nptl/pthread_rwlock_unlock.c

   1 /* Copyright (C) 2003-2017 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>, 2003.
   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 <sysdep.h>
  21 #include <lowlevellock.h>
  22 #include <futex-internal.h>
  23 #include <pthread.h>
  24 #include <pthreadP.h>
  25 #include <stap-probe.h>
  26 #include <elide.h>
  27
  28
  29 /* Unlock RWLOCK.  */
  30 int
  31 __pthread_rwlock_unlock (pthread_rwlock_t *rwlock)
  32 {
  33   int futex_shared =
  34       rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
  35
  36   LIBC_PROBE (rwlock_unlock, 1, rwlock);
  37
  38   /* Trying to elide an unlocked lock may crash the process.  This
  39      is expected and is compatible with POSIX.1-2008: "results are
  40      undefined if the read-write lock rwlock is not held by the
  41      calling thread".  */
  42   if (ELIDE_UNLOCK (rwlock->__data.__writer == 0
  43                     && rwlock->__data.__nr_readers == 0))
  44     return 0;
  45
  46   lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  47   if (rwlock->__data.__writer)
  48     rwlock->__data.__writer = 0;
  49   else
  50     --rwlock->__data.__nr_readers;
  51   /* If there are still readers present, we do not yet need to wake writers
  52      nor are responsible to wake any readers.  */
  53   if (rwlock->__data.__nr_readers == 0)
  54     {
  55       /* Note that if there is a blocked writer, we effectively make it
  56          responsible for waking any readers because we don't wake readers in
  57          this case.  */
  58       if (rwlock->__data.__nr_writers_queued)
  59         {
  60           ++rwlock->__data.__writer_wakeup;
  61           lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  62           futex_wake (&rwlock->__data.__writer_wakeup, 1, futex_shared);
  63           return 0;
  64         }
  65       else if (rwlock->__data.__nr_readers_queued)
  66         {
  67           ++rwlock->__data.__readers_wakeup;
  68           lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  69           futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
  70                       futex_shared);
  71           return 0;
  72         }
  73     }
  74   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  75   return 0;
  76 }
  77
  78 weak_alias (__pthread_rwlock_unlock, pthread_rwlock_unlock)
  79 hidden_def (__pthread_rwlock_unlock)