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

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Date: Thu, 20 Jun 2024 17:22:35 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "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

On Thu, 20 Jun 2024 11:47:08 +0000 Eric Dumazet wrote:
> Move pm_runtime_get_sync() and pm_runtime_put() out of __dev_ethtool
> to dev_ethtool() while RTNL is not yet held.
> 
> These helpers do not depend on RTNL.

The helpers themselves don't, but can we assume no drivers have
implicit dependencies on calling netif_device_detach() under rtnl_lock,
and since the presence checks are under rtnl_lock they are currently
guaranteed not to get any callbacks past detach() + rtnl_unlock()?

I think its better to completely skip PM + presence + ->begin if driver
wants the op to be unlocked, but otherwise keep the locking as is

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?