Re: [patch] fs: aio fix rcu lookup

[Jan Kara]

 On Fri 21-01-11 05:31:53, Nick Piggin wrote:
> On Thu, Jan 20, 2011 at 3:03 PM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, Jan 20, 2011 at 08:20:00AM +1100, Nick Piggin wrote:
> >> On Thu, Jan 20, 2011 at 8:03 AM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
> >> >> I don't know exactly how all programs use io_destroy -- of the small
> >> >> number that do, probably an even smaller number would care here. But I
> >> >> don't think it simplifies things enough to use synchronize_rcu for it.
> >> >
> >> > Above it sounded like you didn't think AIO should be using RCU at all.
> >>
> >> synchronize_rcu of course, not RCU (typo).
> >
> > I think that Nick is suggesting that call_rcu() be used instead.
> > Perhaps also very sparing use of synchronize_rcu_expedited(), which
> > is faster than synchronize_rcu(), but which which uses more CPU time.
> 
> call_rcu() is the obvious alternative, yes.
> 
> Basically, once we give in to synchronize_rcu() we're basically giving
> up. That's certainly a very good tradeoff for something like filesystem
> unregistration or module unload, it buys big simplifications in real
> fastpaths. But I just don't think it should be taken lightly.
So in the end, I've realized I don't need synchronize_rcu() at all and
in fact everything is OK even without call_rcu() if I base my fix on top
of your patch.

Attached is your patch with added comment I proposed and also a patch
fixing the second race. Better?

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR