linux-kernel - Re: fs: locks: WARNING: CPU: 16 PID: 4296 at fs/locks.c:236 locks_free_lock

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Date:	Wed, 21 Jan 2015 08:33:50 -0500
From:	Jeff Layton <jeff.layton@...marydata.com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Jeff Layton <jeff.layton@...marydata.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	dhowells@...hat.com
Subject: Re: fs: locks: WARNING: CPU: 16 PID: 4296 at fs/locks.c:236
 locks_free_lock_context+0x10d/0x240()

On Wed, 21 Jan 2015 08:25:02 -0500
Sasha Levin <sasha.levin@...cle.com> wrote:

> On 01/16/2015 04:16 PM, Jeff Layton wrote:
> > On Fri, 16 Jan 2015 13:53:04 -0500
> > Jeff Layton <jlayton@...marydata.com> wrote:
> > 
> >> > On Fri, 16 Jan 2015 13:10:46 -0500
> >> > Sasha Levin <sasha.levin@...cle.com> wrote:
> >> > 
> >>> > > On 01/16/2015 09:40 AM, Jeff Layton wrote:
> >>>> > > > On Fri, 16 Jan 2015 09:31:23 -0500
> >>>> > > > Sasha Levin <sasha.levin@...cle.com> wrote:
> >>>> > > > 
> >>>>> > > >> On 01/15/2015 03:22 PM, Jeff Layton wrote:
> >>>>>> > > >>> Ok, I tried to reproduce it with that and several variations but it
> >>>>>> > > >>> still doesn't seem to do it for me. Can you try the latest linux-next
> >>>>>> > > >>> tree and see if it's still reproducible there?
> >>>>> > > >>
> >>>>> > > >> It's still not in in today's -next, could you send me a patch for testing
> >>>>> > > >> instead?
> >>>>> > > >>
> >>>> > > > 
> >>>> > > > Seems to be there for me:
> >>>> > > > 
> >>>> > > > ----------------------[snip]-----------------------
> >>>> > > > /*
> >>>> > > >  * This function is called on the last close of an open file.
> >>>> > > >  */
> >>>> > > > void locks_remove_file(struct file *filp)
> >>>> > > > {
> >>>> > > >         /* ensure that we see any assignment of i_flctx */
> >>>> > > >         smp_rmb();
> >>>> > > > 
> >>>> > > >         /* remove any OFD locks */
> >>>> > > >         locks_remove_posix(filp, filp);
> >>>> > > > ----------------------[snip]-----------------------
> >>>> > > > 
> >>>> > > > That's actually the right place to put the barrier, I think. We just
> >>>> > > > need to ensure that this function sees any assignment to i_flctx that
> >>>> > > > occurred before this point. By the time we're here, we shouldn't be
> >>>> > > > getting any new locks that matter to this close since the fcheck call
> >>>> > > > should fail on any new requests.
> >>>> > > > 
> >>>> > > > If that works, then I'll probably make some other changes to the set
> >>>> > > > and re-post it next week.
> >>>> > > > 
> >>>> > > > Many thanks for helping me test this!
> >>> > > 
> >>> > > You're right, I somehow missed that.
> >>> > > 
> >>> > > But it doesn't fix the issue, I still see it happening, but it seems
> >>> > > to be less frequent(?).
> >>> > > 
> >> > 
> >> > Ok, that was my worry (and one of the reasons I really would like to
> >> > find some way to reproduce this on my own). I think what I'll do at
> >> > this point is pull the patchset from linux-next until I can consult
> >> > with someone who understands this sort of cache-coherency problem
> >> > better than I do.
> >> > 
> >> > Once I get it resolved, I'll push it back to my linux-next branch and
> >> > let you know and we can give it another go.
> >> > 
> >> > Thanks for the testing so far!
> > Actually, I take it back. One more try...
> > 
> > I dragooned David Howells into helping me look at this and he talked me
> > into just going back to using the i_lock to protect the i_flctx
> > assignment.
> > 
> > My hope is that will work around whatever strange effect is causing
> > this. Can you test tomorrow's -next tree (once it's been merged) and see
> > whether this is still reproducible?
> 
> I've updated and re-tested with the latest -next, and it seems that the
> issue is gone.
> 
> I'll update if I end up seeing it again.
> 

The change was to rely on the i_lock to protect the i_flctx pointer.
I'm not sure why a cmpxchg() wasn't quite sufficient, but I'll plan to
stick with this for now. It's unlikely to make any real difference in
performance anyway.

Many thanks for testing it, Sasha!
-- 
Jeff Layton <jlayton@...marydata.com>
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