linux-kernel - Re: [PATCH 2/2]block cfq: don't use atomic_t for cfq

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Date:	Thu, 23 Dec 2010 10:18:44 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Jens Axboe <jaxboe@...ionio.com>, jmoyer@...hat.com
Subject: Re: [PATCH 2/2]block cfq: don't use atomic_t for cfq_group

On Thu, Dec 23, 2010 at 10:45:35AM +0800, Shaohua Li wrote:
> cfq_group->ref is used with queue_lock hold, the only exception is
> cfq_set_request, which looks like a bug to me, so ref doesn't need
> to be an atomic and atomic operation is slower.
> 

[..]
>  
> @@ -3683,12 +3685,13 @@ new_queue:
>  
>  	cfqq->allocated[rw]++;
>  	cfqq->ref++;
> +	cfqg = cfq_ref_get_cfqg(cfqq->cfqg);
>  
>  	spin_unlock_irqrestore(q->queue_lock, flags);
>  
>  	rq->elevator_private = cic;
>  	rq->elevator_private2 = cfqq;
> -	rq->elevator_private3 = cfq_ref_get_cfqg(cfqq->cfqg);
> +	rq->elevator_private3 = cfqg;

I think you can move every thing under spinlock. IOW, first set the
rq->elevator_private* fields and delay the release of spinlock. Few
days back I was also looking at wondering that why are we releasing
the spinlock early.

Thanks
Vivek
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