relibc_pthreads-emb/pte_rwlock_check_need_init.c

/*
 * pthread_rwlock_check_need_init.c
 *
 * Description:
 * This translation unit implements read/write lock primitives.
 *
 * --------------------------------------------------------------------------
 *
 *      Pthreads-embedded (PTE) - POSIX Threads Library for embedded systems
 *      Copyright(C) 2008 Jason Schmidlapp
 *
 *      Contact Email: jschmidlapp@users.sourceforge.net
 *
 *
 *      Based upon Pthreads-win32 - POSIX Threads Library for Win32
 *      Copyright(C) 1998 John E. Bossom
 *      Copyright(C) 1999,2005 Pthreads-win32 contributors
 *
 *      Contact Email: rpj@callisto.canberra.edu.au
 *
 *      The original list of contributors to the Pthreads-win32 project
 *      is contained in the file CONTRIBUTORS.ptw32 included with the
 *      source code distribution. The list can also be seen at the
 *      following World Wide Web location:
 *      http://sources.redhat.com/pthreads-win32/contributors.html
 *
 *      This 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 of the License, or (at your option) any later version.
 *
 *      This 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 this library in the file COPYING.LIB;
 *      if not, write to the Free Software Foundation, Inc.,
 *      59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
 */


#include "pthread.h"
#include "implement.h"

int
pte_rwlock_check_need_init (pthread_rwlock_t * rwlock)
{
  int result = 0;

  /*
   * The following guarded test is specifically for statically
   * initialised rwlocks (via PTHREAD_RWLOCK_INITIALIZER).
   *
   * Note that by not providing this synchronisation we risk
   * introducing race conditions into applications which are
   * correctly written.
   *
   * Approach
   * --------
   * We know that static rwlocks will not be PROCESS_SHARED
   * so we can serialise access to internal state using
   * critical sections rather than mutexes.
   *
   * If using a single global lock slows applications down too much,
   * multiple global locks could be created and hashed on some random
   * value associated with each mutex, the pointer perhaps. At a guess,
   * a good value for the optimal number of global locks might be
   * the number of processors + 1.
   *
   */


  pte_osMutexLock (pte_rwlock_test_init_lock);

  /*
   * We got here possibly under race
   * conditions. Check again inside the critical section
   * and only initialise if the rwlock is valid (not been destroyed).
   * If a static rwlock has been destroyed, the application can
   * re-initialise it only by calling pthread_rwlock_init()
   * explicitly.
   */
  if (*rwlock == PTHREAD_RWLOCK_INITIALIZER)
    {
      result = pthread_rwlock_init (rwlock, NULL);
    }
  else if (*rwlock == NULL)
    {
      /*
       * The rwlock has been destroyed while we were waiting to
       * initialise it, so the operation that caused the
       * auto-initialisation should fail.
       */
      result = EINVAL;
    }

  pte_osMutexUnlock(pte_rwlock_test_init_lock);

  return result;
}