netdev - Possible to use both dev_mc_sync and __dev_mc

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Date:   Mon, 21 Mar 2022 18:32:13 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Alexander Duyck <alexanderduyck@...com>,
        Jakub Kicinski <kuba@...nel.org>, Jiri Pirko <jiri@...dia.com>,
        Florian Fainelli <f.fainelli@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Possible to use both dev_mc_sync and __dev_mc_sync?

Hello,

Despite the similar names, the 2 functions above serve quite different
purposes, and as it happens, DSA needs to use both of them, each for its
own purpose.

static void dsa_slave_set_rx_mode(struct net_device *dev)
{
	struct net_device *master = dsa_slave_to_master(dev);
	struct dsa_port *dp = dsa_slave_to_port(dev);
	struct dsa_switch *ds = dp->ds;

	dev_mc_sync(master, dev); // DSA is a stacked device
	dev_uc_sync(master, dev);
	if (dsa_switch_supports_mc_filtering(ds))
		__dev_mc_sync(dev, dsa_slave_sync_mc, dsa_slave_unsync_mc); // DSA is also a hardware device
	if (dsa_switch_supports_uc_filtering(ds))
		__dev_uc_sync(dev, dsa_slave_sync_uc, dsa_slave_unsync_uc);
}

What I'm noticing is that some addresses, for example 33:33:00:00:00:01
(added by addrconf.c as in6addr_linklocal_allnodes) are synced to the
master via dev_mc_sync(), but not to hardware by __dev_mc_sync().

Superficially, this is because dev_mc_sync() -> __hw_addr_sync_one()
will increase ha->sync_cnt to a non-zero value. Then, when __dev_mc_sync()
-> __hw_addr_sync_dev() checks ha->sync_cnt, it sees that it has been
"already synced" (not really), so it doesn't call the "sync" method
(dsa_slave_sync_mc) for this ha.

However I don't understand the deep reasons and I am confused by the
members of struct netdev_hw_addr (synced vs sync_cnt vs refcount).
I can't tell if this was supposed to work, given that "sync address to
another device" is conceptually a different kind of sync than "sync
address to hardware", so I'm a bit surprised that they share the same
variables.

Any ideas?