diff options
Diffstat (limited to 'lib/windows-once.c')
| -rw-r--r-- | lib/windows-once.c | 49 |
1 files changed, 46 insertions, 3 deletions
diff --git a/lib/windows-once.c b/lib/windows-once.c index 17854f5c..a8b9e0f3 100644 --- a/lib/windows-once.c +++ b/lib/windows-once.c @@ -29,7 +29,9 @@ glwthread_once (glwthread_once_t *once_control, void (*initfunction) (void)) { if (once_control->inited <= 0) { - if (InterlockedIncrement (&once_control->started) == 0) + InterlockedIncrement (&once_control->num_threads); + /* If once_control->started is == -1, set it to 0. */ + if (InterlockedCompareExchange (&once_control->started, 0, -1) < 0) { /* This thread is the first one to come to this once_control. */ InitializeCriticalSection (&once_control->lock); @@ -41,8 +43,6 @@ glwthread_once (glwthread_once_t *once_control, void (*initfunction) (void)) } else { - /* Don't let once_control->started grow and wrap around. */ - InterlockedDecrement (&once_control->started); /* Some other thread has already started the initialization. Yield the CPU while waiting for the other thread to finish initializing and taking the lock. */ @@ -58,5 +58,48 @@ glwthread_once (glwthread_once_t *once_control, void (*initfunction) (void)) abort (); } } + /* Here once_control->started == 0 and once_control->inited > 0. */ + if (InterlockedDecrement (&once_control->num_threads) == 0) + /* once_control->num_threads is now zero, and + once_control->started == 0 and once_control->inited > 0. + No other thread will need to use the lock. + We can therefore destroy the lock, to free resources. */ + /* If once_control->inited is == 1, set it to 2. */ + if (InterlockedCompareExchange (&once_control->inited, 2, 1) == 1) + DeleteCriticalSection (&once_control->lock); } + /* Proof of correctness: + * num_threads is incremented and then decremented by some threads. + Therefore, num_threads always stays >= 0, and is == 0 at the end. + * The first thread to go through the once_control->started fence + initializes the lock and moves inited from <= 0 to > 0. The other + threads don't move inited from <= 0 to > 0. + * started, once == 0, stays == 0. + * inited, once > 0, stays > 0 (since at the place where it is assigned 0, + it cannot be > 0). + * inited does not change any more once it is 2. + Therefore, it can be changed from 1 to 2 only once. + * DeleteCriticalSection gets invoked right after inited has been changed + from 1 to 2. Therefore, DeleteCriticalSection gets invoked only once. + * After a moment where num_threads was 0 and started was 0 and + inited was > 0, no thread can reach an InitializeCriticalSection or + EnterCriticalSection invocation. Proof: + - At such a moment, no thread is in the code range between + InterlockedIncrement (&once_control->num_threads) + and + InterlockedDecrement (&once_control->num_threads) + - After such a moment, some thread can increment num_threads, but from + there they cannot reach the InitializeCriticalSection invocation, + because the once_control->started test prevents that, and they cannot + reach the EnterCriticalSection invocation in the other branch because + the + if (once_control->inited <= 0) + test prevents that. + * From this it follows that: + - DeleteCriticalSection cannot be executed while the lock is taken + (because DeleteCriticalSection is only executed after a moment where + num_threads was 0 and started was 0 and inited was > 0). + - Once DeleteCriticalSection has been executed, the lock is not used any + more. + */ } |