linux-kernel - Re: [PATCH] vhost: rcu annotation fixup

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Message-ID: <20110118175500.GA6935@redhat.com>
Date:	Tue, 18 Jan 2011 19:55:00 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.osdl.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vhost: rcu annotation fixup

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@...hat.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 ...

> >  	return acked_features & (1 << bit);
> >  }
> > 
> > -- 
> > 1.7.3.2.91.g446ac
> > --
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