linux-kernel - Re: [RFC] virtio: Support releasing lock during kick

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Message-ID: <20110619135507.GA25728@infradead.org>
Date:	Sun, 19 Jun 2011 09:55:07 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Stefan Hajnoczi <stefanha@...ux.vnet.ibm.com>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	axboe@...nel.dk, linux-kernel@...r.kernel.org, hch@...radead.org
Subject: Re: [RFC] virtio: Support releasing lock during kick

On Sun, Jun 19, 2011 at 10:48:41AM +0300, Michael S. Tsirkin wrote:
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 4ce953f..a8672ec 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -433,6 +433,8 @@ void blk_run_queue(struct request_queue *q)
>  	spin_lock_irqsave(q->queue_lock, flags);
>  	__blk_run_queue(q);
>  	spin_unlock_irqrestore(q->queue_lock, flags);
> +	if (q->request_done)
> +		q->request_done(q);

We have quite a few cases where __blk_run_queue is called directly, and
one more (although not applicable to virtio-blk) that calls ->request_fn
directly.

I think Stefan's way is the way to go for now, releasing and reacquiring
the queue lock once in ->request_fn is much less than the common IDE and
SCSI setups do today.

Eventually ->queue_lock should be split from the driver-internal lock,
and we could do a more efficient calling convention than the one per
request blk_peek_request.  I've started looking into that, but it's
going to take a while.

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