Re: regression 4.4: deadlock in with cgroup percpu_rwsem

[Christoph Hellwig <hch@lst.de>]

On Fri, Jan 22, 2016 at 06:03:13PM -0800, Paul E. McKenney wrote:
> > 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.

FYI, my use case was also related to percpu-ref.  The percpu ref API
is unfortunately really hard to use and will almost always involve
a work queue due to the complex interaction between percpu_ref_kill
and percpu_ref_exit.  One thing that would help a lot of callers would
be a percpu_ref_exit_sync that kills the ref and waits for all references
to go away synchronously.