Re: [PATCHv7 3/3] vhost_net: a kernel-level virtio server

Andi Kleen
2009-11-04T13:15:29Z

On Wed, Nov 04, 2009 at 03:08:28PM +0200, Michael S. Tsirkin wrote:
> On Wed, Nov 04, 2009 at 01:59:57PM +0100, Andi Kleen wrote:
> > > Fine?
> >
> > I cannot say -- are there paths that could drop the device beforehand?
>
> Do you mean drop the mm reference?

No the reference to the device, which owns the mm for you.

>
> > (as in do you hold a reference to it?)
>
> By design I think I always have a reference to mm before I use it.
>
> This works like this:
> ioctl SET_OWNER - calls get_task_mm, I think this gets a reference to mm
> ioctl SET_BACKEND - checks that SET_OWNER was run, starts virtqueue
> ioctl RESET_OWNER - stops virtqueues, drops the reference to mm
> file close - stops virtqueues, if we still have it then drops mm
>
> This is why I think I can call use_mm/unuse_mm while virtqueue is running,
> safely.
> Makes sense?

Do you protect against another thread doing RESET_OWNER in parallel while
RESET_OWNER runs?

-Andi

--
ak@linux.intel.com -- Speaking for myself only.