linux-kernel - Re: [PATCH 1/1] block: rework flush sequencing for blk-mq

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Date:	Fri, 14 Mar 2014 06:23:23 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Hannes Reinecke <hare@...e.de>, Jeff Moyer <jmoyer@...hat.com>,
	Jens Axboe <axboe@...nel.dk>, Shaohua Li <shli@...ionio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dm-devel@...hat.com
Subject: Re: [PATCH 1/1] block: rework flush sequencing for blk-mq

On Fri, Mar 14, 2014 at 09:00:21AM -0400, Mike Snitzer wrote:
> > Getting a little upset, eh?  I didn't say it's broken, I said it gets
> > very little testing.  The regression from me was found like so many
> > before only after it was backported o some enterprise kernel.
> 
> Even _really_ basic dm-multipath testing would've uncovered this bug.

But major testing using dm, md and various low-level drivers didn't.  Do
you really expect everyone to remember to specificly test dm-multipath
for a core block change?  Without a nicely documented way to do it?

That being said I should have remembered that it's special, but even I
didn't and I'm sorry about that.

> > I think the problem here is two-fold:
> >  a) the hardware you use with dm-multipath isn't widely available.
> >  b) it uses a very special code path in the block layer no one else uses
> > 
> > a) might be fixable by having some RDAC or similar emulation in qemu if
> > someone wants to spend the effort.
> 
> The regression from the commit in question was easily reproduced/tested
> using scsi_debug.  Just start the multipathd service and any scsi_debug
> device in the system will get multipath'd.

Really?  That sounds a like a bug in the INQUIRY information returned by
scsi_debug.  Either way please write up these things in Documentation so
you can point people at it easily.

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