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

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Message-ID: <Z5gHAmNpXGBS2Mp8@LQ3V64L9R2>
Date: Mon, 27 Jan 2025 17:21:54 -0500
From: Joe Damato <jdamato@...tly.com>
To: Jakub Kicinski <kuba@...nel.org>
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>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC net-next v3 1/4] net: protect queue -> napi linking with
 netdev_lock()

On Mon, Jan 27, 2025 at 01:37:56PM -0800, Jakub Kicinski wrote:
> 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 :(

OK, I'll wait for something to emerge from that work before
re-spinning this.