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                 futex_async(futex, FUTEX_WAKE,
  59                                 INT_MAX, NULL, NULL, 0);
  60         } else {
  61                 uatomic_set(futex, 0);
  62         }
  63
  64         DBG("Futex wait update active %d", active);
  65 }
  66
  67 /*
  68  * Prepare futex.
  69  */
  70 LTTNG_HIDDEN
  71 void futex_nto1_prepare(int32_t *futex)
  72 {
  73         uatomic_set(futex, -1);
  74         cmm_smp_mb();
  75
  76         DBG("Futex n to 1 prepare done");
  77 }
  78
  79 /*
  80  * Wait futex.
  81  */
  82 LTTNG_HIDDEN
  83 void futex_nto1_wait(int32_t *futex)
  84 {
  85         cmm_smp_mb();
  86
  87         if (uatomic_read(futex) == -1) {
  88                 futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0);
  89         }
  90
  91         DBG("Futex n to 1 wait done");
  92 }
  93
  94 /*
  95  * Wake 1 futex.
  96  */
  97 LTTNG_HIDDEN
  98 void futex_nto1_wake(int32_t *futex)
  99 {
 100         if (caa_unlikely(uatomic_read(futex) == -1)) {
 101                 uatomic_set(futex, 0);
 102                 futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0);
 103         }
 104
 105         DBG("Futex n to 1 wake done");
 106 }