Re: [PATCH] fix-flush_workqueue-vs-cpu_dead-race-update

[Srivatsa Vaddagiri <vatsa@in.ibm.com>]

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
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