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DateMon, 8 Jan 2007 22:01:40 +0530
FromSrivatsa Vaddagiri <>
SubjectRe: [PATCH] fix-flush_workqueue-vs-cpu_dead-race-update
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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
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