linux-kernel - Re: [Stable-review] [93/93] dm mpath: fix stall when requeueing io

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Date:	Wed, 24 Feb 2010 10:12:48 +0900
From:	Kiyoshi Ueda <k-ueda@...jp.nec.com>
To:	torvalds@...ux-foundation.org, Alasdair Kergon <agk@...hat.com>
CC:	Kiyoshi Ueda <k-ueda@...jp.nec.com>,
	Stefan Bader <stefan.bader@...onical.com>,
	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	Mikulas Patocka <mpatocka@...hat.com>, stable@...nel.org,
	Junichi Nomura <j-nomura@...jp.nec.com>,
	akpm@...ux-foundation.org, stable-review@...nel.org,
	alan@...rguk.ukuu.org.uk
Subject: Re: [Stable-review] [93/93] dm mpath: fix stall when requeueing io

Hi Alasdair, Linus,

On 02/24/2010 03:12 AM +0900, Alasdair G Kergon wrote:
> On Mon, Feb 22, 2010 at 07:16:34PM +0900, Kiyoshi Ueda wrote:
>> On 02/22/2010 01:07 AM +0900, Stefan Bader wrote:
>>>> @@ -1568,12 +1575,16 @@ static void dm_request_fn(struct request
>>>>  
>>>>  		blk_start_request(rq);
>>>>  		spin_unlock(q->queue_lock);
>>>> -		map_request(ti, rq, md);
>>>> +		if (map_request(ti, rq, md))
>>>> +			goto requeued;
>>>>  		spin_lock_irq(q->queue_lock);
>>>>  	}
>> In the current device-mapper code, I would like to go with
>> spin_unlock/lock here.
>> However, there was a case to enable irq in map_requst() for request
>> allocation, and this spin_lock_irq was a work-around for the case.
>> Now, there is no such case in the device-mapper code, so spin_lock should
>> be enough here.  But I'm still using spin_lock_irq for safeness, since
>> there might be some more cases to enable irq during request submission
>> to underlying devices.
>> I'll remove the _irq in the future after lots of testings.
>  
> So, have I understood your reasoning?
> 
> - This function (dm_request_fn) is always called with local interrupts disabled.
> E.g. from generic_unplug_device() or blk_run_queue().
> 
> - The 'map_request()' function was found to re-enable interrupts in one case, but
> that case got fixed.
> 
> - The code still uses spin_lock_irq to ensure they remain disabled as protection
> against there being other cases.  This should be changed to spin_lock as a clean-up
> but you are not aware of any current breakage.

That's correct.
I think the spin_lock_irq can be changed to spin_lock as a clean-up.
But I don't want to break things in this late stage of 2.6.33-rc
and/or the stable tree.
So I'll send the clean-up patch for 2.6.34 once I make sure it's ok.

Thanks,
Kiyoshi Ueda
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