Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression

Jeff Layton
2020-03-19T17:52:03Z

On Mon, 2020-03-16 at 10:26 -0700, Linus Torvalds wrote:
> On Mon, Mar 16, 2020 at 4:07 AM Jeff Layton <jlayton@kernel.org> wrote:
> >
> > + /*
> > + * If fl_blocker is NULL, it won't be set again as this thread "owns"
> > + * the lock and is the only one that might try to claim the lock.
> > + * Because fl_blocker is explicitly set last during a delete, it's
> > + * safe to locklessly test to see if it's NULL. If it is, then we know
> > + * that no new locks can be inserted into its fl_blocked_requests list,
> > + * and we can therefore avoid doing anything further as long as that
> > + * list is empty.
> > + */
> > + if (!smp_load_acquire(&waiter->fl_blocker) &&
> > + list_empty(&waiter->fl_blocked_requests))
> > + return status;
>
> Ack. This looks sane to me now.
>
> yangerkun - how did you find the original problem?
>
> Would you mind using whatever stress test that caused commit
> 6d390e4b5d48 ("locks: fix a potential use-after-free problem when
> wakeup a waiter") with this patch? And if you did it analytically,
> you're a champ and should look at this patch too!
>

Thanks for all the help with this.

Yangerkun gave me his Reviewed-by and I sent you the most recent version
of the patch yesterday (cc'ing the relevant mailing lists). I left you
as author as the original patch was yours.

Let me know if you'd prefer I send a pull request instead.

Cheers,
--
Jeff Layton <jlayton@kernel.org>