linux-kernel - Re: [RFC net-next v3 1/4] net: protect queue -> napi linking with netdev

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Message-ID: <20250127133756.413efb24@kernel.org>
Date: Mon, 27 Jan 2025 13:37:56 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Joe Damato <jdamato@...tly.com>
Cc: netdev@...r.kernel.org, gerhard@...leder-embedded.com,
 jasowang@...hat.com, leiyang@...hat.com, xuanzhuo@...ux.alibaba.com,
 mkarsten@...terloo.ca, Andrew Lunn <andrew+netdev@...n.ch>, "David S.
 Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo
 Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Kuniyuki
 Iwashima <kuniyu@...zon.com>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, Lorenzo Bianconi <lorenzo@...nel.org>, Alexander
 Lobakin <aleksander.lobakin@...el.com>, linux-kernel@...r.kernel.org (open
 list)
Subject: Re: [RFC net-next v3 1/4] net: protect queue -> napi linking with
 netdev_lock()

On Tue, 21 Jan 2025 19:10:41 +0000 Joe Damato wrote:
> From: Jakub Kicinski <kuba@...nel.org>
> 
> netdev netlink is the only reader of netdev_{,rx_}queue->napi,
> and it already holds netdev->lock. Switch protection of the
> writes to netdev->lock as well.
> 
> Add netif_queue_set_napi_locked() for API completeness,
> but the expectation is that most current drivers won't have
> to worry about locking any more. Today they jump thru hoops
> to take rtnl_lock.

I started having second thoughts about this patch, sorry to say.
NAPI objects were easy to protect with the lock because there's
a clear registration and unregistration API. Queues OTOH are made
visible by the netif_set_real_num_queues() call, which is tricky 
to protect with the instance lock. Queues are made visible, then
we configure them.

My thinking changed a bit, I think we should aim to protect all
ndos and ethtool ops with the instance lock. Stanislav and Saeed
seem to be working on that:
https://lore.kernel.org/all/Z5LhKdNMO5CvAvZf@mini-arch/
so hopefully that doesn't cause too much of a delay.
But you may need to rework this series further :(