Messages in this thread | | | Date | Mon, 8 Jan 2007 20:52:11 +0530 | From | Srivatsa Vaddagiri <> | Subject | Re: [PATCH] fix-flush_workqueue-vs-cpu_dead-race-update |
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On Mon, Jan 08, 2007 at 12:51:03AM +0300, Oleg Nesterov wrote: > Change flush_workqueue() to use for_each_possible_cpu(). This means that > flush_cpu_workqueue() may hit CPU which is already dead. However in that > case > > if (!list_empty(&cwq->worklist) || cwq->current_work != NULL) > > means that CPU_DEAD in progress, it will do kthread_stop() + take_over_work() > so we can proceed and insert a barrier. We hold cwq->lock, so we are safe. > > This patch replaces fix-flush_workqueue-vs-cpu_dead-race.patch which was > broken by switching to preempt_disable (now we don't need locking at all). > Note that migrate_sequence (was hotplug_sequence) is incremented under > cwq->lock. Since flush_workqueue does lock/unlock of cwq->lock on all CPUs, > it must see the new value if take_over_work() happened before we checked > this cwq, and this is the case we should worry about: otherwise we added > a barrier. > > Srivatsa?
This is head-spinning :)
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.
2.
CPU_DEAD->cleanup_workqueue_thread->(cwq->thread = NULL)->kthread_stop() .. ^^^^^^^^^^^^^^^^^^^^ |___ Problematic
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.
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.
Alternatively, for the second race, I guess we can avoid setting cwq->thread = NULL in cleanup_workqueue_thread() till the thread has exited, but I am not sure if that opens up any other race. The first race seems harder to fix ..
I wonder if spin (spinroot.com) or some other formal model can make this job of spotting-races easier for us ..
-- Regards, vatsa - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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