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, gret = 0;
  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         ret = pthread_mutex_lock(&__urcu_compat_futex_lock);
  73         assert(!ret);
  74         switch (op) {
  75         case FUTEX_WAIT:
  76                 /*
  77                  * Wait until *uaddr is changed to something else than "val".
  78                  * Comparing *uaddr content against val figures out which
  79                  * thread has been awakened.
  80                  */
  81                 while (CMM_LOAD_SHARED(*uaddr) == val)
  82                         pthread_cond_wait(&__urcu_compat_futex_cond,
  83                                 &__urcu_compat_futex_lock);
  84                 break;
  85         case FUTEX_WAKE:
  86                 /*
  87                  * Each wake is sending a broadcast, thus attempting wakeup of
  88                  * all awaiting threads, independently of their respective
  89                  * uaddr.
  90                  */
  91                 pthread_cond_broadcast(&__urcu_compat_futex_cond);
  92                 break;
  93         default:
  94                 gret = -EINVAL;
  95         }
  96         ret = pthread_mutex_unlock(&__urcu_compat_futex_lock);
  97         assert(!ret);
  98         return gret;
  99 }
 100
 101 /*
 102  * _ASYNC SIGNAL-SAFE_.
 103  * For now, timeout, uaddr2 and val3 are unused.
 104  * Waiter will busy-loop trying to read the condition.
 105  */
 106
 107 int compat_futex_async(int32_t *uaddr, int op, int32_t val,
 108         const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
 109 {
 110         /*
 111          * Check if NULL. Don't let users expect that they are taken into
 112          * account.
 113          */
 114         assert(!timeout);
 115         assert(!uaddr2);
 116         assert(!val3);
 117
 118         /*
 119          * Ensure previous memory operations on uaddr have completed.
 120          */
 121         cmm_smp_mb();
 122
 123         switch (op) {
 124         case FUTEX_WAIT:
 125                 while (CMM_LOAD_SHARED(*uaddr) == val)
 126                         poll(NULL, 0, 10);
 127                 break;
 128         case FUTEX_WAKE:
 129                 break;
 130         default:
 131                 return -EINVAL;
 132         }
 133         return 0;
 134 }