Subject: Re: [PATCH v2] macvtap: rcu_dereference outside read-lock section

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 12 Aug 2012 11:49:29 +0300

On Sun, Aug 12, 2012 at 10:26:31AM +0400, Denis Efremov wrote:
> rcu_dereference occurs in update section. Replacement by
> rcu_dereference_protected in order to prevent lockdep
> complaint.
> 
> Found by Linux Driver Verification project (linuxtesting.org)
> 
> Signed-off-by: Denis Efremov <[email-masked]>
> ---
>  drivers/net/macvtap.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> index 0737bd4..0f0f9ce 100644
> --- a/drivers/net/macvtap.c
> +++ b/drivers/net/macvtap.c
> @@ -94,7 +94,8 @@ static int get_slot(struct macvlan_dev *vlan, struct macvtap_queue *q)
>  	int i;
>  
>  	for (i = 0; i < MAX_MACVTAP_QUEUES; i++) {
> -		if (rcu_dereference(vlan->taps[i]) == q)
> +		if (rcu_dereference_protected(vlan->taps[i],
> +					      lockdep_is_held(&macvtap_lock)) == q)

And now it's too long.  Maybe a temporary would be better?

		struct macvtap_queue *t;
		t = rcu_dereference_protected(vlan->taps[i],
					      lockdep_is_held(&macvtap_lock));
		if (t == q)
  			return i;
		
?

Anyway -

Acked-by: Michael S. Tsirkin <[email-masked]>

>  			return i;
>  	}
> -- 
> 1.7.7