nptl/pthread_rwlock_timedrdlock.c

   1 /* Copyright (C) 2003-2016 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 (pthread_rwlock_t *rwlock,
  32                             const struct timespec *abstime)
  33 {
  34   int result = 0;
  35   bool wake = false;
  36   int futex_shared =
  37       rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
  38
  39   /* Make sure we are alone.  */
  40   lll_lock(rwlock->__data.__lock, rwlock->__data.__shared);
  41
  42   while (1)
  43     {
  44       int err;
  45
  46       /* Get the rwlock if there is no writer...  */
  47       if (rwlock->__data.__writer == 0
  48           /* ...and if either no writer is waiting or we prefer readers.  */
  49           && (!rwlock->__data.__nr_writers_queued
  50               || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
  51         {
  52           /* Increment the reader counter.  Avoid overflow.  */
  53           if (++rwlock->__data.__nr_readers == 0)
  54             {
  55               /* Overflow on number of readers.  */
  56               --rwlock->__data.__nr_readers;
  57               result = EAGAIN;
  58             }
  59           else
  60             {
  61               /* See pthread_rwlock_rdlock.  */
  62               if (rwlock->__data.__nr_readers == 1
  63                   && rwlock->__data.__nr_readers_queued > 0
  64                   && rwlock->__data.__nr_writers_queued > 0)
  65                 {
  66                   ++rwlock->__data.__readers_wakeup;
  67                   wake = true;
  68                 }
  69             }
  70
  71           break;
  72         }
  73
  74       /* Make sure we are not holding the rwlock as a writer.  This is
  75          a deadlock situation we recognize and report.  */
  76       if (__builtin_expect (rwlock->__data.__writer
  77                             == THREAD_GETMEM (THREAD_SELF, tid), 0))
  78         {
  79           result = EDEADLK;
  80           break;
  81         }
  82
  83       /* Make sure the passed in timeout value is valid.  Ideally this
  84          test would be executed once.  But since it must not be
  85          performed if we would not block at all simply moving the test
  86          to the front is no option.  Replicating all the code is
  87          costly while this test is not.  */
  88       if (__builtin_expect (abstime->tv_nsec >= 1000000000
  89                             || abstime->tv_nsec < 0, 0))
  90         {
  91           result = EINVAL;
  92           break;
  93         }
  94
  95       /* Remember that we are a reader.  */
  96       if (++rwlock->__data.__nr_readers_queued == 0)
  97         {
  98           /* Overflow on number of queued readers.  */
  99           --rwlock->__data.__nr_readers_queued;
 100           result = EAGAIN;
 101           break;
 102         }
 103
 104       int waitval = rwlock->__data.__readers_wakeup;
 105
 106       /* Free the lock.  */
 107       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 108
 109       /* Wait for the writer to finish.  We handle ETIMEDOUT below; on other
 110          return values, we decide how to continue based on the state of the
 111          rwlock.  */
 112       err = futex_abstimed_wait (&rwlock->__data.__readers_wakeup, waitval,
 113                                  abstime, futex_shared);
 114
 115       /* Get the lock.  */
 116       lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
 117
 118       --rwlock->__data.__nr_readers_queued;
 119
 120       /* Did the futex call time out?  */
 121       if (err == ETIMEDOUT)
 122         {
 123           /* Yep, report it.  */
 124           result = ETIMEDOUT;
 125           break;
 126         }
 127     }
 128
 129   /* We are done, free the lock.  */
 130   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 131
 132   if (wake)
 133     futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
 134
 135   return result;
 136 }