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

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Date:	Thu, 04 Apr 2013 22:24:56 +0800
From:	Aaron Lu <aaron.lu@...el.com>
To:	Tejun Heo <tj@...nel.org>
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 04/04/2013 10:20 PM, Tejun Heo wrote:
> 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.

OK, thanks for the suggestion!

-Aaron

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