linux-kernel - Re: FW: [PATCH 008 of 9] md: Fix possible raid1/raid10 deadlock on read error during resync.

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Message-ID: <18408.29826.857813.735627@notabene.brown>
Date:	Tue, 25 Mar 2008 14:41:54 +1100
From:	Neil Brown <neilb@...e.de>
To:	"John Z." <md2sf@...l.com>
Cc:	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: FW: [PATCH 008 of 9] md: Fix possible raid1/raid10 deadlock on
 read error during resync.

On Monday March 24, md2sf@...l.com wrote:
> 
> > 
> > -----Original Message-----
> > From: linux-kernel-owner@...r.kernel.org
> > [mailto:linux-kernel-owner@...r.kernel.org] On Behalf Of NeilBrown
> > Sent: Sunday, March 02, 2008 5:18 PM
> > To: Andrew Morton
> > Cc: linux-raid@...r.kernel.org; linux-kernel@...r.kernel.org; K.Tanaka
> > Subject: [PATCH 008 of 9] md: Fix possible raid1/raid10 deadlock on read
> > error during resync.
> > 
> 
> > diff .prev/drivers/md/raid1.c ./drivers/md/raid1.c
> > --- .prev/drivers/md/raid1.c	2008-03-03 11:03:39.000000000 +1100
> > +++ ./drivers/md/raid1.c	2008-03-03 09:56:52.000000000 +1100
> > @@ -704,13 +704,20 @@ static void freeze_array(conf_t *conf)
> >   	/* stop syncio and normal IO and wait for everything to
> >   	 * go quite.
> >   	 * We increment barrier and nr_waiting, and then
> > -	 * wait until barrier+nr_pending match nr_queued+2
> > +	 * wait until nr_pending match nr_queued+1
> > +	 * This is called in the context of one normal IO request
> > +	 * that has failed. Thus any sync request that might be pending
> > +	 * will be blocked by nr_pending, and we need to wait for
> > +	 * pending IO requests to complete or be queued for re-try.
> > +	 * Thus the number queued (nr_queued) plus this request (1)
> > +	 * must match the number of pending IOs (nr_pending) before
> > +	 * we continue.
> >   	 */
> >   	spin_lock_irq(&conf->resync_lock);
> >   	conf->barrier++;
> >   	conf->nr_waiting++;
> >   	wait_event_lock_irq(conf->wait_barrier,
> > -			    conf->barrier+conf->nr_pending ==
> > conf->nr_queued+2,
> > +			    conf->nr_pending == conf->nr_queued+1,
> >   			    conf->resync_lock,
> >   			    ({ flush_pending_writes(conf);
> >   			       raid1_unplug(conf->mddev->queue); }));
> > --
> 
> When we call freeze_array, it is after reschedule_retry, during which conf->nr_queued is already incremented.
> Should we use conf->nr_pending == conf->nr_pending here?
> 

Can I assume you mean
          conf->nr_pending == conf->nr_queued
??

Yes, it is after reschedule_retry which increments ->nr_queued, but
also after
       conf->nr_queued--;
in raid1d when the request is removed from the queue.

Does that make sense?

NeilBrown
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