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SubjectRe: Question on vhost_has_feature()
On Tue, Mar 04, 2014 at 05:01:51PM -0800, Paul E. McKenney wrote:
> 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
> >

Or if you see a better way to document the use of rcu here
than the comment above, I'll gladly do it too.



--
MST


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