pthread_rwlock_timedrdlock.c

/* Copyright (C) 2003-2016 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>, 2003.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

#include <errno.h>
#include <sysdep.h>
#include <lowlevellock.h>
#include <futex-internal.h>
#include <pthread.h>
#include <pthreadP.h>
#include <sys/time.h>
#include <stdbool.h>


/* Try to acquire read lock for RWLOCK or return after specfied time.  */
int
pthread_rwlock_timedrdlock (pthread_rwlock_t *rwlock,
			    const struct timespec *abstime)
{
  int result = 0;
  bool wake = false;
  int futex_shared =
      rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;

  /* Make sure we are alone.  */
  lll_lock(rwlock->__data.__lock, rwlock->__data.__shared);

  while (1)
    {
      int err;

      /* Get the rwlock if there is no writer...  */
      if (rwlock->__data.__writer == 0
	  /* ...and if either no writer is waiting or we prefer readers.  */
	  && (!rwlock->__data.__nr_writers_queued
	      || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
	{
	  /* Increment the reader counter.  Avoid overflow.  */
	  if (++rwlock->__data.__nr_readers == 0)
	    {
	      /* Overflow on number of readers.	 */
	      --rwlock->__data.__nr_readers;
	      result = EAGAIN;
	    }
	  else
	    {
	      /* See pthread_rwlock_rdlock.  */
	      if (rwlock->__data.__nr_readers == 1
		  && rwlock->__data.__nr_readers_queued > 0
		  && rwlock->__data.__nr_writers_queued > 0)
		{
		  ++rwlock->__data.__readers_wakeup;
		  wake = true;
		}
	    }

	  break;
	}

      /* Make sure we are not holding the rwlock as a writer.  This is
	 a deadlock situation we recognize and report.  */
      if (__builtin_expect (rwlock->__data.__writer
			    == THREAD_GETMEM (THREAD_SELF, tid), 0))
	{
	  result = EDEADLK;
	  break;
	}

      /* Make sure the passed in timeout value is valid.  Ideally this
	 test would be executed once.  But since it must not be
	 performed if we would not block at all simply moving the test
	 to the front is no option.  Replicating all the code is
	 costly while this test is not.  */
      if (__builtin_expect (abstime->tv_nsec >= 1000000000
                            || abstime->tv_nsec < 0, 0))
	{
	  result = EINVAL;
	  break;
	}

      /* Remember that we are a reader.  */
      if (++rwlock->__data.__nr_readers_queued == 0)
	{
	  /* Overflow on number of queued readers.  */
	  --rwlock->__data.__nr_readers_queued;
	  result = EAGAIN;
	  break;
	}

      int waitval = rwlock->__data.__readers_wakeup;

      /* Free the lock.  */
      lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);

      /* Wait for the writer to finish.  We handle ETIMEDOUT below; on other
	 return values, we decide how to continue based on the state of the
	 rwlock.  */
      err = futex_abstimed_wait (&rwlock->__data.__readers_wakeup, waitval,
				 abstime, futex_shared);

      /* Get the lock.  */
      lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);

      --rwlock->__data.__nr_readers_queued;

      /* Did the futex call time out?  */
      if (err == ETIMEDOUT)
	{
	  /* Yep, report it.  */
	  result = ETIMEDOUT;
	  break;
	}
    }

  /* We are done, free the lock.  */
  lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);

  if (wake)
    futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);

  return result;
}