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

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Message-ID: <20130404142058.GF9425@htj.dyndns.org>
Date:	Thu, 4 Apr 2013 07:20:58 -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

Hello,

On Thu, Apr 04, 2013 at 10:16:39PM +0800, Aaron Lu wrote:
> It probably wouldn't happen for normal user, only for some SCSI driver
> developers like me, e.g. the sr_mod will normally load during boot, and
> when I made some changes to the code, I'll unload the driver and reload
> it. This is found when I was developing ZPODD code and I just found some
> time to see what happened.

Does that unregister and register the same queue?  Ah, crap, it does.

> The queue for the scsi device will always be there, the queue(and the
> device) will not go away on driver unregistration. So it will be left in
> normal mode, and on next blk_register_queue call, the warning will show
> up.
> 
> > 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.
> 
> By expensive, do you mean the drain of the queue? Since the queue is to
> be unregistered, I suppose the queue has to be drained somewhere?

The problematic one is synchronize_rcu().  It adds up pretty quickly.
I'd suggest just skipping blk_queue_bypass_end() during registration
if the queue is not bypassing.

Thanks.

-- 
tejun
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