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