linux-kernel - Re: [patch-mm] Workaround for RAID breakage

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Date:	Tue, 16 Jan 2007 11:41:36 +1100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Michal Piotrowski <michal.k.k.piotrowski@...il.com>,
	Andrew Morton <akpm@...l.org>,
	Neil Brown <neilb@....unsw.edu.au>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch-mm] Workaround for RAID breakage

On Mon, Jan 15 2007, Thomas Gleixner wrote:
> On Mon, 2007-01-15 at 09:08 +0100, Thomas Gleixner wrote:
> > > Thomas saw something similar yesterday and he the partial results that 
> > > git.block (between rc2-mm1 and rc4-mm1) breaks certain disk drivers or 
> > > filesystems drivers. For me it worked fine, so it must be only on some 
> > > combinations. The changes to ll_rw_block.c look quite extensive.
> > 
> > Yes. Jens Axboe confirmed yesterday that the plug changes broke RAID.
> 
> I tracked this down and found two problems:
> 
> - The new plug/unplug code does not check for underruns. That allows the
> plug count (ioc->plugged) to become negative. This gets triggered from
> various places. 
>
> AFAICS this is intentional to avoid checks all over the place, but the
> underflow check is missing. All we need to do is make sure, that in case
> of ioc->plugged == 0 we return early and bug, if there is either a queue
> plugged in or the plugged_list is not empty.
> 
> Jens ?

It should not go negative, that would be a bug elsewhere. So it's
interesting if it does, we should definitely put a WARN_ON() check in
there for that.

> - The raid1 code has no bitmap set in remount r/w. So the
> pending_bio_list gets not processed for quite a time. The workaround is
> to kick mddev->thread, so the list is processed. Not sure about that.
> 
> Neil ?

Super, thanks for that Thomas! I'll merge it in the plug branch.

-- 
Jens Axboe

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