Re: [RFC PATCH 3/3] hw/virtio: Have virtqueue_get_avail_bytes() pass caches arg to callees

[Stefan Hajnoczi]

On Wed, Sep 01, 2021 at 05:55:38PM +0200, Stefano Garzarella wrote:
> On Thu, Aug 26, 2021 at 07:26:58PM +0200, Philippe Mathieu-Daudé wrote:
> > Both virtqueue_packed_get_avail_bytes() and
> > virtqueue_split_get_avail_bytes() access the region cache, but
> > their caller also does. Simplify by having virtqueue_get_avail_bytes
> > calling both with RCU lock held, and passing the caches as argument.
> > 
> > Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> > ---
> > RFC because I'm not sure this is safe enough
> 
> It seems safe to me.
> 
> While reviewing I saw that vring_get_region_caches() has
> /* Called within rcu_read_lock().  */ comment, but it seems to me that we
> call that function in places where we haven't acquired it, which shouldn't
> be a problem, but should we remove that comment?

Do you have specific examples? That sounds worrying because the caller
can't do much with the returned pointer if it was fetched outside the
RCU read lock.

Stefan

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