linux-kernel - Re: [locks] 6d390e4b5d: will-it-scale.per_process

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Date:   Thu, 19 Mar 2020 13:51:59 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     NeilBrown <neilb@...e.de>, yangerkun <yangerkun@...wei.com>,
        kernel test robot <rong.a.chen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        Bruce Fields <bfields@...ldses.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6%
 regression

On Mon, 2020-03-16 at 10:26 -0700, Linus Torvalds wrote:
> On Mon, Mar 16, 2020 at 4:07 AM Jeff Layton <jlayton@...nel.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@...nel.org>