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

Joakim Johansson jocke at tbricks.com
Mon Feb 14 04:20:23 PST 2011


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):

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 (dispatch_atomic_cmpxchg(&qc->dgq_pending, 1, 0)) // zero out, ensure visibility
	_dispatch_debug("got new worker thread for global queue: %p", dq); 
	_dispatch_debug(“unexpectedly got a new worker thread for global queue: %p", dq); 

Any enlightenment appreciated.



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