nptl/pthread_rwlock_rdlock.c

   1 /* Copyright (C) 2003,2004,2007,2011 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 <pthread.h>
  23 #include <pthreadP.h>
  24
  25
  26 /* Acquire read lock for RWLOCK.  */
  27 int
  28 __pthread_rwlock_rdlock (rwlock)
  29      pthread_rwlock_t *rwlock;
  30 {
  31   int result = 0;
  32
  33   /* Make sure we are alone.  */
  34   lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  35
  36   while (1)
  37     {
  38       /* Get the rwlock if there is no writer...  */
  39       if (rwlock->__data.__writer == 0
  40           /* ...and if either no writer is waiting or we prefer readers.  */
  41           && (!rwlock->__data.__nr_writers_queued
  42               || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
  43         {
  44           /* Increment the reader counter.  Avoid overflow.  */
  45           if (__builtin_expect (++rwlock->__data.__nr_readers == 0, 0))
  46             {
  47               /* Overflow on number of readers.  */
  48               --rwlock->__data.__nr_readers;
  49               result = EAGAIN;
  50             }
  51
  52           break;
  53         }
  54
  55       /* Make sure we are not holding the rwlock as a writer.  This is
  56          a deadlock situation we recognize and report.  */
  57       if (__builtin_expect (rwlock->__data.__writer
  58                             == THREAD_GETMEM (THREAD_SELF, tid), 0))
  59         {
  60           result = EDEADLK;
  61           break;
  62         }
  63
  64       /* Remember that we are a reader.  */
  65       if (__builtin_expect (++rwlock->__data.__nr_readers_queued == 0, 0))
  66         {
  67           /* Overflow on number of queued readers.  */
  68           --rwlock->__data.__nr_readers_queued;
  69           result = EAGAIN;
  70           break;
  71         }
  72
  73       int waitval = rwlock->__data.__readers_wakeup;
  74
  75       /* Free the lock.  */
  76       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  77
  78       /* Wait for the writer to finish.  */
  79       lll_futex_wait (&rwlock->__data.__readers_wakeup, waitval,
  80                       rwlock->__data.__shared);
  81
  82       /* Get the lock.  */
  83       lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  84
  85       --rwlock->__data.__nr_readers_queued;
  86     }
  87
  88   /* We are done, free the lock.  */
  89   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  90
  91   return result;
  92 }
  93
  94 weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
  95 strong_alias (__pthread_rwlock_rdlock, __pthread_rwlock_rdlock_internal)