Re: [PATCH v3 net-next 08/11] net: bridge: put SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS on the blocking call chain

[Nikolay Aleksandrov]

On 10/02/2021 11:14, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@xxxxxxx>
> 
> Since we would like br_switchdev_set_port_flag to not use an atomic
> notifier, it should be called from outside spinlock context.
> 
> We can temporarily drop br->lock, but that creates some concurrency
> complications (example below is given for sysfs):
> - There might be an "echo 1 > multicast_flood" simultaneous with an
>   "echo 0 > multicast_flood". The result of this is nondeterministic
>   either way, so I'm not too concerned as long as the result is
>   consistent (no other flags have changed).
> - There might be an "echo 1 > multicast_flood" simultaneous with an
>   "echo 0 > learning". My expectation is that none of the two writes are
>   "eaten", and the final flags contain BR_MCAST_FLOOD=1 and BR_LEARNING=0
>   regardless of the order of execution. That is actually possible if, on
>   the commit path, we don't do a trivial "p->flags = flags" which might
>   overwrite bits outside of our mask, but instead we just change the
>   flags corresponding to our mask.
> 

Not sure I follow here, how do we get any concurrency issues with sysfs or netlink
when both take rtnl before doing any changes ?

> Now that br_switchdev_set_port_flag is never called from under br->lock,
> it runs in sleepable context.
> 
> All switchdev drivers handle SWITCHDEV_PORT_ATTR_SET as both blocking
> and atomic, so no changes are needed on that front.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
> ---
> Changes in v3:
> - Drop the br->lock around br_switchdev_set_port_flag in this patch, for
>   both sysfs and netlink.
> - Only set/restore the masked bits in p->flags to avoid concurrency
>   issues.
> 
> Changes in v2:
> Patch is new.
> 
>  net/bridge/br_netlink.c   | 10 +++++++---
>  net/bridge/br_switchdev.c |  5 ++---
>  net/bridge/br_sysfs_if.c  | 22 ++++++++++++++--------
>  3 files changed, 23 insertions(+), 14 deletions(-)
> 
> diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
> index b7731614c036..8f09106966c4 100644
> --- a/net/bridge/br_netlink.c
> +++ b/net/bridge/br_netlink.c
> @@ -869,7 +869,7 @@ static void br_set_port_flag(struct net_bridge_port *p, struct nlattr *tb[],
>  static int br_setport(struct net_bridge_port *p, struct nlattr *tb[],
>  		      struct netlink_ext_ack *extack)
>  {
> -	unsigned long old_flags, changed_mask;
> +	unsigned long flags, old_flags, changed_mask;
>  	bool br_vlan_tunnel_old;
>  	int err;
>  
> @@ -896,10 +896,14 @@ static int br_setport(struct net_bridge_port *p, struct nlattr *tb[],
>  	br_set_port_flag(p, tb, IFLA_BRPORT_ISOLATED, BR_ISOLATED);
>  
>  	changed_mask = old_flags ^ p->flags;
> +	flags = p->flags;
>  
> -	err = br_switchdev_set_port_flag(p, p->flags, changed_mask, extack);
> +	spin_unlock_bh(&p->br->lock);
> +	err = br_switchdev_set_port_flag(p, flags, changed_mask, extack);
> +	spin_lock_bh(&p->br->lock);
>  	if (err) {
> -		p->flags = old_flags;
> +		p->flags &= ~changed_mask;
> +		p->flags |= (old_flags & changed_mask);
>  		goto out;
>  	}
>  
> diff --git a/net/bridge/br_switchdev.c b/net/bridge/br_switchdev.c
> index dbd94156960f..a79164ee65b9 100644
> --- a/net/bridge/br_switchdev.c
> +++ b/net/bridge/br_switchdev.c
> @@ -79,9 +79,8 @@ int br_switchdev_set_port_flag(struct net_bridge_port *p,
>  	attr.u.brport_flags.val = flags & mask;
>  	attr.u.brport_flags.mask = mask;
>  
> -	/* We run from atomic context here */
> -	err = call_switchdev_notifiers(SWITCHDEV_PORT_ATTR_SET, p->dev,
> -				       &info.info, extack);
> +	err = call_switchdev_blocking_notifiers(SWITCHDEV_PORT_ATTR_SET, p->dev,
> +						&info.info, extack);
>  	err = notifier_to_errno(err);
>  	if (err == -EOPNOTSUPP)
>  		return 0;
> diff --git a/net/bridge/br_sysfs_if.c b/net/bridge/br_sysfs_if.c
> index 72e92376eef1..3f21fdd1cdaa 100644
> --- a/net/bridge/br_sysfs_if.c
> +++ b/net/bridge/br_sysfs_if.c
> @@ -68,16 +68,22 @@ static int store_flag(struct net_bridge_port *p, unsigned long v,
>  	else
>  		flags &= ~mask;
>  
> -	if (flags != p->flags) {
> -		err = br_switchdev_set_port_flag(p, flags, mask, &extack);
> -		if (err) {
> -			netdev_err(p->dev, "%s\n", extack._msg);
> -			return err;
> -		}
> +	if (flags == p->flags)
> +		return 0;
>  
> -		p->flags = flags;
> -		br_port_flags_change(p, mask);
> +	spin_unlock_bh(&p->br->lock);
> +	err = br_switchdev_set_port_flag(p, flags, mask, &extack);
> +	spin_lock_bh(&p->br->lock);
> +	if (err) {
> +		netdev_err(p->dev, "%s\n", extack._msg);
> +		return err;
>  	}
> +
> +	p->flags &= ~mask;
> +	p->flags |= (flags & mask);
> +
> +	br_port_flags_change(p, mask);
> +
>  	return 0;
>  }
>  
>