netdev - RE: [PATCH net-next] xen-netback: stop vif thread spinning if frontend is unresponsive

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Date:	Wed, 8 Jan 2014 09:55:13 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'David Miller' <davem@...emloft.net>,
	"paul.durrant@...rix.com" <paul.durrant@...rix.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"wei.liu2@...rix.com" <wei.liu2@...rix.com>,
	"ian.campbell@...rix.com" <ian.campbell@...rix.com>,
	"david.vrabel@...rix.com" <david.vrabel@...rix.com>
Subject: RE: [PATCH net-next] xen-netback: stop vif thread spinning if
 frontend is unresponsive

> From: David Miller
...
> > -	if (!npo.copy_prod)
> > +	if (!npo.copy_prod) {
> > +		if (ring_full)
> > +			vif->rx_queue_stopped = true;
> >  		goto done;
> > +	}
> > +
> > +	vif->rx_queue_stopped = false;
> 
> And then you can code this as:
> 
> 	vif->rx_queue_stopped = (!npo.copy_prod && ring_full);
> 	if (!npo.copy_prod)
> 		goto done;

Which isn't quite the same...
1) It always writes vif->rx_queue_stopped, the old code could
   leave it unchanged.
2) If 'npo' is global then the compiler can't assume that 'vif'
   doesn't alias it so may have to re-read it following the
   write to 'vif->rx_queue_stopped'.

	David



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