On Mon, Jan 08, 2007 at 06:56:38PM +0300, Oleg Nesterov wrote: > > Spotted atleast these problems: > > > > 1. run_workqueue()->work.func()->flush_work()->mutex_lock(workqueue_mutex) > > deadlocks if we are blocked in cleanup_workqueue_thread()->kthread_stop() > > for the same worker thread to exit. > > > > Looks possible in practice to me. > > Yes, this is the same (old) problem as we have/had with flush_workqueue(). > We can convert flush_work() to use preempt_disable (this is not > straightforward, > but easy), or forbid to call flush_work() from work.func().
I think I noticed other problems of avoiding workqueue_mutex() in flush_work() ..don't recall the exact problem. > > 2. > > > > CPU_DEAD->cleanup_workqueue_thread->(cwq->thread = NULL)->kthread_stop() .. > > ^^^^^^^^^^^^^^^^^^^^ > > |___ Problematic > > Hmm... This should not be possible? cwq->thread != NULL on CPU_DEAD event. sure, cwq->thread != NULL at CPU_DEAD event. However cleanup_workqueue_thread() will set it to NULL and block in kthread_stop(), waiting for the kthread to finish run_workqueue and exit. If one of the work functions being run by run_workqueue() calls flush_workqueue()->flush_cpu_workqueue() now, flush_cpu_workqueue() can fail to recognize that "keventd is trying to flush its own queue" which can cause deadlocks. > > Now while we are blocked here, if a work->func() calls > > flush_workqueue->flush_cpu_workqueue, we clearly cant identify that event > > thread is trying to flush its own queue (cwq->thread == current test > > fails) and hence we will deadlock. > > Could you clarify? I believe cwq->thread == current test always works, we > never > "substitute" cwq->thread. The test fails in the window described above. > > A lock_cpu_hotplug(), or any other ability to block concurrent hotplug > > operations from happening, in run_workqueue would have avoided both the > > above > > races. > > I still don't think this is a good idea. We also need > is_cpu_down_waits_for_lock_cpu_hotplug() > > helper, otherwise we have a deadlock if work->func() sleeps and re-queues > itself. Can you elaborate this a bit? > > Alternatively, for the second race, I guess we can avoid setting > > cwq->thread = NULL in cleanup_workqueue_thread() till the thread has exited, > > Yes, http://marc.theaimsgroup.com/?l=linux-kernel&m=116818097927685, I believe > we can do this later. This way workqueue will have almost zero interaction > with cpu-hotplug, and cpu UP/DOWN event won't be delayed by sleeping > work.func(). > take_over_work() can go away, this also allows us to simplify things. I agree it minimizes the interactions. Maybe worth attempting. However I suspect it may not be as simple as it appears :) -- Regards, vatsa - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
