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

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Date:	Wed, 01 Jun 2016 13:49:23 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...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 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!
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.

>>
>> 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
>>