linux-kernel - Re: [PATCH] cgroup/cpuset: fix circular locking dependency

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Message-Id: <20180102174408.GM7829@linux.vnet.ibm.com>
Date:   Tue, 2 Jan 2018 09:44:08 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Prateek Sood <prsood@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>, avagin@...il.com,
        mingo@...nel.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, sramana@...eaurora.org
Subject: Re: [PATCH] cgroup/cpuset: fix circular locking dependency

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:
> > 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
>