nptl/pthread_rwlock_rdlock.c

   1 /* Copyright (C) 2003, 2004, 2007 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, write to the Free
  17    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  18    02111-1307 USA.  */
  19
  20 #include <errno.h>
  21 #include <sysdep.h>
  22 #include <lowlevellock.h>
  23 #include <pthread.h>
  24 #include <pthreadP.h>
  25
  26
  27 /* Acquire read lock for RWLOCK.  */
  28 int
  29 __pthread_rwlock_rdlock (rwlock)
  30      pthread_rwlock_t *rwlock;
  31 {
  32   int result = 0;
  33
  34   /* Make sure we are along.  */
  35   lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  36
  37   while (1)
  38     {
  39       /* Get the rwlock if there is no writer...  */
  40       if (rwlock->__data.__writer == 0
  41           /* ...and if either no writer is waiting or we prefer readers.  */
  42           && (!rwlock->__data.__nr_writers_queued
  43               || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
  44         {
  45           /* Increment the reader counter.  Avoid overflow.  */
  46           if (__builtin_expect (++rwlock->__data.__nr_readers == 0, 0))
  47             {
  48               /* Overflow on number of readers.  */
  49               --rwlock->__data.__nr_readers;
  50               result = EAGAIN;
  51             }
  52
  53           break;
  54         }
  55
  56       /* Make sure we are not holding the rwlock as a writer.  This is
  57          a deadlock situation we recognize and report.  */
  58       if (__builtin_expect (rwlock->__data.__writer
  59                             == THREAD_GETMEM (THREAD_SELF, tid), 0))
  60         {
  61           result = EDEADLK;
  62           break;
  63         }
  64
  65       /* Remember that we are a reader.  */
  66       if (__builtin_expect (++rwlock->__data.__nr_readers_queued == 0, 0))
  67         {
  68           /* Overflow on number of queued readers.  */
  69           --rwlock->__data.__nr_readers_queued;
  70           result = EAGAIN;
  71           break;
  72         }
  73
  74       int waitval = rwlock->__data.__readers_wakeup;
  75
  76       /* Free the lock.  */
  77       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  78
  79       /* Wait for the writer to finish.  */
  80       lll_futex_wait (&rwlock->__data.__readers_wakeup, waitval,
  81                       rwlock->__data.__shared);
  82
  83       /* Get the lock.  */
  84       lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  85
  86       --rwlock->__data.__nr_readers_queued;
  87     }
  88
  89   /* We are done, free the lock.  */
  90   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  91
  92   return result;
  93 }
  94
  95 weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
  96 strong_alias (__pthread_rwlock_rdlock, __pthread_rwlock_rdlock_internal)