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Date:	Wed, 19 Dec 2012 09:55:58 -0200
From:	Rafael Aquini <aquini@...hat.com>
To:	Luiz Capitulino <lcapitulino@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org, riel@...hat.com,
	mst@...hat.com, amit.shah@...hat.com, agl@...ibm.com
Subject: Re: [RFC 1/2] virtio_balloon: move locking to the balloon thread

On Tue, Dec 18, 2012 at 06:17:29PM -0200, Luiz Capitulino wrote:
> Today, the balloon_lock mutex is taken and released by fill_balloon()
> and leak_balloon() when both functions are entered and when they
> return.
> 
> This commit moves the locking to the caller instead, which is
> the balloon() thread. The balloon thread is the sole caller of those
> functions today.
> 
> The reason for this move is that the next commit will introduce
> a shrinker callback for the balloon driver, which will also call
> leak_balloon() but will require different locking semantics.
> 
> Signed-off-by: Luiz Capitulino <lcapitulino@...hat.com>
> ---
>  drivers/virtio/virtio_balloon.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 2a70558..877e695 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -133,7 +133,6 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
>  	/* We can only do one array worth at a time. */
>  	num = min(num, ARRAY_SIZE(vb->pfns));
>  
> -	mutex_lock(&vb->balloon_lock);
>  	for (vb->num_pfns = 0; vb->num_pfns < num;
>  	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
>  		struct page *page = balloon_page_enqueue(vb_dev_info);
> @@ -155,7 +154,6 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
>  	/* Did we get any? */
>  	if (vb->num_pfns != 0)
>  		tell_host(vb, vb->inflate_vq);
> -	mutex_unlock(&vb->balloon_lock);
>  }
>

Since you're removing the locking scheme from within this function, I think it
would be a good idea introduce a comment stating its caller must held the mutex
vb->balloon_lock.

  
>  static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
> @@ -177,7 +175,6 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
>  	/* We can only do one array worth at a time. */
>  	num = min(num, ARRAY_SIZE(vb->pfns));
>  
> -	mutex_lock(&vb->balloon_lock);
>  	for (vb->num_pfns = 0; vb->num_pfns < num;
>  	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
>  		page = balloon_page_dequeue(vb_dev_info);
> @@ -193,7 +190,6 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
>  	 * is true, we *have* to do it in this order
>  	 */
>  	tell_host(vb, vb->deflate_vq);
> -	mutex_unlock(&vb->balloon_lock);
>  	release_pages_by_pfn(vb->pfns, vb->num_pfns);
>  }
>

ditto

  
> @@ -306,11 +302,13 @@ static int balloon(void *_vballoon)
>  					 || freezing(current));
>  		if (vb->need_stats_update)
>  			stats_handle_request(vb);
> +		mutex_lock(&vb->balloon_lock);
>  		if (diff > 0)
>  			fill_balloon(vb, diff);
>  		else if (diff < 0)
>  			leak_balloon(vb, -diff);
>  		update_balloon_size(vb);
> +		mutex_unlock(&vb->balloon_lock);
>  	}
>  	return 0;
>  }

Just a nitpick:
As leak_balloon() is also called at remove_common(), you'll need to introduce the
mutex there, similarly.


Thanks for move this forward.

Cheers!
-- Rafael
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