linux-kernel - Re: [patch] cciss: Fix race between disk-adding code and interrupt handler

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Message-ID: <20080414173720.GQ12774@kernel.dk>
Date:	Mon, 14 Apr 2008 19:37:20 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	scameron@...rdog.cca.cpqcorp.net
Cc:	linux-kernel@...r.kernel.org, mike.miller@...com,
	mikem@...rdog.cca.cpqcorp.net
Subject: Re: [patch] cciss: Fix race between disk-adding code and interrupt handler

On Mon, Apr 14 2008, scameron@...rdog.cca.cpqcorp.net wrote:
> 
> 
> > On Mon, Apr 14 2008, scameron@...rdog.cca.cpqcorp.net wrote:
> > > 
> > > 
> > > Fix race condition between cciss_init_one(), cciss_update_drive_info(),
> > > and cciss_check_queues().  cciss_softirq_done would try to start
> > > queues which were not quite ready to be started, as its checks for
> > > readiness were not sufficiently synchronized with the queue initializing
> > > code in cciss_init_one and cciss_update_drive_info.  Slow cpu and
> > > large numbers of logical drives seem to make the race more likely 
> > > to cause a problem.
> > 
> > Hmm, this seems backwards to me.  cciss_softirq_done() isn't going to
> > start the queues, until an irq has triggered for instance. Why isn't the
> > init properly ordered instead of band-aiding around this with a
> > 'queue_ready' variable?
> >
> 
> Each call to add_disk() will trigger some interrupts, 
> and earlier added disks may cause the queues of later,
> not-yet-completely added disks to be started.
> 
> I suppose the init routine might be reorganized to initialize all
> the queues, then have second loop call add_disk() for all
> of them.  Is that what you had in mind by "properly ordered?"

Yep precisely, don't call add_disk() until everything is set up.

> Disks may be added at run time though, and I think this tears
> down all but the first disk, and re-adds them all, if I remember
> right, so there is some complication there to think about.

Well, other drivers manage quite fine without resorting to work-arounds
:-)

-- 
Jens Axboe

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