src/common/futex.cpp

   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.hpp>
  16
  17 #include "futex.hpp"
  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 void futex_wait_update(int32_t *futex, int active)
  42 {
  43         if (active) {
  44                 uatomic_set(futex, 1);
  45                 if (futex_async(futex, FUTEX_WAKE,
  46                                 INT_MAX, NULL, NULL, 0) < 0) {
  47                         PERROR("futex_async");
  48                         abort();
  49                 }
  50         } else {
  51                 uatomic_set(futex, 0);
  52         }
  53
  54         DBG("Futex wait update active %d", active);
  55 }
  56
  57 /*
  58  * Prepare futex.
  59  */
  60 void futex_nto1_prepare(int32_t *futex)
  61 {
  62         uatomic_set(futex, -1);
  63         cmm_smp_mb();
  64
  65         DBG("Futex n to 1 prepare done");
  66 }
  67
  68 /*
  69  * Wait futex.
  70  */
  71 void futex_nto1_wait(int32_t *futex)
  72 {
  73         cmm_smp_mb();
  74
  75         while (uatomic_read(futex) == -1) {
  76                 if (!futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
  77                         /*
  78                          * Prior queued wakeups queued by unrelated code
  79                          * using the same address can cause futex wait to
  80                          * return 0 even through the futex value is still
  81                          * -1 (spurious wakeups). Check the value again
  82                          * in user-space to validate whether it really
  83                          * differs from -1.
  84                          */
  85                         continue;
  86                 }
  87                 switch (errno) {
  88                 case EAGAIN:
  89                         /* Value already changed. */
  90                         goto end;
  91                 case EINTR:
  92                         /* Retry if interrupted by signal. */
  93                         break;  /* Get out of switch. Check again. */
  94                 default:
  95                         /* Unexpected error. */
  96                         PERROR("futex_async");
  97                         abort();
  98                 }
  99         }
 100 end:
 101         DBG("Futex n to 1 wait done");
 102 }
 103
 104 /*
 105  * Wake 1 futex.
 106  */
 107 void futex_nto1_wake(int32_t *futex)
 108 {
 109         if (caa_unlikely(uatomic_read(futex) != -1))
 110                 goto end;
 111         uatomic_set(futex, 0);
 112         if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
 113                 PERROR("futex_async");
 114                 abort();
 115         }
 116 end:
 117         DBG("Futex n to 1 wake done");
 118 }