ltt-sessiond/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
   6  * modify it under the terms of the GNU General Public License
   7  * as published by the Free Software Foundation; only version 2
   8  * of the License.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License
  16  * along with this program; if not, write to the Free Software
  17  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
  18  */
  19
  20 #define _GNU_SOURCE
  21 #include <sys/syscall.h>
  22 #include <unistd.h>
  23 #include <urcu.h>
  24 #include <urcu/futex.h>
  25
  26 #include <lttngerr.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  * Prepare futex.
  48  */
  49 void futex_nto1_prepare(int32_t *futex)
  50 {
  51         uatomic_set(futex, -1);
  52         cmm_smp_mb();
  53
  54         DBG("Futex n to 1 prepare done");
  55 }
  56
  57 /*
  58  * Wait futex.
  59  */
  60 void futex_nto1_wait(int32_t *futex)
  61 {
  62         cmm_smp_mb();
  63
  64         if (uatomic_read(futex) == -1) {
  65                 futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0);
  66         }
  67
  68         DBG("Futex n to 1 wait done");
  69 }
  70
  71 /*
  72  * Wake 1 futex.
  73  */
  74 void futex_nto1_wake(int32_t *futex)
  75 {
  76         if (unlikely(uatomic_read(futex) == -1)) {
  77                 uatomic_set(futex, 0);
  78                 futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0);
  79         }
  80
  81         DBG("Futex n to 1 wake done");
  82 }