From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Campbell <Ian.Campbell@citrix.com>
Subject: Re: [Pv-ops][PATCH] Netback multiple tasklet support
Date: Thu, 10 Dec 2009 18:07:24 +0000
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To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Steven Smith <Steven.Smith@eu.citrix.com>, "Xu,
	Dongxiao" <dongxiao.xu@intel.com>, "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>, Ian Pratt <Ian.Pratt@eu.citrix.com>
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On Thu, 2009-12-10 at 17:54 +0000, Jeremy Fitzhardinge wrote: 
> On 12/10/09 01:07, Ian Campbell wrote:
> > Subject: xen: ensure locking gnttab_copy_grant_page is safe against interrupts.
> >
> > Now that netback processing occurs in a thread instead of a tasklet
> > gnttab_copy_grant_page needs to be safe against interrupts.
> >
> > The code is currently commented out in this tree but on 2.6.18 we observed a
> > deadlock where the netback thread called gnttab_copy_grant_page, locked
> > gnttab_dma_lock for writing, was interrupted and on return from interrupt the
> > network stack's TX tasklet ended up calling __gnttab_dma_map_page via the
> > hardware driver->swiotlb and tries to take gnttab_dma_lock for reading.
> >
> > Correct the commented code so we don't get bitten if/when it is re-enabled.
> >    
> 
> What's the issue here?

a deadlock if someone naively uncomments the existing code.

>   I'm inclined to just remove the commented-out 
> code if it isn't being used, and re-evaluate the locking if/when it 
> becomes necessary.

that would be fine also.

Ian.