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

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Date:	Fri, 01 Jun 2012 11:31:37 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	Asias He <asias@...hat.com>, 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

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.

-- 
Jens Axboe

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