On Tue, Mar 04, 2014 at 09:56:42AM +0200, Michael S. Tsirkin wrote:
> On Mon, Mar 03, 2014 at 11:44:23AM -0800, Paul E. McKenney wrote:
> > Hello, Michael,
> >
> > Just curious about the purpose of the rcu_dereference_index_check() in
> > vhost_has_feature(). I don't see how it fits in. The closest thing
> > I see if the use in handle_rx(), where it selects vq->log or NULL. But
> > in that case, I would expect the usual RCU usage pattern to wrap an
> > rcu_dereference() around the vq->log.
> >
> > Enlightenment?
> >
> > Thanx, Paul
>
> Hi Paul,
>
> Yes, it's weird. As you say the use is in handle_rx.
> The way it's supposed to work is that readers take vq mutex, and
> writers change the value and then take and release
> vq mutex.
> We did it like this because there are thinkably multiple vqs.
>
> I tried to document it in vhost.h :
> /* Readers use RCU to access memory table pointer
> * log base pointer and features.
> * Writers use mutex below.*/
>
> If this is a problem, it's possible to restructure the code to avoid
> this pattern for vhost_has_feature, pls let me know.
I am not yet sure whether or not it is a problem, it just looked a bit
strange. ;-)
Thanx, Paul
> Thanks a lot for paying attention!
>
>
> --
> MST
>
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