Re: [Xen-devel] [PATCH 1/6] xen-gntdev: Fix circular locking dependency

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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

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