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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
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