[libdispatch-dev] _dispatch_queue_wakeup_global/_dispatch_worker_thread2 use of dgq_pending - question

[Joakim Johansson]

Hi,

I am spending a bit of time investigating possible improvements to libpthread_workqueue and reviewed some of the libdispatch code and found one small thing I didn’t quite understand (but it might simply be my lack of understanding the semantics of the gcc atomic ops fully):

------------
queue.c:
1490: void 
1491: _dispatch_worker_thread2(void *context) { 
.
.
.
1502:	qc->dgq_pending = 0; 
——————

Shouldn’t this be set using atomic primitives to ensure visibility of the update of dgq_pending?
(_dispatch_queue_wakeup_global() uses dispatch_atomic_cmpxchg() to set this)

I would have expected something like (#ifdef:ed here to not warn when running without pthread workqueues, as it would be called by a path that doesn’t set dgq_pending):
——
#if HAVE_PTHREAD_WORKQUEUES
if (dispatch_atomic_cmpxchg(&qc->dgq_pending, 1, 0)) // zero out, ensure visibility
{
	_dispatch_debug("got new worker thread for global queue: %p", dq); 
}
else
{
	_dispatch_debug(“unexpectedly got a new worker thread for global queue: %p", dq); 
}
#endif
——

Any enlightenment appreciated.

Thanks,

Joakim