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    SubjectRe: [PATCH] vhost: rcu annotation fixup
    On Tue, Jan 18, 2011 at 11:02:33AM -0800, Paul E. McKenney wrote:
    > On Tue, Jan 18, 2011 at 07:55:00PM +0200, Michael S. Tsirkin wrote:
    > > 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@redhat.com>
    > > > > ---
    > > > > 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 ...
    >
    > I suppose you could pass a pointer to the lockdep map structure.
    > Not sure if this makes sense, but it would handle the situation.

    Will it compile with lockdep disabled too? What will the pointer be?

    > Alternatively, create a helper function that checks the possibilities
    > and screams if none of them are in effect.
    >
    > Thanx, Paul

    The problem here is the callee needs to know about all callers.

    > > > > return acked_features & (1 << bit);
    > > > > }
    > > > >
    > > > > --
    > > > > 1.7.3.2.91.g446ac
    > > > > --
    > > > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    > > > > the body of a message to majordomo@vger.kernel.org
    > > > > More majordomo info at http://vger.kernel.org/majordomo-info.html
    > > > > Please read the FAQ at http://www.tux.org/lkml/


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