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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 16 Dec 2010, Jeremy Fitzhardinge wrote:
> 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.
 
Agreed. I'll add it in the next version.

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