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

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Message-ID: <20110118174834.GF2193@linux.vnet.ibm.com>
Date:	Tue, 18 Jan 2011 09:48:34 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.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 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?

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

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