On Tue, Jan 02, 2018 at 08:16:56AM -0800, Tejun Heo wrote:

Hello,

On Fri, Dec 29, 2017 at 02:07:16AM +0530, Prateek Sood wrote:

quoted

task T is waiting for cpuset_mutex acquired
by kworker/2:1

sh ==> cpuhp/2 ==> kworker/2:1 ==> sh 

kworker/2:3 ==> kthreadd ==> Task T ==> kworker/2:1

It seems that my earlier patch set should fix this scenario:
1) Inverting locking order of cpuset_mutex and cpu_hotplug_lock.
2) Make cpuset hotplug work synchronous.

Could you please share your feedback.

Hmm... this can also be resolved by adding WQ_MEM_RECLAIM to the
synchronize rcu workqueue, right?  Given the wide-spread usages of
synchronize_rcu and friends, maybe that's the right solution, or at
least something we also need to do, for this particular deadlock?

To make WQ_MEM_RECLAIM work, I need to dynamically allocate RCU's
workqueues, correct?  Or is there some way to mark a statically
allocated workqueue as WQ_MEM_RECLAIM after the fact?

I can dynamically allocate them, but I need to carefully investigate
boot-time use.  So if it is possible to be lazy, I do want to take
the easy way out.  ;-)

							Thanx, Paul

Again, I don't have anything against making the domain rebuliding part
of cpuset operations synchronous and these tricky deadlock scenarios
do indicate that doing so would probably be beneficial.  That said,
tho, these scenarios seem more of manifestations of other problems
exposed through kthreadd dependency than anything else.

Thanks.

-- 
tejun