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

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Message-Id: <1168936038.2941.182.camel@localhost.localdomain>
Date:	Tue, 16 Jan 2007 09:27:18 +0100
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jens Axboe <jens.axboe@...cle.com>
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 Tue, 2007-01-16 at 11:41 +1100, Jens Axboe wrote:
> > 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.

Well. All offenders come via queue_sync_plugs(). queue_sync_plugs()
calls blk_unplug_current().

One path which triggers is blk_sync_queue(). This happens e.g. in the
cleanup of the floppy check. There are other call pathes too.

The other is raid md_super_write(). It is not plugged and calls with the
barrier bit set, which executes the unlikely path in __make_request():

        if (unlikely(bio_barrier(bio))) {
                queue_sync_plugs(q);
                spin_lock_irq(q->queue_lock);
                goto get_rq;
        }

So you either need checks all over the place to avoid calling
blk_unplug_current(), or you prevent the unplug below 0 like I did. The
BUG_ON()s I added should catch any real invalid callers. But it's up to
you.

	tglx


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