netdev - Re: [PATCHv2 net-next] xen-netback: fix race between napi

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Message-ID: <20140516083946.GA17887@zion.uk.xensource.com>
Date:	Fri, 16 May 2014 09:39:46 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	David Vrabel <david.vrabel@...rix.com>, <netdev@...r.kernel.org>,
	<xen-devel@...ts.xenproject.org>, Wei Liu <wei.liu2@...rix.com>
Subject: Re: [PATCHv2 net-next] xen-netback: fix race between napi_complete()
 and interrupt handler

On Fri, May 16, 2014 at 09:30:09AM +0100, Ian Campbell wrote:
> On Thu, 2014-05-15 at 18:17 +0100, David Vrabel wrote:
> > When the NAPI budget was not all used, xenvif_poll() would call
> > napi_complete() /after/ enabling the interrupt.  This resulted in a
> > race between the napi_complete() and the napi_schedule() in the
> > interrupt handler.  The use of local_irq_save/restore() avoided by
> > race iff the handler is running on the same CPU but not if it was
> > running on a different CPU.
> > 
> > Fix this properly by calling napi_complete() before reenabling
> > interrupts (in the xenvif_napi_schedule_or_enable_irq() call).
> > 
> > Signed-off-by: David Vrabel <david.vrabel@...rix.com>
> > ---
> > v2:
> > - Rename xenvif_check_rx_xenvif() to
> >   xenvif_napi_schedule_or_enable_irq() to make it more obvious what it
> >   does.
> 
> Which is to update the event pointer such that IRQs are generated again
> rather than enabling IRQs as such (enable_irq makes me think of
> EFLAGS.IF). But given that I can't think of a better way to describe it:
> Acked-by: Ian Campbell <ian.campbell@...rix.com>
> (although in this area Wei's ack is more important than mine IMHO).
> 
> I think this is intended as a replacement to Wei's "xen-netback: don't
> move event pointer in TX credit timeout callback" rather than in
> addition to, correct?
> 

That patch of mine is incorrect and it's not needed anymore.

Wei.

> Ian.
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