN) {
  80                         break;
  81                 }
  82                 caa_cpu_relax();
  83         }
  84         while (!(uatomic_read(&waiter->state) & WAITER_TEARDOWN)) {
  85                 poll(NULL, 0, 10);
  86         }
  87         LTTNG_ASSERT(uatomic_read(&waiter->state) & WAITER_TEARDOWN);
  88         DBG("End of waiter wait period");
  89 }
  90
  91 /*
  92  * Note: lttng_waiter_wake needs waiter to stay allocated throughout its
  93  * execution. In this scheme, the waiter owns the node memory, and we only allow
  94  * it to free this memory when it sees the WAITER_TEARDOWN flag.
  95  */
  96 LTTNG_HIDDEN
  97 void lttng_waiter_wake_up(struct lttng_waiter *waiter)
  98 {
  99         cmm_smp_mb();
 100         LTTNG_ASSERT(uatomic_read(&waiter->state) == WAITER_WAITING);
 101         uatomic_set(&waiter->state, WAITER_WOKEN_UP);
 102         if (!(uatomic_read(&waiter->state) & WAITER_RUNNING)) {
 103                 if (futex_noasync(&waiter->state, FUTEX_WAKE, 1,
 104                                 NULL, NULL, 0) < 0) {
 105                         PERROR("futex_noasync");
 106                         abort();
 107                 }
 108         }
 109         /* Allow teardown of struct urcu_wait memory. */
 110         uatomic_or(&waiter->state, WAITER_TEARDOWN);
 111 }