nptl/pthread_rwlock_rdlock.c

   1 /* Copyright (C) 2003-2015 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 #include <elide.h>
  26
  27
  28 /* Acquire read lock for RWLOCK.  Slow path.  */
  29 static int __attribute__((noinline))
  30 __pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
  31 {
  32   int result = 0;
  33
  34   /* Lock is taken in caller.  */
  35
  36   while (1)
  37     {
  38       /* Make sure we are not holding the rwlock as a writer.  This is
  39          a deadlock situation we recognize and report.  */
  40       if (__builtin_expect (rwlock->__data.__writer
  41                             == THREAD_GETMEM (THREAD_SELF, tid), 0))
  42         {
  43           result = EDEADLK;
  44           break;
  45         }
  46
  47       /* Remember that we are a reader.  */
  48       if (__glibc_unlikely (++rwlock->__data.__nr_readers_queued == 0))
  49         {
  50           /* Overflow on number of queued readers.  */
  51           --rwlock->__data.__nr_readers_queued;
  52           result = EAGAIN;
  53           break;
  54         }
  55
  56       int waitval = rwlock->__data.__readers_wakeup;
  57
  58       /* Free the lock.  */
  59       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  60
  61       /* Wait for the writer to finish.  */
  62       lll_futex_wait (&rwlock->__data.__readers_wakeup, waitval,
  63                       rwlock->__data.__shared);
  64
  65       /* Get the lock.  */
  66       lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  67
  68       --rwlock->__data.__nr_readers_queued;
  69
  70       /* Get the rwlock if there is no writer...  */
  71       if (rwlock->__data.__writer == 0
  72           /* ...and if either no writer is waiting or we prefer readers.  */
  73           && (!rwlock->__data.__nr_writers_queued
  74               || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
  75         {
  76           /* Increment the reader counter.  Avoid overflow.  */
  77           if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
  78             {
  79               /* Overflow on number of readers.  */
  80               --rwlock->__data.__nr_readers;
  81               result = EAGAIN;
  82             }
  83           else
  84             LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
  85
  86           break;
  87         }
  88     }
  89
  90   /* We are done, free the lock.  */
  91   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  92
  93   return result;
  94 }
  95
  96
  97 /* Fast path of acquiring read lock on RWLOCK.  */
  98
  99 int
 100 __pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
 101 {
 102   int result = 0;
 103
 104   LIBC_PROBE (rdlock_entry, 1, rwlock);
 105
 106   if (ELIDE_LOCK (rwlock->__data.__rwelision,
 107                   rwlock->__data.__lock == 0
 108                   && rwlock->__data.__writer == 0
 109                   && rwlock->__data.__nr_readers == 0))
 110     return 0;
 111
 112   /* Make sure we are alone.  */
 113   lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
 114
 115   /* Get the rwlock if there is no writer...  */
 116   if (rwlock->__data.__writer == 0
 117       /* ...and if either no writer is waiting or we prefer readers.  */
 118       && (!rwlock->__data.__nr_writers_queued
 119           || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
 120     {
 121       /* Increment the reader counter.  Avoid overflow.  */
 122       if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
 123         {
 124           /* Overflow on number of readers.      */
 125           --rwlock->__data.__nr_readers;
 126           result = EAGAIN;
 127         }
 128       else
 129         LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
 130
 131       /* We are done, free the lock.  */
 132       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 133
 134       return result;
 135     }
 136
 137   return __pthread_rwlock_rdlock_slow (rwlock);
 138 }
 139
 140 weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
 141 hidden_def (__pthread_rwlock_rdlock)