compat_futex.c

   1 /*
   2  * compat_futex.c
   3  *
   4  * Userspace RCU library - sys_futex compatibility code
   5  *
   6  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   7  *
   8  * This library is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public
  10  * License as published by the Free Software Foundation; either
  11  * version 2.1 of the License, or (at your option) any later version.
  12  *
  13  * This library is distributed in the hope that it will be useful,
  14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16  * Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public
  19  * License along with this library; if not, write to the Free Software
  20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  21  */
  22
  23 #include <stdio.h>
  24 #include <pthread.h>
  25 #include <signal.h>
  26 #include <assert.h>
  27 #include <errno.h>
  28 #include <poll.h>
  29 #include <stdint.h>
  30
  31 #include <urcu/arch.h>
  32 #include <urcu/futex.h>
  33 #include <urcu/system.h>
  34
  35 /*
  36  * Using attribute "weak" for __urcu_compat_futex_lock and
  37  * __urcu_compat_futex_cond. Those are globally visible by the entire
  38  * program, even though many shared objects may have their own version.
  39  * The first version that gets loaded will be used by the entire program
  40  * (executable and all shared objects).
  41  */
  42
  43 __attribute__((weak))
  44 pthread_mutex_t __urcu_compat_futex_lock = PTHREAD_MUTEX_INITIALIZER;
  45 __attribute__((weak))
  46 pthread_cond_t __urcu_compat_futex_cond = PTHREAD_COND_INITIALIZER;
  47
  48 /*
  49  * _NOT SIGNAL-SAFE_. pthread_cond is not signal-safe anyway. Though.
  50  * For now, timeout, uaddr2 and val3 are unused.
  51  * Waiter will relinquish the CPU until woken up.
  52  */
  53
  54 int compat_futex_noasync(int32_t *uaddr, int op, int32_t val,
  55         const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
  56 {
  57         int ret = 0, lockret;
  58
  59         /*
  60          * Check if NULL. Don't let users expect that they are taken into
  61          * account.
  62          */
  63         assert(!timeout);
  64         assert(!uaddr2);
  65         assert(!val3);
  66
  67         /*
  68          * memory barriers to serialize with the previous uaddr modification.
  69          */
  70         cmm_smp_mb();
  71
  72         lockret = pthread_mutex_lock(&__urcu_compat_futex_lock);
  73         if (lockret) {
  74                 errno = lockret;
  75                 ret = -1;
  76                 goto end;
  77         }
  78         switch (op) {
  79         case FUTEX_WAIT:
  80                 /*
  81                  * Wait until *uaddr is changed to something else than "val".
  82                  * Comparing *uaddr content against val figures out which
  83                  * thread has been awakened.
  84                  */
  85                 while (CMM_LOAD_SHARED(*uaddr) == val)
  86                         pthread_cond_wait(&__urcu_compat_futex_cond,
  87                                 &__urcu_compat_futex_lock);
  88                 break;
  89         case FUTEX_WAKE:
  90                 /*
  91                  * Each wake is sending a broadcast, thus attempting wakeup of
  92                  * all awaiting threads, independently of their respective
  93                  * uaddr.
  94                  */
  95                 pthread_cond_broadcast(&__urcu_compat_futex_cond);
  96                 break;
  97         default:
  98                 errno = EINVAL;
  99                 ret = -1;
 100         }
 101         lockret = pthread_mutex_unlock(&__urcu_compat_futex_lock);
 102         if (lockret) {
 103                 errno = lockret;
 104                 ret = -1;
 105         }
 106 end:
 107         return ret;
 108 }
 109
 110 /*
 111  * _ASYNC SIGNAL-SAFE_.
 112  * For now, timeout, uaddr2 and val3 are unused.
 113  * Waiter will busy-loop trying to read the condition.
 114  * It is OK to use compat_futex_async() on a futex address on which
 115  * futex() WAKE operations are also performed.
 116  */
 117
 118 int compat_futex_async(int32_t *uaddr, int op, int32_t val,
 119         const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
 120 {
 121         int ret = 0;
 122
 123         /*
 124          * Check if NULL. Don't let users expect that they are taken into
 125          * account.
 126          */
 127         assert(!timeout);
 128         assert(!uaddr2);
 129         assert(!val3);
 130
 131         /*
 132          * Ensure previous memory operations on uaddr have completed.
 133          */
 134         cmm_smp_mb();
 135
 136         switch (op) {
 137         case FUTEX_WAIT:
 138                 while (CMM_LOAD_SHARED(*uaddr) == val) {
 139                         if (poll(NULL, 0, 10) < 0) {
 140                                 ret = -1;
 141                                 /* Keep poll errno. Caller handles EINTR. */
 142                                 goto end;
 143                         }
 144                 }
 145                 break;
 146         case FUTEX_WAKE:
 147                 break;
 148         default:
 149                 errno = EINVAL;
 150                 ret = -1;
 151         }
 152 end:
 153         return ret;
 154 }