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

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Date:	Thu, 2 Jun 2016 10:30:09 -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 Wed, Jun 01, 2016 at 01:49:23PM +0800, Bob Liu wrote:
> 
> On 06/01/2016 04:33 AM, Konrad Rzeszutek Wilk wrote:
> > 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?
> 
> 	blkfront 					blkback
> blkfront_resume()                                   
>  > talk_to_blkback()
>   > Set blkfront to XenbusStateInitialised
> 						Front changed()
>   						 > Connect()
> 						  > Set blkback to XenbusStateConnected
> 
> blkback_changed()
>  > Skip talk_to_blkback()
>    because frontstate == XenbusStateInitialised
>  > blkfront_connect()
>   > Set blkfront to XenbusStateConnected
> 
> 
> ------------------------------------------------------------------
> But sometimes blkfront receives
> blkback_changed() event more than once!

Could the control stack (xend) be doing this?

> Not sure why.
> 
> blkback_changed()
>  > because now frontstate != XenbusStateInitialised
>    talk_to_blkback() is also called again
>   > blkfront state changed from 
>     XenbusStateConnected to XenbusStateInitialised
>     (Which is not correct!)
> 
> 
>     						Front_changed():
> 						 > Do nothing because blkback
> 						   already in XenbusStateConnected
> 
> 
> Now blkback is XenbusStateConnected but blkfront still XenbusStateInitialised.

Ah!
> 
> >>
> >> 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
> >>