/*
* pthread_once.c
*
* Description:
* This translation unit implements miscellaneous thread functions.
*
* --------------------------------------------------------------------------
*
* 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 current list of contributors is contained
* in the file CONTRIBUTORS 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
*/
/*
* NOTES:
* pthread_once() performs a very simple task. So why is this implementation
* so complicated?
*
* The original implementation WAS very simple, but it relied on Windows random
* priority boosting to resolve starvation problems. Windows priority boosting
* does not occur for realtime priority classes (levels 16 to 31).
*
* You can check back to previous versions of code in the CVS repository or
* search the mailing list archives for discussion.
*
* Version A
* ---------
* Waiting threads would resume and suspend again using Sleep(0) until the
* init_routine had completed, but a higher priority waiter could hog the CPU and
* starve the initter thread until Windows randomly boosted it's priority, or forever
* for realtime applications.
*
* Version B
* ---------
* This was fixed by introducing a per once_control manual-reset event that is
* created and destroyed dynamically only if there are waiters. The design did not
* need global critical sections. Each once_control remained independent. A waiter
* could be confident that if the event was not null then it did not need to create
* the event.
*
* Version C
* ---------
* Since a change in ABI would result from version B, it was decided to take
* the opportunity and make pthread_once() fully compliant with the Single Unix
* Specification (version 3 at the time). This required allowing the init_routine
* to be a cancelation point. A cancelation meant that at least some waiting threads
* if any had to be woken so that one might become the new initter thread.
* Waiters could no longer simply assume that, if the event was not null, it did
* not need to create an event.
*
* Also, the cancelled init thread needed to set the event, and the
* new init thread (the winner of the race between any newly arriving threads and
* waking waiters) would need to reset it again. In the meantime, threads could be
* happily looping around until they either suspended on the reset event, or exited
* because the init thread had completed. It was also once again possible for a higher
* priority waiter to starve the init thread.
*
* Version D
* ---------
* There were now two options considered:
* - use an auto-reset event; OR
* - add our own priority boosting.
*
* An auto-reset event would stop threads from looping ok, but it makes threads
* dependent on earlier threads to successfully set the event in turn when it's time
* to wake up, and this serialises threads unecessarily on MP systems. It also adds
* an extra kernel call for each waking thread. If one waiter wakes and dies (async
* cancelled or killed) before it can set the event, then all remaining waiters are
* stranded.
*
* Priority boosting is a standard method for solving priority inversion and
* starvation problems. Furthermore, all of the priority boost logic can
* be restricted to the post cancellation tracks. That is, it need not slow
* the normal cancel-free behaviour. Threads remain independent of other threads.
*
* Version E
* ---------
* Substituting a semaphore in place of the event achieves the same effect as an
* auto-reset event in the post cancellation phase, and a manual-reset event in the
* normal exit phase. The new initter thread does not need to do any post-cancellation
* operations, and waiters only need to check that there is a new initter running
* before starting to wait. All priority issues and adjustments disappear.
*/
#include "pthread.h"
#include "implement.h"
static void PTW32_CDECL
ptw32_once_init_routine_cleanup(void * arg)
{
pthread_once_t * once_control = (pthread_once_t *) arg;
(void) PTW32_INTERLOCKED_EXCHANGE((LPLONG)&once_control->started, (LONG)PTW32_FALSE);
if (InterlockedExchangeAdd((LPLONG)&once_control->semaphore, 0L)) /* MBR fence */
{
ReleaseSemaphore(once_control->semaphore, 1, NULL);
}
}
int
pthread_once (pthread_once_t * once_control, void (*init_routine) (void))
/*
* ------------------------------------------------------
* DOCPUBLIC
* If any thread in a process with a once_control parameter
* makes a call to pthread_once(), the first call will summon
* the init_routine(), but subsequent calls will not. The
* once_control parameter determines whether the associated
* initialization routine has been called. The init_routine()
* is complete upon return of pthread_once().
* This function guarantees that one and only one thread
* executes the initialization routine, init_routine when
* access is controlled by the pthread_once_t control
* key.
*
* pthread_once() is not a cancelation point, but the init_routine
* can be. If it's cancelled then the effect on the once_control is
* as if pthread_once had never been entered.
*
*
* PARAMETERS
* once_control
* pointer to an instance of pthread_once_t
*
* init_routine
* pointer to an initialization routine
*
*
* DESCRIPTION
* See above.
*
* RESULTS
* 0 success,
* EINVAL once_control or init_routine is NULL
*
* ------------------------------------------------------
*/
{
int result;
HANDLE sema;
if (once_control == NULL || init_routine == NULL)
{
result = EINVAL;
goto FAIL0;
}
else
{
result = 0;
}
while (!InterlockedExchangeAdd((LPLONG)&once_control->done, 0L)) /* Atomic Read */
{
if (!PTW32_INTERLOCKED_EXCHANGE((LPLONG)&once_control->started, (LONG)PTW32_TRUE))
{
#ifdef _MSC_VER
#pragma inline_depth(0)
#endif
pthread_cleanup_push(ptw32_once_init_routine_cleanup, (void *) once_control);
(*init_routine)();
pthread_cleanup_pop(0);
#ifdef _MSC_VER
#pragma inline_depth()
#endif
(void) PTW32_INTERLOCKED_EXCHANGE((LPLONG)&once_control->done, (LONG)PTW32_TRUE);
/*
* we didn't create the semaphore.
* it is only there if there is someone waiting.
*/
if (InterlockedExchangeAdd((LPLONG)&once_control->semaphore, 0L)) /* MBR fence */
{
ReleaseSemaphore(once_control->semaphore, once_control->numSemaphoreUsers, NULL);
}
}
else
{
InterlockedIncrement((LPLONG)&once_control->numSemaphoreUsers);
if (!InterlockedExchangeAdd((LPLONG)&once_control->semaphore, 0L)) /* MBR fence */
{
sema = CreateSemaphore(NULL, 0, INT_MAX, NULL);
if (PTW32_INTERLOCKED_COMPARE_EXCHANGE((PTW32_INTERLOCKED_LPLONG)&once_control->semaphore,
(PTW32_INTERLOCKED_LONG)sema,
(PTW32_INTERLOCKED_LONG)0))
{
CloseHandle(sema);
}
}
/*
* Check 'done' and 'started' again in case the initting thread has finished or cancelled
* and left before seeing that there was a semaphore to release.
*/
if (InterlockedExchangeAdd((LPLONG)&once_control->done, 0L) /* Done immediately, or */
|| !InterlockedExchangeAdd((LPLONG)&once_control->started, 0L) /* No initter yet, or */
|| WaitForSingleObject(once_control->semaphore, INFINITE)) /* Done or Cancelled */
{
if (0 == InterlockedDecrement((LPLONG)&once_control->numSemaphoreUsers))
{
/* we were last */
if ((sema = (HANDLE) PTW32_INTERLOCKED_EXCHANGE((LPLONG)&once_control->semaphore,
(LONG)0)))
{
CloseHandle(sema);
}
}
}
}
}
/*
* ------------
* Failure Code
* ------------
*/
FAIL0:
return (result);
} /* pthread_once */