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  * The code is adapted from the adaptative busy wait/wake_up scheme used in
  34  * liburcu.
  35  */
  36
  37 /* Number of busy-loop attempts before waiting on futex. */
  38 #define FUTEX_WAIT_ATTEMPTS 1000
  39
  40 enum futex_wait_state {
  41         /* FUTEX_WAIT_WAITING is compared directly (futex() compares it). */
  42         FUTEX_WAIT_WAITING =    0,
  43         /* non-zero are used as masks. */
  44         FUTEX_WAIT_WAKEUP =     (1 << 0),
  45         FUTEX_WAIT_RUNNING =    (1 << 1),
  46         FUTEX_WAIT_TEARDOWN =   (1 << 2),
  47 };
  48
  49 /*
  50  * Update futex according to active or not. This scheme is used to wake every
  51  * libust waiting on the shared memory map futex hence the INT_MAX used in the
  52  * futex() call. If active, we set the value and wake everyone else we indicate
  53  * that we are gone (cleanup() case).
  54  */
  55 LTTNG_HIDDEN
  56 void futex_wait_update(int32_t *futex, int active)
  57 {
  58         if (active) {
  59                 uatomic_set(futex, 1);
  60                 if (futex_async(futex, FUTEX_WAKE,
  61                                 INT_MAX, NULL, NULL, 0) < 0) {
  62                         PERROR("futex_async");
  63                         abort();
  64                 }
  65         } else {
  66                 uatomic_set(futex, 0);
  67         }
  68
  69         DBG("Futex wait update active %d", active);
  70 }
  71
  72 /*
  73  * Prepare futex.
  74  */
  75 LTTNG_HIDDEN
  76 void futex_nto1_prepare(int32_t *futex)
  77 {
  78         uatomic_set(futex, FUTEX_WAIT_WAITING);
  79         cmm_smp_mb();
  80
  81         DBG("Futex n to 1 prepare done");
  82 }
  83
  84 /*
  85  * Wait futex.
  86  */
  87 LTTNG_HIDDEN
  88 void futex_nto1_wait(int32_t *futex)
  89 {
  90         unsigned int i;
  91
  92         /* Load and test condition before read state */
  93         cmm_smp_rmb();
  94         for (i = 0; i < FUTEX_WAIT_ATTEMPTS; i++) {
  95                 if (uatomic_read(futex) != FUTEX_WAIT_WAITING)
  96                         goto skip_futex_wait;
  97                 caa_cpu_relax();
  98         }
  99         while (futex_noasync(futex, FUTEX_WAIT, FUTEX_WAIT_WAITING,
 100                         NULL, NULL, 0)) {
 101                 switch (errno) {
 102                 case EWOULDBLOCK:
 103                         /* Value already changed. */
 104                         goto skip_futex_wait;
 105                 case EINTR:
 106                         /* Retry if interrupted by signal. */
 107                         break;  /* Get out of switch. */
 108                 default:
 109                         /* Unexpected error. */
 110                         PERROR("futex");
 111                         abort();
 112                 }
 113         }
 114 skip_futex_wait:
 115
 116         /* Tell waker thread than we are running. */
 117         uatomic_or(futex, FUTEX_WAIT_RUNNING);
 118
 119         /*
 120          * Wait until waker thread lets us know it's ok to tear down
 121          * memory allocated for the futex.
 122          */
 123         for (i = 0; i < FUTEX_WAIT_ATTEMPTS; i++) {
 124                 if (uatomic_read(futex) & FUTEX_WAIT_TEARDOWN)
 125                         break;
 126                 caa_cpu_relax();
 127         }
 128         while (!(uatomic_read(futex) & FUTEX_WAIT_TEARDOWN))
 129                 poll(NULL, 0, 10);
 130         assert(uatomic_read(futex) & FUTEX_WAIT_TEARDOWN);
 131         DBG("Futex n to 1 wait done");
 132 }
 133
 134 /*
 135  * Wake 1 futex.
 136  */
 137 LTTNG_HIDDEN
 138 void futex_nto1_wake(int32_t *futex)
 139 {
 140         cmm_smp_mb();
 141         uatomic_set(futex, FUTEX_WAIT_WAKEUP);
 142         if (!(uatomic_read(futex) & FUTEX_WAIT_RUNNING)) {
 143                 if (futex_noasync(futex, FUTEX_WAKE, 1,
 144                                 NULL, NULL, 0) < 0) {
 145                         PERROR("futex_noasync");
 146                         abort();
 147                 }
 148         }
 149         /* Allow teardown of futex. */
 150         uatomic_or(futex, FUTEX_WAIT_TEARDOWN);
 151         DBG("Futex n to 1 wake done");
 152 }