From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Jan Beulich" <JBeulich@novell.com>
Subject: Re: [Pv-ops][PATCH] Netback multiple tasklet
	 support
Date: Fri, 11 Dec 2009 08:34:07 +0000
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To: Ian Campbell <Ian.Campbell@citrix.com>, Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Steven Smith <Steven.Smith@eu.citrix.com>, Ian Pratt <Ian.Pratt@eu.citrix.com>, "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>, Dongxiao Xu <dongxiao.xu@intel.com>
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>>> Ian Campbell <Ian.Campbell@citrix.com> 10.12.09 19:07 >>>
>On Thu, 2009-12-10 at 17:54 +0000, Jeremy Fitzhardinge wrote:=20
>> 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.
>> >   =20
>>=20
>> What's the issue here?
>
>a deadlock if someone naively uncomments the existing code.

Btw., can any of you explain why 2.6.18 needs this (and the related) code,
but pv-ops doesn't?

Thanks, Jan