src/common/futex.c

   1 /*
   2  * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
   3  * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   4  *
   5  * SPDX-License-Identifier: GPL-2.0-only
   6  *
   7  */
   8
   9 #define _LGPL_SOURCE
  10 #include <limits.h>
  11 #include <unistd.h>
  12 #include <urcu.h>
  13 #include <urcu/futex.h>
  14
  15 #include <common/common.h>
  16
  17 #include "futex.h"
  18
  19 /*
  20  * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
  21  * "nto1" added to all function signature.
  22  *
  23  * Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
  24  * git tree for a detail example of this scheme being used. futex_async() is
  25  * the urcu wrapper over the futex() sycall.
  26  *
  27  * There is also a formal verification available in the git tree.
  28  *
  29  *   branch: formal-model
  30  *   commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
  31  *
  32  * Ref: git://git.lttng.org/userspace-rcu.git
  33  */
  34
  35 /*
  36  * Update futex according to active or not. This scheme is used to wake every
  37  * libust waiting on the shared memory map futex hence the INT_MAX used in the
  38  * futex() call. If active, we set the value and wake everyone else we indicate
  39  * that we are gone (cleanup() case).
  40  */
  41 LTTNG_HIDDEN
  42 void futex_wait_update(int32_t *futex, int active)
  43 {
  44         if (active) {
  45                 uatomic_set(futex, 1);
  46                 if (futex_async(futex, FUTEX_WAKE,
  47                                 INT_MAX, NULL, NULL, 0) < 0) {
  48                         PERROR("futex_async");
  49                         abort();
  50                 }
  51         } else {
  52                 uatomic_set(futex, 0);
  53         }
  54
  55         DBG("Futex wait update active %d", active);
  56 }
  57
  58 /*
  59  * Prepare futex.
  60  */
  61 LTTNG_HIDDEN
  62 void futex_nto1_prepare(int32_t *futex)
  63 {
  64         uatomic_set(futex, -1);
  65         cmm_smp_mb();
  66
  67         DBG("Futex n to 1 prepare done");
  68 }
  69
  70 /*
  71  * Wait futex.
  72  */
  73 LTTNG_HIDDEN
  74 void futex_nto1_wait(int32_t *futex)
  75 {
  76         cmm_smp_mb();
  77
  78         if (uatomic_read(futex) != -1)
  79                 goto end;
  80         while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
  81                 switch (errno) {
  82                 case EWOULDBLOCK:
  83                         /* Value already changed. */
  84                         goto end;
  85                 case EINTR:
  86                         /* Retry if interrupted by signal. */
  87                         break;  /* Get out of switch. */
  88                 default:
  89                         /* Unexpected error. */
  90                         PERROR("futex_async");
  91                         abort();
  92                 }
  93         }
  94 end:
  95         DBG("Futex n to 1 wait done");
  96 }
  97
  98 /*
  99  * Wake 1 futex.
 100  */
 101 LTTNG_HIDDEN
 102 void futex_nto1_wake(int32_t *futex)
 103 {
 104         if (caa_unlikely(uatomic_read(futex) != -1))
 105                 goto end;
 106         uatomic_set(futex, 0);
 107         if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
 108                 PERROR("futex_async");
 109                 abort();
 110         }
 111 end:
 112         DBG("Futex n to 1 wake done");
 113 }