fbtl/pthread_rwlock_rdlock.c

   1 /* Copyright (C) 2003-2013 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 (pthread_rwlock_t *rwlock)
  30 {
  31   int result = 0;
  32
  33   LIBC_PROBE (rdlock_entry, 1, rwlock);
  34
  35   /* Make sure we are alone.  */
  36   lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  37
  38   while (1)
  39     {
  40       /* Get the rwlock if there is no writer...  */
  41       if (rwlock->__data.__writer == 0
  42           /* ...and if either no writer is waiting or we prefer readers.  */
  43           && (!rwlock->__data.__nr_writers_queued
  44               || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
  45         {
  46           /* Increment the reader counter.  Avoid overflow.  */
  47           if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
  48             {
  49               /* Overflow on number of readers.  */
  50               --rwlock->__data.__nr_readers;
  51               result = EAGAIN;
  52             }
  53           else
  54             LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
  55
  56           break;
  57         }
  58
  59       /* Make sure we are not holding the rwlock as a writer.  This is
  60          a deadlock situation we recognize and report.  */
  61       if (__builtin_expect (rwlock->__data.__writer
  62                             == THREAD_GETMEM (THREAD_SELF, tid), 0))
  63         {
  64           result = EDEADLK;
  65           break;
  66         }
  67
  68       /* Remember that we are a reader.  */
  69       if (__glibc_unlikely (++rwlock->__data.__nr_readers_queued == 0))
  70         {
  71           /* Overflow on number of queued readers.  */
  72           --rwlock->__data.__nr_readers_queued;
  73           result = EAGAIN;
  74           break;
  75         }
  76
  77       int waitval = rwlock->__data.__readers_wakeup;
  78
  79       /* Free the lock.  */
  80       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  81
  82       /* Wait for the writer to finish.  */
  83       lll_futex_wait (&rwlock->__data.__readers_wakeup, waitval,
  84                       rwlock->__data.__shared);
  85
  86       /* Get the lock.  */
  87       lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  88
  89       --rwlock->__data.__nr_readers_queued;
  90     }
  91
  92   /* We are done, free the lock.  */
  93   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  94
  95   return result;
  96 }
  97
  98 weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
  99 hidden_def (__pthread_rwlock_rdlock)