Re: [PATCH] generic dynamic per cpu refcounting

[Kent Overstreet <koverstreet@google.com>]

On Mon, Jan 28, 2013 at 01:18:32PM -0800, Tejun Heo wrote:
> Hello, Kent.
> 
> On Mon, Jan 28, 2013 at 12:55:40PM -0800, Kent Overstreet wrote:
> > > I don't understand why we need two stages. What prevents the killing
> > > thread from fetching percpu counters after dying passes one
> > > synchronize_sched()?
> > 
> > It does. The second synchronize_sched() is needed after we set state :=
> > dead, and before we drop the initial ref. Otherwise the ref could hit 0
> > before percpu_ref_put knows to check for it.
> 
> Still a bit confused.  Why do we need to make the two steps separate?
> What prevents us from doing the following?
> 
> 	set dying;
> 	synchronize_sched();
> 	collect percpu refs into global atomic_t;
> 	put the base ref;

After you set state := dying, percpu_ref_put() decrements the atomic_t,
but it can't check if it's 0 yet because the thread that's collecting
the percpu refs might not be done yet.

So percpu_ref_put can't check for ref == 0 until after state == dead.
But the put in your example might have made ref 0. When did you set
state to dead?