netdev - Re: [PATCH] vhost: rcu annotation fixup

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Date:	Tue, 18 Jan 2011 22:10:31 +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 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@...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 ...
> 
> 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
> > > > --
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