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From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: Bob Liu <bob.liu@oracle.com>
Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
	roger.pau@citrix.com
Subject: Re: [PATCH 1/2] xen-blkfront: don't call talk_to_blkback when already connected to blkback
Date: Thu, 2 Jun 2016 10:30:09 -0400	[thread overview]
Message-ID: <20160602143009.GB23598__9003.69915548271$1464878016$gmane$org@char.us.oracle.com> (raw)
In-Reply-To: <574E7763.9060001@oracle.com>

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@oracle.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
> >>

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  parent reply	other threads:[~2016-06-02 14:30 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <1464685157-30738-1-git-send-email-bob.liu@oracle.com>
2016-05-31  8:59 ` [PATCH 2/2] xen-blkfront: fix resume issues Bob Liu
2016-05-31 20:33 ` [PATCH 1/2] xen-blkfront: don't call talk_to_blkback when already connected to blkback Konrad Rzeszutek Wilk
     [not found] ` <20160531203307.GC23808@char.us.oracle.com>
2016-06-01  5:49   ` Bob Liu
     [not found]   ` <574E7763.9060001@oracle.com>
2016-06-02 14:30     ` Konrad Rzeszutek Wilk [this message]
2016-06-07 15:25     ` Konrad Rzeszutek Wilk
     [not found]     ` <20160607152524.GA10281@localhost.localdomain>
2016-06-08  6:46       ` Bob Liu
     [not found]       ` <5757BF4E.9080307@oracle.com>
2016-06-08 14:47         ` Konrad Rzeszutek Wilk
     [not found]         ` <20160608144739.GD17027@char.us.oracle.com>
2016-06-15  8:39           ` Ross Lagerwall
     [not found]           ` <57611424.2020802@citrix.com>
2016-06-15 14:08             ` Konrad Rzeszutek Wilk
2016-05-31  8:59 Bob Liu

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