src/common/futex.c

   1 /*
   2  * Copyright (C)  2011 - David Goulet <david.goulet@polymtl.ca>
   3  *                       Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License, version 2 only,
   7  * as published by the Free Software Foundation.
   8  *
   9  * This program is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  * GNU General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU General Public License along
  15  * with this program; if not, write to the Free Software Foundation, Inc.,
  16  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  17  */
  18
  19 #define _LGPL_SOURCE
  20 #include <limits.h>
  21 #include <sys/syscall.h>
  22 #include <unistd.h>
  23 #include <urcu.h>
  24 #include <urcu/futex.h>
  25
  26 #include <common/common.h>
  27
  28 #include "futex.h"
  29
  30 /*
  31  * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
  32  * "nto1" added to all function signature.
  33  *
  34  * Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
  35  * git tree for a detail example of this scheme being used. futex_async() is
  36  * the urcu wrapper over the futex() sycall.
  37  *
  38  * There is also a formal verification available in the git tree.
  39  *
  40  *   branch: formal-model
  41  *   commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
  42  *
  43  * Ref: git://git.lttng.org/userspace-rcu.git
  44  */
  45
  46 /*
  47  * Update futex according to active or not. This scheme is used to wake every
  48  * libust waiting on the shared memory map futex hence the INT_MAX used in the
  49  * futex() call. If active, we set the value and wake everyone else we indicate
  50  * that we are gone (cleanup() case).
  51  */
  52 LTTNG_HIDDEN
  53 void futex_wait_update(int32_t *futex, int active)
  54 {
  55         if (active) {
  56                 uatomic_set(futex, 1);
  57                 if (futex_async(futex, FUTEX_WAKE,
  58                                 INT_MAX, NULL, NULL, 0) < 0) {
  59                         PERROR("futex_async");
  60                         abort();
  61                 }
  62         } else {
  63                 uatomic_set(futex, 0);
  64         }
  65
  66         DBG("Futex wait update active %d", active);
  67 }
  68
  69 /*
  70  * Prepare futex.
  71  */
  72 LTTNG_HIDDEN
  73 void futex_nto1_prepare(int32_t *futex)
  74 {
  75         uatomic_set(futex, -1);
  76         cmm_smp_mb();
  77
  78         DBG("Futex n to 1 prepare done");
  79 }
  80
  81 /*
  82  * Wait futex.
  83  */
  84 LTTNG_HIDDEN
  85 void futex_nto1_wait(int32_t *futex)
  86 {
  87         cmm_smp_mb();
  88
  89         if (uatomic_read(futex) != -1)
  90                 goto end;
  91         while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
  92                 switch (errno) {
  93                 case EWOULDBLOCK:
  94                         /* Value already changed. */
  95                         goto end;
  96                 case EINTR:
  97                         /* Retry if interrupted by signal. */
  98                         break;  /* Get out of switch. */
  99                 default:
 100                         /* Unexpected error. */
 101                         PERROR("futex_async");
 102                         abort();
 103                 }
 104         }
 105 end:
 106         DBG("Futex n to 1 wait done");
 107 }
 108
 109 /*
 110  * Wake 1 futex.
 111  */
 112 LTTNG_HIDDEN
 113 void futex_nto1_wake(int32_t *futex)
 114 {
 115         if (caa_unlikely(uatomic_read(futex) != -1))
 116                 goto end;
 117         uatomic_set(futex, 0);
 118         if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
 119                 PERROR("futex_async");
 120                 abort();
 121         }
 122 end:
 123         DBG("Futex n to 1 wake done");
 124 }