src/bin/lttng-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 <limits.h>
  22 #include <sys/syscall.h>
  23 #include <unistd.h>
  24 #include <urcu.h>
  25 #include <urcu/futex.h>
  26
  27 #include <common/error.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 void futex_wait_update(int32_t *futex, int active)
  54 {
  55         if (active) {
  56                 uatomic_set(futex, 1);
  57                 futex_async(futex, FUTEX_WAKE,
  58                                 INT_MAX, NULL, NULL, 0);
  59         } else {
  60                 uatomic_set(futex, 0);
  61         }
  62
  63         DBG("Futex wait update active %d", active);
  64 }
  65
  66 /*
  67  * Prepare futex.
  68  */
  69 void futex_nto1_prepare(int32_t *futex)
  70 {
  71         uatomic_set(futex, -1);
  72         cmm_smp_mb();
  73
  74         DBG("Futex n to 1 prepare done");
  75 }
  76
  77 /*
  78  * Wait futex.
  79  */
  80 void futex_nto1_wait(int32_t *futex)
  81 {
  82         cmm_smp_mb();
  83
  84         if (uatomic_read(futex) == -1) {
  85                 futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0);
  86         }
  87
  88         DBG("Futex n to 1 wait done");
  89 }
  90
  91 /*
  92  * Wake 1 futex.
  93  */
  94 void futex_nto1_wake(int32_t *futex)
  95 {
  96         if (caa_unlikely(uatomic_read(futex) == -1)) {
  97                 uatomic_set(futex, 0);
  98                 futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0);
  99         }
 100
 101         DBG("Futex n to 1 wake done");
 102 }