On Tue, Jan 18, 2011 at 09:48:34AM -0800, Paul E. McKenney wrote:
> On Tue, Jan 18, 2011 at 01:08:45PM +0200, Michael S. Tsirkin wrote:
> > When built with rcu checks enabled, vhost triggers
> > bogus warnings as vhost features are read without
> > dev->mutex sometimes.
> > Fixing it properly is not trivial as vhost.h does not
> > know which lockdep classes it will be used under.
> > Disable the warning by stubbing out the check for now.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > ---
> > drivers/vhost/vhost.h | 4 +---
> > 1 files changed, 1 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 2af44b7..2d03a31 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -173,9 +173,7 @@ static inline int vhost_has_feature(struct vhost_dev *dev, int bit)
> > {
> > unsigned acked_features;
> >
> > - acked_features =
> > - rcu_dereference_index_check(dev->acked_features,
> > - lockdep_is_held(&dev->mutex));
> > + acked_features = rcu_dereference_index_check(dev->acked_features, 1);
>
> Ouch!!!
>
> Could you please at least add a comment?
Yes, OK.
> Alternatively, pass in the lock that is held and check for that? Given
> that this is a static inline, the compiler should be able to optimize
> the argument away when !PROVE_RCU, correct?
>
> Thanx, Paul
Hopefully, yes. We don't always have a lock: the idea was
to create a lockdep for these cases. But we can't pass
the pointer to that ...
> > return acked_features & (1 << bit);
> > }
> >
> > --
> > 1.7.3.2.91.g446ac
> > --
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