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SubjectRe: [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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