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