netdev - Re: [RFC PATCH v2 net-next 3/4] net: dsa: hold rtnl_mutex when calling dsa_master

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Date:   Fri, 10 Dec 2021 20:17:02 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Ansuel Smith <ansuelsmth@...il.com>
Subject: Re: [RFC PATCH v2 net-next 3/4] net: dsa: hold rtnl_mutex when
 calling dsa_master_{setup,teardown}

On Thu, Dec 09, 2021 at 07:39:26PM +0200, Vladimir Oltean wrote:
> DSA needs to simulate master tracking events when a binding is first
> with a DSA master established and torn down, in order to give drivers
> the simplifying guarantee that ->master_up and ->master_going_down calls
> are made in exactly this order. To avoid races, we need to block the
> reception of NETDEV_UP/NETDEV_GOING_DOWN events in the netdev notifier
> chain while we are changing the master's dev->dsa_ptr (this changes what
> netdev_uses_dsa(dev) reports).

This paragraph needs to be updated. For one, "->master_up and
->master_going down calls are made in exactly this order" needs to
become something like "->master_change calls are made only when the
master's readiness state to pass traffic changes". Then, "block the
reception of NETDEV_UP/NETDEV_GOING_DOWN" must also mention
"NETDEV_CHANGE".

> 
> The dsa_master_setup() and dsa_master_teardown() functions optionally
> require the rtnl_mutex to be held, if the tagger needs the master to be
> promiscuous, these functions call dev_set_promiscuity(). Move the
> rtnl_lock() from that function and make it top-level.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> ---
>  net/dsa/dsa2.c   | 8 ++++++++
>  net/dsa/master.c | 4 ++--
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/net/dsa/dsa2.c b/net/dsa/dsa2.c
> index a6cb3470face..6d4422c9e334 100644
> --- a/net/dsa/dsa2.c
> +++ b/net/dsa/dsa2.c
> @@ -1015,6 +1015,8 @@ static int dsa_tree_setup_master(struct dsa_switch_tree *dst)
>  	struct dsa_port *dp;
>  	int err;
>  
> +	rtnl_lock();
> +
>  	list_for_each_entry(dp, &dst->ports, list) {
>  		if (dsa_port_is_cpu(dp)) {
>  			err = dsa_master_setup(dp->master, dp);
> @@ -1023,6 +1025,8 @@ static int dsa_tree_setup_master(struct dsa_switch_tree *dst)
>  		}
>  	}
>  
> +	rtnl_unlock();
> +
>  	return 0;
>  }
>  
> @@ -1030,9 +1034,13 @@ static void dsa_tree_teardown_master(struct dsa_switch_tree *dst)
>  {
>  	struct dsa_port *dp;
>  
> +	rtnl_lock();
> +
>  	list_for_each_entry(dp, &dst->ports, list)
>  		if (dsa_port_is_cpu(dp))
>  			dsa_master_teardown(dp->master);
> +
> +	rtnl_unlock();
>  }
>  
>  static int dsa_tree_setup_lags(struct dsa_switch_tree *dst)
> diff --git a/net/dsa/master.c b/net/dsa/master.c
> index f4efb244f91d..2199104ca7df 100644
> --- a/net/dsa/master.c
> +++ b/net/dsa/master.c
> @@ -267,9 +267,9 @@ static void dsa_master_set_promiscuity(struct net_device *dev, int inc)
>  	if (!ops->promisc_on_master)
>  		return;
>  
> -	rtnl_lock();
> +	ASSERT_RTNL();
> +
>  	dev_set_promiscuity(dev, inc);
> -	rtnl_unlock();
>  }
>  
>  static ssize_t tagging_show(struct device *d, struct device_attribute *attr,
> -- 
> 2.25.1
>