nptl/pthread_rwlock_rdlock.c

   1 /* Copyright (C) 2003-2012 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 #include <stap-probe.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   LIBC_PROBE (rdlock_entry, 1, rwlock);
  35
  36   /* Make sure we are alone.  */
  37   lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  38
  39   while (1)
  40     {
  41       /* Get the rwlock if there is no writer...  */
  42       if (rwlock->__data.__writer == 0
  43           /* ...and if either no writer is waiting or we prefer readers.  */
  44           && (!rwlock->__data.__nr_writers_queued
  45               || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
  46         {
  47           /* Increment the reader counter.  Avoid overflow.  */
  48           if (__builtin_expect (++rwlock->__data.__nr_readers == 0, 0))
  49             {
  50               /* Overflow on number of readers.  */
  51               --rwlock->__data.__nr_readers;
  52               result = EAGAIN;
  53             }
  54           else
  55             LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
  56
  57           break;
  58         }
  59
  60       /* Make sure we are not holding the rwlock as a writer.  This is
  61          a deadlock situation we recognize and report.  */
  62       if (__builtin_expect (rwlock->__data.__writer
  63                             == THREAD_GETMEM (THREAD_SELF, tid), 0))
  64         {
  65           result = EDEADLK;
  66           break;
  67         }
  68
  69       /* Remember that we are a reader.  */
  70       if (__builtin_expect (++rwlock->__data.__nr_readers_queued == 0, 0))
  71         {
  72           /* Overflow on number of queued readers.  */
  73           --rwlock->__data.__nr_readers_queued;
  74           result = EAGAIN;
  75           break;
  76         }
  77
  78       int waitval = rwlock->__data.__readers_wakeup;
  79
  80       /* Free the lock.  */
  81       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  82
  83       /* Wait for the writer to finish.  */
  84       lll_futex_wait (&rwlock->__data.__readers_wakeup, waitval,
  85                       rwlock->__data.__shared);
  86
  87       /* Get the lock.  */
  88       lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  89
  90       --rwlock->__data.__nr_readers_queued;
  91     }
  92
  93   /* We are done, free the lock.  */
  94   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  95
  96   return result;
  97 }
  98
  99 weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
 100 hidden_def (__pthread_rwlock_rdlock)