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 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 #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/error.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 void futex_wait_update(int32_t *futex, int active)
  53 {
  54         if (active) {
  55                 uatomic_set(futex, 1);
  56                 futex_async(futex, FUTEX_WAKE,
  57                                 INT_MAX, NULL, NULL, 0);
  58         } else {
  59                 uatomic_set(futex, 0);
  60         }
  61
  62         DBG("Futex wait update active %d", active);
  63 }
  64
  65 /*
  66  * Prepare futex.
  67  */
  68 void futex_nto1_prepare(int32_t *futex)
  69 {
  70         uatomic_set(futex, -1);
  71         cmm_smp_mb();
  72
  73         DBG("Futex n to 1 prepare done");
  74 }
  75
  76 /*
  77  * Wait futex.
  78  */
  79 void futex_nto1_wait(int32_t *futex)
  80 {
  81         cmm_smp_mb();
  82
  83         if (uatomic_read(futex) == -1) {
  84                 futex_async(futex, FUTEX_WAIT, -1, NULL, NULL, 0);
  85         }
  86
  87         DBG("Futex n to 1 wait done");
  88 }
  89
  90 /*
  91  * Wake 1 futex.
  92  */
  93 void futex_nto1_wake(int32_t *futex)
  94 {
  95         if (caa_unlikely(uatomic_read(futex) == -1)) {
  96                 uatomic_set(futex, 0);
  97                 futex_async(futex, FUTEX_WAKE, 1, NULL, NULL, 0);
  98         }
  99
 100         DBG("Futex n to 1 wake done");
 101 }