linux-kernel - Re: [PATCH] blk: start bypass mode in blk_unregister

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Date:	Thu, 4 Apr 2013 07:03:27 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Aaron Lu <aaron.lu@...el.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH] blk: start bypass mode in blk_unregister_queue

On Thu, Apr 04, 2013 at 10:01:14PM +0800, Aaron Lu wrote:
> In blk_register_queue, we will end bypass mode for the queue; but in
> blk_unregister_queue, we didn't start bypass mode for it. This would
> cause the WARN_ON_ONCE(q->bypass_depth < 0) to trigger if the queue gets
> registered, unregistered and then again registered, e.g. unload scsi
> cdrom module driver sr_mod and then reload it will trigger such a
> warning.
> 
> Signed-off-by: Aaron Lu <aaron.lu@...el.com>

Is this something which actually happens?  Why would an unregistered
queue registered again?  Do we even support that?

Starting a bypass mode can be very expensive and some drivers create
and destroy a lot of queues during probing.  We don't want a call to
blk_queue_bypass_start() on every queue creation / destruction cycle.

Thanks.

-- 
tejun
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