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

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Message-ID: <4FC5BDF5.2040000@redhat.com>
Date:	Wed, 30 May 2012 14:28:05 +0800
From:	Asias He <asias@...hat.com>
To:	Tim Gardner <rtg.canonical@...il.com>
CC:	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
	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 05/29/2012 09:45 PM, Tim Gardner wrote:
> On 05/28/2012 07:39 PM, Asias He wrote:
>
> <snip>
>
>> @@ -440,6 +435,11 @@ void blk_cleanup_queue(struct request_queue *q)
>>   	del_timer_sync(&q->backing_dev_info.laptop_mode_wb_timer);
>>   	blk_sync_queue(q);
>>
>> +	spin_lock_irq(lock);
>> +	if (q->queue_lock !=&q->__queue_lock)
>> +		q->queue_lock =&q->__queue_lock;
>> +	spin_unlock_irq(lock);
>> +
>
> 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.

-- 
Asias
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