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

[yangerkun]

On 2020/3/18 0:07, Jeff Layton wrote:
> 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@xxxxxxxxxx> 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.

Yeah, you can add:

Reviewed-by: yangerkun <yangerkun@xxxxxxxxxx>

> Cheers,