linux-kernel - Re: [PATCH V3] block: Mitigate lock unbalance caused by lock switching

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Date:	Wed, 06 Jun 2012 10:12:43 +0800
From:	Asias He <asias@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
CC:	Tejun Heo <tj@...nel.org>, Tim Gardner <rtg.canonical@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	tim.gardner@...onical.com
Subject: Re: [PATCH V3] block: Mitigate lock unbalance caused by lock switching

Hello, Jens

On 06/01/2012 05:31 PM, Jens Axboe wrote:
> On 05/30/2012 08:28 AM, Tejun Heo wrote:
>> Hello,
>>
>> On Wed, May 30, 2012 at 3:28 PM, Asias He<asias@...hat.com>  wrote:
>>>> Isn't the 'if' clause superfluous ? You could just do the assignment,
>>>> e.g.,
>>>>
>>>> +       spin_lock_irq(lock);
>>>> +       q->queue_lock =&q->__queue_lock;
>>>> +       spin_unlock_irq(lock);
>>>
>>>
>>> Well, this saves a if clause but adds an unnecessary assignment if the lock
>>> is already internal lock.
>>
>> It's not hot path. Dirtying the cacheline there doesn't mean anything.
>> I don't really care either way but making optimization argument is
>> pretty silly here.
>
> And more importantly, dropping the if loses information as well. That's
> a lot more important than any misguided optimization attempts. So I
> agree, the if stays.

Could you pick this patch in your tree?

-- 
Asias
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