/* * 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 */