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

[Jens Axboe <axboe@kernel.dk>]

On 05/30/2012 08:28 AM, Tejun Heo wrote:
> Hello,
> 
> On Wed, May 30, 2012 at 3:28 PM, Asias He <asias@redhat.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