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

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Date:   Tue, 17 Mar 2020 12:07:15 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     yangerkun <yangerkun@...wei.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     NeilBrown <neilb@...e.de>,
        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 Tue, 2020-03-17 at 22:05 +0800, yangerkun wrote:
> 
> On 2020/3/17 9:41, yangerkun wrote:
> > 
> > On 2020/3/17 1:26, 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?\
> > 
> > While try to fix CVE-2019-19769, add some log in __locks_wake_up_blocks 
> > help me to rebuild the problem soon. This help me to discern the problem 
> > soon.
> > 
> > > 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!
> > 
> > I will try to understand this patch, and if it's looks good to me, will 
> > do the performance test!
> 
> This patch looks good to me, with this patch, the bug '6d390e4b5d48 
> ("locks: fix a potential use-after-free problem when wakeup a waiter")' 
> describes won't happen again. Actually, I find that syzkaller has report 
> this bug before[1], and the log of it can help us to reproduce it with 
> some latency in __locks_wake_up_blocks!
> 
> Also, some ltp testcases describes in [2] pass too with the patch!
> 
> For performance test, I have try to understand will-it-scale/lkp, but it 
> seem a little complex to me, and may need some more time. So, Rong Chen, 
> can you help to do this? Or the results may come a little later...
> 
> Thanks,
> ----
> [1] https://syzkaller.appspot.com/bug?extid=922689db06e57b69c240
> [2] https://lkml.org/lkml/2020/3/11/578

Thanks yangerkun. Let me know if you want to add your Reviewed-by tag.

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