[net-next,2/6] net: bridge: Populate the pvid flag in br_vlan_get_info

Currently this simplified code snippet fails:

	br_vlan_get_pvid(netdev, &pvid);
	br_vlan_get_info(netdev, pvid, &vinfo);
	ASSERT(!(vinfo.flags & BRIDGE_VLAN_INFO_PVID));

It is intuitive that the pvid of a netdevice should have the
BRIDGE_VLAN_INFO_PVID flag set.

However I can't seem to pinpoint a commit where this behavior was
introduced. It seems like it's been like that since forever.

At a first glance it would make more sense to just handle the
BRIDGE_VLAN_INFO_PVID flag in __vlan_add_flags. However, as Nikolay
explains:

  There are a few reasons why we don't do it, most importantly because
  we need to have only one visible pvid at any single time, even if it's
  stale - it must be just one. Right now that rule will not be violated
  by this change, but people will try using this flag and could see two
  pvids simultaneously. You can see that the pvid code is even using
  memory barriers to propagate the new value faster and everywhere the
  pvid is read only once.  That is the reason the flag is set
  dynamically when dumping entries, too.  A second (weaker) argument
  against would be given the above we don't want another way to do the
  same thing, specifically if it can provide us with two pvids (e.g. if
  walking the vlan list) or if it can provide us with a pvid different
  from the one set in the vg. [Obviously, I'm talking about RCU
  pvid/vlan use cases similar to the dumps.  The locked cases are fine.
  I would like to avoid explaining why this shouldn't be relied upon
  without locking]

So instead of introducing the above change and making sure of the pvid
uniqueness under RCU, simply dynamically populate the pvid flag in
br_vlan_get_info().

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
---
 net/bridge/br_vlan.c | 2 ++
 1 file changed, 2 insertions(+)

On 8/20/19 2:59 AM, Vladimir Oltean wrote:
> Currently this simplified code snippet fails:
> 
> 	br_vlan_get_pvid(netdev, &pvid);
> 	br_vlan_get_info(netdev, pvid, &vinfo);
> 	ASSERT(!(vinfo.flags & BRIDGE_VLAN_INFO_PVID));
> 
> It is intuitive that the pvid of a netdevice should have the
> BRIDGE_VLAN_INFO_PVID flag set.
> 
> However I can't seem to pinpoint a commit where this behavior was
> introduced. It seems like it's been like that since forever.
> 
> At a first glance it would make more sense to just handle the
> BRIDGE_VLAN_INFO_PVID flag in __vlan_add_flags. However, as Nikolay
> explains:
> 
>   There are a few reasons why we don't do it, most importantly because
>   we need to have only one visible pvid at any single time, even if it's
>   stale - it must be just one. Right now that rule will not be violated
>   by this change, but people will try using this flag and could see two
>   pvids simultaneously. You can see that the pvid code is even using
>   memory barriers to propagate the new value faster and everywhere the
>   pvid is read only once.  That is the reason the flag is set
>   dynamically when dumping entries, too.  A second (weaker) argument
>   against would be given the above we don't want another way to do the
>   same thing, specifically if it can provide us with two pvids (e.g. if
>   walking the vlan list) or if it can provide us with a pvid different
>   from the one set in the vg. [Obviously, I'm talking about RCU
>   pvid/vlan use cases similar to the dumps.  The locked cases are fine.
>   I would like to avoid explaining why this shouldn't be relied upon
>   without locking]
> 
> So instead of introducing the above change and making sure of the pvid
> uniqueness under RCU, simply dynamically populate the pvid flag in
> br_vlan_get_info().
> 
> Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
> ---
>  net/bridge/br_vlan.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/bridge/br_vlan.c b/net/bridge/br_vlan.c
> index f5b2aeebbfe9..bb98984cd27d 100644
> --- a/net/bridge/br_vlan.c
> +++ b/net/bridge/br_vlan.c
> @@ -1281,6 +1281,8 @@ int br_vlan_get_info(const struct net_device *dev, u16 vid,
>  
>  	p_vinfo->vid = vid;
>  	p_vinfo->flags = v->flags;
> +	if (vid == br_get_pvid(vg))
> +		p_vinfo->flags |= BRIDGE_VLAN_INFO_PVID;
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(br_vlan_get_info);
> 

Looks good, thanks!
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>