linux-kernel - Re: BUG at scsi_lib.c:1108 [Was: mmotm 2009-11-24-16-47 uploaded]

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Date:	Fri, 27 Nov 2009 15:17:21 +1100
From:	Neil Brown <neilb@...e.de>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	James Bottomley <James.Bottomley@...e.de>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mm-commits@...r.kernel.org, linux-scsi@...r.kernel.org,
	Jens Axboe <Jens.Axboe@...cle.com>
Subject: Re: BUG at scsi_lib.c:1108 [Was: mmotm 2009-11-24-16-47 uploaded]

On Thu, 26 Nov 2009 08:13:59 +1100
Neil Brown <neilb@...e.de> wrote:

> On Wed, 25 Nov 2009 21:22:57 +0100
> Jiri Slaby <jirislaby@...il.com> wrote:
> 
> > It doesn't make sense at all. How can empty merge cause a regression?
> > And md/for-next doesn't produce the BUG.
> 
> I've hit that situation before.  Two separate changes in separate
> branches conspire to cause a problem.
> 
> md/for-next contains code to handle barriers properly for all levels,
> not just RAID1.
> It is possible I got this wrong in some way, and some new sanity check
> in a separate branch is firing, or it is possible some other bug in
> barrier handling has been added and now that MD sends barriers, it is
> being triggered.

And it looks like I did get it wrong in some way.
I handled a barrier by:
   - send an empty barrier to each component
   - schedule to write with the barrier flag cleared
   - send another empty barrier

If an empty barrier is received, step 2 sends an empty non-barrier,
which caused the BUG.
I have revised the code to special case empty barriers and only perform
the first step.
This should appear in the next -next.

Thanks,
NeilBrown
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