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 _GNU_SOURCE
  20 #define _LGPL_SOURCE
  21 #include <limits.h>
  22 #include <sys/syscall.h>
  23 #include <unistd.h>
  24 #include <urcu.h>
  25 #include <urcu/futex.h>
  26
  27 #include <common/common.h>
  28
  29 #include "futex.h"
  30
  31 /*
  32  * This futex wait/wake scheme only works for N wakers / 1 waiters. Hence the
  33  * "nto1" added to all function signature.
  34  *
  35  * Please see wait_gp()/update_counter_and_wait() calls in urcu.c in the urcu
  36  * git tree for a detail example of this scheme being used. futex_async() is
  37  * the urcu wrapper over the futex() sycall.
  38  *
  39  * There is also a formal verification available in the git tree.
  40  *
  41  *   branch: formal-model
  42  *   commit id: 2a8044f3493046fcc8c67016902dc7beec6f026a
  43  *
  44  * Ref: git://git.lttng.org/userspace-rcu.git
  45  */
  46
  47 /*
  48  * Update futex according to active or not. This scheme is used to wake every
  49  * libust waiting on the shared memory map futex hence the INT_MAX used in the
  50  * futex() call. If active, we set the value and wake everyone else we indicate
  51  * that we are gone (cleanup() case).
  52  */
  53 LTTNG_HIDDEN
  54 void futex_wait_update(int32_t *futex, int active)
  55 {
  56         if (active) {
  57                 uatomic_set(futex, 1);
  58                 if (futex_async(futex, FUTEX_WAKE,
  59                                 INT_MAX, NULL, NULL, 0) < 0) {
  60                         PERROR("futex_async");
  61                         abort();
  62                 }
  63         } else {
  64                 uatomic_set(futex, 0);
  65         }
  66
  67         DBG("Futex wait update active %d", active);
  68 }
  69
  70 /*
  71  * Prepare futex.
  72  */
  73 LTTNG_HIDDEN
  74 void futex_nto1_prepare(int32_t *futex)
  75 {
  76         uatomic_set(futex, -1);
  77         cmm_smp_mb();
  78
  79         DBG("Futex n to 1 prepare done");
  80 }
  81
  82 /*
  83  * Wait futex.
  84  */
  85 LTTNG_HIDDEN
  86 void futex_nto1_wait(int32_t *futex)
  87 {
  88         cmm_smp_mb();
  89
  90         if (uatomic_read(futex) != -1)
  91                 goto end;
  92         while (futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0)) {
  93                 switch (errno) {
  94                 case EWOULDBLOCK:
  95                         /* Value already changed. */
  96                         goto end;
  97                 case EINTR:
  98                         /* Retry if interrupted by signal. */
  99                         break;  /* Get out of switch. */
 100                 default:
 101                         /* Unexpected error. */
 102                         PERROR("futex_async");
 103                         abort();
 104                 }
 105         }
 106 end:
 107         DBG("Futex n to 1 wait done");
 108 }
 109
 110 /*
 111  * Wake 1 futex.
 112  */
 113 LTTNG_HIDDEN
 114 void futex_nto1_wake(int32_t *futex)
 115 {
 116         if (caa_unlikely(uatomic_read(futex) != -1))
 117                 goto end;
 118         uatomic_set(futex, 0);
 119         if (futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0) < 0) {
 120                 PERROR("futex_async");
 121                 abort();
 122         }
 123 end:
 124         DBG("Futex n to 1 wake done");
 125 }