nptl/pthread_rwlock_timedrdlock.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 <futex-internal.h>
  23 #include <pthread.h>
  24 #include <pthreadP.h>
  25 #include <sys/time.h>
  26 #include <stdbool.h>
  27
  28
  29 /* Try to acquire read lock for RWLOCK or return after specfied time.  */
  30 int
  31 pthread_rwlock_timedrdlock (rwlock, abstime)
  32      pthread_rwlock_t *rwlock;
  33      const struct timespec *abstime;
  34 {
  35   int result = 0;
  36   bool wake = false;
  37   int futex_shared =
  38       rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
  39
  40   /* Make sure we are alone.  */
  41   lll_lock(rwlock->__data.__lock, rwlock->__data.__shared);
  42
  43   while (1)
  44     {
  45       int err;
  46
  47       /* Get the rwlock if there is no writer...  */
  48       if (rwlock->__data.__writer == 0
  49           /* ...and if either no writer is waiting or we prefer readers.  */
  50           && (!rwlock->__data.__nr_writers_queued
  51               || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
  52         {
  53           /* Increment the reader counter.  Avoid overflow.  */
  54           if (++rwlock->__data.__nr_readers == 0)
  55             {
  56               /* Overflow on number of readers.  */
  57               --rwlock->__data.__nr_readers;
  58               result = EAGAIN;
  59             }
  60           else
  61             {
  62               /* See pthread_rwlock_rdlock.  */
  63               if (rwlock->__data.__nr_readers == 1
  64                   && rwlock->__data.__nr_readers_queued > 0
  65                   && rwlock->__data.__nr_writers_queued > 0)
  66                 {
  67                   ++rwlock->__data.__readers_wakeup;
  68                   wake = true;
  69                 }
  70             }
  71
  72           break;
  73         }
  74
  75       /* Make sure we are not holding the rwlock as a writer.  This is
  76          a deadlock situation we recognize and report.  */
  77       if (__builtin_expect (rwlock->__data.__writer
  78                             == THREAD_GETMEM (THREAD_SELF, tid), 0))
  79         {
  80           result = EDEADLK;
  81           break;
  82         }
  83
  84       /* Make sure the passed in timeout value is valid.  Ideally this
  85          test would be executed once.  But since it must not be
  86          performed if we would not block at all simply moving the test
  87          to the front is no option.  Replicating all the code is
  88          costly while this test is not.  */
  89       if (__builtin_expect (abstime->tv_nsec >= 1000000000
  90                             || abstime->tv_nsec < 0, 0))
  91         {
  92           result = EINVAL;
  93           break;
  94         }
  95
  96       /* Remember that we are a reader.  */
  97       if (++rwlock->__data.__nr_readers_queued == 0)
  98         {
  99           /* Overflow on number of queued readers.  */
 100           --rwlock->__data.__nr_readers_queued;
 101           result = EAGAIN;
 102           break;
 103         }
 104
 105       int waitval = rwlock->__data.__readers_wakeup;
 106
 107       /* Free the lock.  */
 108       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 109
 110       /* Wait for the writer to finish.  We handle ETIMEDOUT below; on other
 111          return values, we decide how to continue based on the state of the
 112          rwlock.  */
 113       err = futex_abstimed_wait (&rwlock->__data.__readers_wakeup, waitval,
 114                                  abstime, futex_shared);
 115
 116       /* Get the lock.  */
 117       lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
 118
 119       --rwlock->__data.__nr_readers_queued;
 120
 121       /* Did the futex call time out?  */
 122       if (err == ETIMEDOUT)
 123         {
 124           /* Yep, report it.  */
 125           result = ETIMEDOUT;
 126           break;
 127         }
 128     }
 129
 130   /* We are done, free the lock.  */
 131   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 132
 133   if (wake)
 134     futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
 135
 136   return result;
 137 }