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
  34 /*
  35  * Using attribute "weak" for __urcu_compat_futex_lock and
  36  * __urcu_compat_futex_cond. Those are globally visible by the entire
  37  * program, even though many shared objects may have their own version.
  38  * The first version that gets loaded will be used by the entire program
  39  * (executable and all shared objects).
  40  */
  41
  42 __attribute__((weak))
  43 pthread_mutex_t __urcu_compat_futex_lock = PTHREAD_MUTEX_INITIALIZER;
  44 __attribute__((weak))
  45 pthread_cond_t __urcu_compat_futex_cond = PTHREAD_COND_INITIALIZER;
  46
  47 /*
  48  * _NOT SIGNAL-SAFE_. pthread_cond is not signal-safe anyway. Though.
  49  * For now, timeout, uaddr2 and val3 are unused.
  50  * Waiter will relinquish the CPU until woken up.
  51  */
  52
  53 int compat_futex_noasync(int32_t *uaddr, int op, int32_t val,
  54         const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
  55 {
  56         int ret, gret = 0;
  57
  58         /*
  59          * Check if NULL. Don't let users expect that they are taken into
  60          * account.
  61          */
  62         assert(!timeout);
  63         assert(!uaddr2);
  64         assert(!val3);
  65
  66         /*
  67          * memory barriers to serialize with the previous uaddr modification.
  68          */
  69         cmm_smp_mb();
  70
  71         ret = pthread_mutex_lock(&__urcu_compat_futex_lock);
  72         assert(!ret);
  73         switch (op) {
  74         case FUTEX_WAIT:
  75                 /*
  76                  * Wait until *uaddr is changed to something else than "val".
  77                  * Comparing *uaddr content against val figures out which
  78                  * thread has been awakened.
  79                  */
  80                 while (*uaddr == val)
  81                         pthread_cond_wait(&__urcu_compat_futex_cond,
  82                                 &__urcu_compat_futex_lock);
  83                 break;
  84         case FUTEX_WAKE:
  85                 /*
  86                  * Each wake is sending a broadcast, thus attempting wakeup of
  87                  * all awaiting threads, independently of their respective
  88                  * uaddr.
  89                  */
  90                 pthread_cond_broadcast(&__urcu_compat_futex_cond);
  91                 break;
  92         default:
  93                 gret = -EINVAL;
  94         }
  95         ret = pthread_mutex_unlock(&__urcu_compat_futex_lock);
  96         assert(!ret);
  97         return gret;
  98 }
  99
 100 /*
 101  * _ASYNC SIGNAL-SAFE_.
 102  * For now, timeout, uaddr2 and val3 are unused.
 103  * Waiter will busy-loop trying to read the condition.
 104  */
 105
 106 int compat_futex_async(int32_t *uaddr, int op, int32_t val,
 107         const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
 108 {
 109         /*
 110          * Check if NULL. Don't let users expect that they are taken into
 111          * account.
 112          */
 113         assert(!timeout);
 114         assert(!uaddr2);
 115         assert(!val3);
 116
 117         /*
 118          * Ensure previous memory operations on uaddr have completed.
 119          */
 120         cmm_smp_mb();
 121
 122         switch (op) {
 123         case FUTEX_WAIT:
 124                 while (*uaddr == val)
 125                         poll(NULL, 0, 10);
 126                 break;
 127         case FUTEX_WAKE:
 128                 break;
 129         default:
 130                 return -EINVAL;
 131         }
 132         return 0;
 133 }