netdev - Re: [PATCH net-next 3/6] net: ethtool: perform pm duties outside of rtnl lock

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Date: Fri, 21 Jun 2024 02:59:54 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Eric Dumazet <edumazet@...gle.com>,
	"David S . Miller" <davem@...emloft.net>,
	Paolo Abeni <pabeni@...hat.com>, Ziwei Xiao <ziweixiao@...gle.com>,
	Praveen Kaligineedi <pkaligineedi@...gle.com>,
	Harshitha Ramamurthy <hramamurthy@...gle.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Jeroen de Borst <jeroendb@...gle.com>,
	Shailend Chand <shailend@...gle.com>, netdev@...r.kernel.org,
	eric.dumazet@...il.com
Subject: Re: [PATCH net-next 3/6] net: ethtool: perform pm duties outside of
 rtnl lock

> I also keep wondering whether we shouldn't use this as an opportunity
> to introduce a "netdev instance lock". I think you mentioned we should
> move away from rtnl for locking ethtool and ndos since most drivers
> don't care at all about global state. Doing that is a huge project, 
> but maybe this is where we start?

Is there much benefit to the average system?

Embedded systems typically have 1 or 2 netdevs. Laptops, desktops and
the like have one, maybe two netdevs. VMs typically have one netdev.
So we are talking about high end switches with lots of ports and
servers hosting lots of VMs. So of the around 500 netdev drivers we
have, only maybe a dozen drivers would benefit?

It seems unlikely those 500 drivers will be reviewed and declared safe
to not take RTNL. So maybe a better way forward is that struct
ethtool_ops gains a flag indicating its ops can be called without
first talking RTNL. Somebody can then look at those dozen drivers, and
we leave the other 490 alone and don't need to worry about
regressions.

	Andrew