nptl/pthread_rwlock_tryrdlock.c

   1 /* Copyright (C) 2002-2015 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <errno.h>
  20 #include "pthreadP.h"
  21 #include <lowlevellock.h>
  22 #include <futex-internal.h>
  23 #include <elide.h>
  24 #include <stdbool.h>
  25
  26
  27 int
  28 __pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock)
  29 {
  30   int result = EBUSY;
  31   bool wake = false;
  32   int futex_shared =
  33       rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
  34
  35   if (ELIDE_TRYLOCK (rwlock->__data.__rwelision,
  36                      rwlock->__data.__lock == 0
  37                      && rwlock->__data.__nr_readers == 0
  38                      && rwlock->__data.__writer, 0))
  39     return 0;
  40
  41   lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  42
  43   if (rwlock->__data.__writer == 0
  44       && (rwlock->__data.__nr_writers_queued == 0
  45           || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
  46     {
  47       if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
  48         {
  49           --rwlock->__data.__nr_readers;
  50           result = EAGAIN;
  51         }
  52       else
  53         {
  54           result = 0;
  55           /* See pthread_rwlock_rdlock.  */
  56           if (rwlock->__data.__nr_readers == 1
  57               && rwlock->__data.__nr_readers_queued > 0
  58               && rwlock->__data.__nr_writers_queued > 0)
  59             {
  60               ++rwlock->__data.__readers_wakeup;
  61               wake = true;
  62             }
  63         }
  64     }
  65
  66   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  67
  68   if (wake)
  69     futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
  70
  71   return result;
  72 }
  73 strong_alias (__pthread_rwlock_tryrdlock, pthread_rwlock_tryrdlock)