netdev - Re: for_each_netdev_rcu() protected by RTNL and CONFIG_PROVE_RCU

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Message-ID: <20250207-adamant-copper-jackrabbit-27e9fc@leitao>
Date: Fri, 7 Feb 2025 03:26:14 -0800
From: Breno Leitao <leitao@...ian.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Kuniyuki Iwashima <kuniyu@...zon.com>, andrew+netdev@...n.ch,
	kernel-team@...a.com, kuba@...nel.org, netdev@...r.kernel.org,
	ushankar@...estorage.com
Subject: Re: for_each_netdev_rcu() protected by RTNL and CONFIG_PROVE_RCU_LIST

Hello Eric,

On Fri, Feb 07, 2025 at 11:56:53AM +0100, Eric Dumazet wrote:

> > I suppose we will need to move some of definitions around, but, I am
> > NOT confident in which way.
> 
> Note that we have different accessors like rtnl_dereference() and
> rcu_dereference_rtnl()

Makes sense. I suppose that would be a for_each_netdev_rtnl().

> It helps to differentiate expectations, and as self describing code.

The problem with this approach, is that we don't know what lock the
caller of dev_getbyhwaddr_rcu() is using, thus, we cannot leverage
a possible for_each_netdev_rtnl() inside dev_getbyhwaddr_rcu().

> I would not change  for_each_netdev_rcu(), and instead add a new
> dev_getbyhwaddr_rtnl()
> function for contexts holding RTNL.

Initially, I had reservations about this approach, but after further
consideration, it seems that creating separate variants of
dev_getbyhwaddr() might be the most effective solution.

By doing so, we can introduce dev_getbyhwaddr_rcu() and
dev_getbyhwaddr_rtnl(), each tailored to specific locking mechanisms.

To explore this idea further, I'll create a proof-of-concept
implementation to see how these new functions would look in practice.

This will help us determine whether this approach is indeed the best way
forward. Thanks for the suggestion.

> Alternatively, add one rcu_read_lock()/rcu_read_unlock() pair as some
> dev_getbyhwaddr_rcu() callers already do.

Fair, we can do that as well, but, it seemed weird to me to have
something like:

	rtnl_lock();
	rcu_read_lock();
	dev_getbyhwaddr_rcu();

Thanks for chiming in
--breno