Re: regression 4.4: deadlock in with cgroup percpu_rwsem

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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