linux-kernel - Re: regression 4.4: deadlock in with cgroup percpu

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Message-ID: <20160123020313.GA4915@linux.vnet.ibm.com>
Date:	Fri, 22 Jan 2016 18:03:13 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"linux-kernel@...r.kernel.org >> Linux Kernel Mailing List" 
	<linux-kernel@...r.kernel.org>,
	linux-s390 <linux-s390@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>, hch@....de
Subject: Re: regression 4.4: deadlock in with cgroup percpu_rwsem

On Wed, Jan 20, 2016 at 10:30:07AM -0500, Tejun Heo wrote:
> Hello,
> 
> On Wed, Jan 20, 2016 at 11:47:58AM +0100, Peter Zijlstra wrote:
> > TJ, is css_offline guaranteed to be called in hierarchical order? I
> 
> No, they aren't.  The ancestors of a css are guaranteed to stay around
> until css_free is called on the css and that's the only ordering
> guarantee.
> 
> > got properly lost in the whole cgroup destroy code. There's endless
> > workqueues and rcu callbacks there.
> 
> Yeah, it's hairy.  I wondered about adding support for bouncing to
> workqueue in both percpu_ref and rcu which would make things easier to
> follow.  Not sure how often this pattern happens tho.

This came up recently offlist for call_rcu(), so that a call to (say)
call_rcu_schedule_work() would do a schedule_work() after a grace period
elapsed, invoking the function passed in to call_rcu_schedule_work().
There are several existing cases that do this, so special-casing it seems
worthwhile.  Perhaps something vaguely similar would work for percpu_ref.

							Thanx, Paul