linux-kernel - Re: [PATCH 1/2] xen-blkfront: don't call talk_to_blkback when already connected to blkback

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Date:	Tue, 31 May 2016 16:33:07 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Bob Liu <bob.liu@...cle.com>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	roger.pau@...rix.com
Subject: Re: [PATCH 1/2] xen-blkfront: don't call talk_to_blkback when
 already connected to blkback

On Tue, May 31, 2016 at 04:59:16PM +0800, Bob Liu wrote:
> Sometimes blkfont may receive twice blkback_changed() notification after
> migration, then talk_to_blkback() will be called twice too and confused
> xen-blkback.

Could you enlighten the patch description by having some form of
state transition here? I am curious how you got the frontend
to get in XenbusStateConnected (via blkif_recover right) and then
the backend triggering the update once more?

Or is just a simple race - the backend moves from XenbusStateConnected->
XenbusStateConnected - which retriggers the frontend to hit in
blkback_changed the XenbusStateConnected state and go in there?
(That would be in conenct_ring changing the state). But I don't
see how the frontend_changed code get there as we have:

 770                 /*
 771                  * Ensure we connect even when two watches fire in
 772                  * close succession and we miss the intermediate value
 773                  * of frontend_state.
 774                  */
 775                 if (dev->state == XenbusStateConnected)
 776                         break;
 777 

?

Now what about 'blkfront_connect' being called on the second time?

Ah, info->connected is probably by then in BLKIF_STATE_CONNECTED
(as blkif_recover changed) and we just reread the size of the disk.

Is that how about the flow goes?
> 
> Signed-off-by: Bob Liu <bob.liu@...cle.com>
> ---
>  drivers/block/xen-blkfront.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index ca13df8..01aa460 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -2485,7 +2485,8 @@ static void blkback_changed(struct xenbus_device *dev,
>  		break;
>  
>  	case XenbusStateConnected:
> -		if (dev->state != XenbusStateInitialised) {
> +		if ((dev->state != XenbusStateInitialised) &&
> +			(dev->state != XenbusStateConnected)) {
>  			if (talk_to_blkback(dev, info))
>  				break;
>  		}
> -- 
> 2.7.4
>