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

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

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