src/common/futex.c

   1 /*
   2  * Copyright (C) 2011 EfficiOS Inc.
   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         while (uatomic_read(futex) == -1) {
  79                 if (!futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
  80                         /*
  81                          * Prior queued wakeups queued by unrelated code
  82                          * using the same address can cause futex wait to
  83                          * return 0 even through the futex value is still
  84                          * -1 (spurious wakeups). Check the value again
  85                          * in user-space to validate whether it really
  86                          * differs from -1.
  87                          */
  88                         continue;
  89                 }
  90                 switch (errno) {
  91                 case EAGAIN:
  92                         /* Value already changed. */
  93                         goto end;
  94                 case EINTR:
  95                         /* Retry if interrupted by signal. */
  96                         break;  /* Get out of switch. Check again. */
  97                 default:
  98                         /* Unexpected error. */
  99                         PERROR("futex_async");
 100                         abort();
 101                 }
 102         }
 103 end:
 104         DBG("Futex n to 1 wait done");
 105 }
 106
 107 /*
 108  * Wake 1 futex.
 109  */
 110 LTTNG_HIDDEN
 111 void futex_nto1_wake(int32_t *futex)
 112 {
 113         if (caa_unlikely(uatomic_read(futex) != -1))
 114                 goto end;
 115         uatomic_set(futex, 0);
 116         if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
 117                 PERROR("futex_async");
 118                 abort();
 119         }
 120 end:
 121         DBG("Futex n to 1 wake done");
 122 }