netdev - RE: [PATCH net v3 1/1] xen-netback: Handle backend state transitions in a more robust way

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Message-ID: <9AAE0902D5BC7E449B7C8E4E778ABCD0111C7D@AMSPEX01CL01.citrite.net>
Date:	Tue, 24 Sep 2013 15:30:36 +0000
From:	Paul Durrant <Paul.Durrant@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Wei Liu <wei.liu2@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>
Subject: RE: [PATCH net v3 1/1] xen-netback: Handle backend state
 transitions in a more robust way

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@...rix.com]
> Sent: 24 September 2013 16:26
> To: Paul Durrant
> Cc: xen-devel@...ts.xen.org; netdev@...r.kernel.org; Ian Campbell; Wei Liu;
> David Vrabel
> Subject: Re: [PATCH net v3 1/1] xen-netback: Handle backend state
> transitions in a more robust way
> 
> On Tue, Sep 24, 2013 at 03:51:22PM +0100, Paul Durrant wrote:
> > When the frontend state changes metback now specifies its desired state
> to
>                                   netback
> > a new function, set_backend_state(), which transitions through any
> [...]
> > +/* Handle backend state transitions:
> > + *
> > + * The backend state starts in InitWait and the following transtions are
>                                                              transitions
> > + * allowed.
> >
> [...]
> > @@ -363,7 +448,9 @@ static void hotplug_status_changed(struct
> xenbus_watch *watch,
> >  	if (IS_ERR(str))
> >  		return;
> >  	if (len == sizeof("connected")-1 && !memcmp(str, "connected", len))
> {
> > -		xenbus_switch_state(be->dev, XenbusStateConnected);
> > +		/* Complete any pending state change */
> > +		xenbus_switch_state(be->dev, be->state);
> > +
> 
> The state transition takes place iff hotplug status is "connected", is
> this desirable? What if hotplug fails?
> 

The xenbus state will remain in InitWait but the backend state will be Connected.

> If it cycles through connect again it looks like it will trigger that
> BUG_ON in connect()?
> 

No, I don't think so because be->state is not the same as dev->state. The frontend can go to Closing/Closed (which will take dev->state to Closing/Closed) and this should be fine.

  Paul
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