From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: [PATCH 1/6] xen-gntdev: Fix circular
	locking	dependency
Date: Wed, 15 Dec 2010 16:28:31 -0800
Message-ID: <4D095D2F.8070002@goop.org>
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	<1292406442.32368.2613.camel@zakaz.uk.xensource.com>
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To: Ian Campbell <Ian.Campbell@eu.citrix.com>
Cc: Daniel De Graaf <dgdegra@tycho.nsa.gov>, xen-devel <xen-devel@lists.xensource.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>
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On 12/15/2010 01:47 AM, Ian Campbell wrote:
> On Tue, 2010-12-14 at 21:40 +0000, Daniel De Graaf wrote:
>> On 12/14/2010 04:11 PM, Jeremy Fitzhardinge wrote:
>>> On 12/14/2010 06:55 AM, Daniel De Graaf wrote:
>>>> apply_to_page_range will acquire PTE lock while priv->lock is held, and
>>>> mn_invl_range_start tries to acquire priv->lock with PTE already held.
>>>> Fix by not holding priv->lock during the entire map operation.
>>> Is priv->lock needed to protect the contents of map?
>>>
>>>     J
>> No, since the map can only be mapped once (checked by map->vma assignment
>> while the lock is held). The unmap ioctl will return -EBUSY until
>> an munmap() is called on the area, so it also can't race, and the other
>> users are only active once the mmap operation completes.
> Sounds reasonable enough to me. There are a few unlocked accesses to
> vma->map: 
>         gntdev_del_map (when called from gntdev_ioctl_map_grant_ref)
>         gntdev_vma_close
> are these safe? If so then it would be worth a comment about why.
>
> Anyway this patch appears to resolve the lockdep warning I was seeing
> with 2.6.37 with qemu-xen backed block devices.

Good.  Stefano should stick this on his patch queue.

    J