diff options
| -rw-r--r-- | ChangeLog | 6 | ||||
| -rw-r--r-- | pthread_once.c | 83 |
2 files changed, 56 insertions, 33 deletions
@@ -1,3 +1,9 @@ +2005-03-14 Ross Johnson <ross at callisto.canberra.edu.au> + + * pthread_once.c (pthread_once): Downgrade interlocked operations to simple + memory operations where these are protected by the critical section; edit + comments. + 2005-03-13 Ross Johnson <rpj at callisto.canberra.edu.au> * pthread_once.c (pthread_once): Completely redesigned; a change was diff --git a/pthread_once.c b/pthread_once.c index c15c5c1..1c0e01f 100644 --- a/pthread_once.c +++ b/pthread_once.c @@ -38,19 +38,23 @@ #include "implement.h" -void ptw32_once_init_routine_cleanup(void * arg) +static void +ptw32_once_init_routine_cleanup(void * arg) { pthread_once_t * once_control = (pthread_once_t *) arg; (void) PTW32_INTERLOCKED_EXCHANGE((LPLONG)&once_control->state, (LONG)PTW32_ONCE_CANCELLED); - (void) PTW32_INTERLOCKED_EXCHANGE((LPLONG)&once_control->started, (LONG)PTW32_FALSE); - // There are waiters, wake some up - // We're deliberately not using PulseEvent. It's iffy, and deprecated. EnterCriticalSection(&ptw32_once_event_lock); if (once_control->event) - SetEvent(once_control->event); + { + /* + * There are waiters, wake some up + * We're deliberately not using PulseEvent. It's iffy, and deprecated. + */ + SetEvent(once_control->event); + } LeaveCriticalSection(&ptw32_once_event_lock); } @@ -108,20 +112,27 @@ pthread_once (pthread_once_t * once_control, void (*init_routine) (void)) result = 0; } - while (!(InterlockedExchangeAdd((LPLONG)&once_control->state, 0L) & (LONG)PTW32_ONCE_DONE)) // Atomic Read + while (!(InterlockedExchangeAdd((LPLONG)&once_control->state, 0L) /* Atomic Read */ + & (LONG)PTW32_ONCE_DONE)) { if (!PTW32_INTERLOCKED_EXCHANGE((LPLONG)&once_control->started, (LONG)PTW32_TRUE)) { - // Clear residual state from a cancelled init_routine - // (and DONE still hasn't been set of course). + /* + * Clear residual state from a cancelled init_routine + * (and DONE still hasn't been set of course). + */ if (PTW32_INTERLOCKED_EXCHANGE((LPLONG)&once_control->state, (LONG)PTW32_ONCE_CLEAR) & PTW32_ONCE_CANCELLED) { - // The previous initter was cancelled. - // We now have a new initter (us) and we need to make the rest wait again. + /* + * The previous initter was cancelled. + * We now have a new initter (us) and we need to make the rest wait again. + */ EnterCriticalSection(&ptw32_once_event_lock); if (once_control->event) - ResetEvent(once_control->event); + { + ResetEvent(once_control->event); + } LeaveCriticalSection(&ptw32_once_event_lock); /* @@ -133,67 +144,73 @@ pthread_once (pthread_once_t * once_control, void (*init_routine) (void)) } pthread_cleanup_push(ptw32_once_init_routine_cleanup, (void *) once_control); - (*init_routine)(); - pthread_cleanup_pop(0); (void) PTW32_INTERLOCKED_EXCHANGE((LPLONG)&once_control->state, (LONG)PTW32_ONCE_DONE); - // we didn't create the event. - // it is only there if there is someone waiting + /* + * we didn't create the event. + * it is only there if there is someone waiting + */ EnterCriticalSection(&ptw32_once_event_lock); if (once_control->event) - SetEvent(once_control->event); + { + SetEvent(once_control->event); + } LeaveCriticalSection(&ptw32_once_event_lock); } else { - // wait for init. - // while waiting, create an event to wait on + /* + * wait for init. + * while waiting, create an event to wait on + */ EnterCriticalSection(&ptw32_once_event_lock); - (void) InterlockedIncrement((LPLONG)&once_control->eventUsers); + once_control->eventUsers++; /* + * RE CANCELLATION: * If we are the first thread after the initter thread, and the init_routine is cancelled * while we're suspended at this point in the code:- * - state will not get set to PTW32_ONCE_DONE; * - cleanup will not see an event and cannot set it; - * - therefore, we will eventually resume, create an event and wait on it; + * - therefore, we will eventually resume, create an event and wait on it, maybe forever; * Remedy: cleanup must set state == CANCELLED before checking for an event, so that * we will see it and avoid waiting (as for state == DONE). We will go around again and * we may become the initter. * If we are still the only other thread when we get to the end of this block, we will * have closed the event (good). If another thread beats us to be initter, then we will - * re-enter here (good). If, when we reach lights out, other threads have reached this - * point, we will not close the event. The eventUsers counter will still correctly reflect - * the real number of waiters, the old event will remain in use. It will be reset - * by the new initter after clearing the CANCELLED state, causing any threads that are + * re-enter here (good). In case the old event is reused, the event is always reset by + * the new initter after clearing the CANCELLED state, causing any threads that are * cycling around the loop to wait again. */ - if (!InterlockedExchangeAdd((LPLONG)&once_control->event, 0L)) // Atomic Read + if (!once_control->event) { once_control->event = CreateEvent(NULL, PTW32_TRUE, PTW32_FALSE, NULL); } LeaveCriticalSection(&ptw32_once_event_lock); - // check 'state' again in case the initting thread has finished or cancelled - // and left before seeing that there was an event to trigger. - // (Now that the event IS created, if init gets finished AFTER this, - // then the event handle is guaranteed to be seen and triggered). + /* + * check 'state' again in case the initting thread has finished or cancelled + * and left before seeing that there was an event to trigger. + * (Now that the event IS created, if init gets finished AFTER this, + * then the event handle is guaranteed to be seen and triggered). + */ - if (!InterlockedExchangeAdd((LPLONG)&once_control->state, 0L)) // Atomic Reads + if (!InterlockedExchangeAdd((LPLONG)&once_control->state, 0L)) /* Atomic Read */ { - // Neither DONE nor CANCELLED + /* Neither DONE nor CANCELLED */ (void) WaitForSingleObject(once_control->event, INFINITE); } - // last one out shut off the lights: + /* last one out shut off the lights */ EnterCriticalSection(&ptw32_once_event_lock); - if (InterlockedDecrement((LPLONG)&once_control->eventUsers) == 0) // we were last + if (0 == --once_control->eventUsers) { + /* we were last */ CloseHandle(once_control->event); once_control->event = 0; } |