linux-kernel - Re: [PATCH net] netdevsim: disable local BH when scheduling NAPI

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Message-ID: <20250214-daffy-athletic-kakapo-ba9b1b@leitao>
Date: Fri, 14 Feb 2025 05:01:25 -0800
From: Breno Leitao <leitao@...ian.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Paolo Abeni <pabeni@...hat.com>, David Wei <dw@...idwei.uk>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	paulmck@...nel.org, kernel-team@...a.com, stable@...r.kernel.org
Subject: Re: [PATCH net] netdevsim: disable local BH when scheduling NAPI

Hello Eric,

On Wed, Feb 12, 2025 at 07:55:32PM +0100, Eric Dumazet wrote:
> On Wed, Feb 12, 2025 at 7:34 PM Breno Leitao <leitao@...ian.org> wrote:
> >
> > --- a/drivers/net/netdevsim/netdev.c
> > +++ b/drivers/net/netdevsim/netdev.c
> > @@ -87,7 +87,9 @@ static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
> >         if (unlikely(nsim_forward_skb(peer_dev, skb, rq) == NET_RX_DROP))
> >                 goto out_drop_cnt;
> >
> > +       local_bh_disable();
> >         napi_schedule(&rq->napi);
> > +       local_bh_enable();
> >
> 
> I thought all ndo_start_xmit() were done under local_bh_disable()

I think it depends on the path?

> Could you give more details ?

There are several paths to ndo_start_xmit(), and please correct me if
I am reading the code wrongly here.

Common path:

	__dev_direct_xmit()
		local_bh_disable();
			netdev_start_xmit()
				__netdev_start_xmit()
					ops->ndo_start_xmit(skb, dev);


But, in some other cases, I see:

	netpoll_start_xmit()
		netdev_start_xmit()
			....

My reading is that not all cases have local_bh_disable() disabled before
calling ndo_start_xmit().

Question: Must BH be disabled before calling ndo_start_xmit()? If so,
the problem might be in the netpoll code!? Also, is it worth adding
a DEBUG_NET_WARN_ON_ONCE()?

Note: Jakub gave another suggestion on how to fix this, so, I send a v2
with a different approach:

	https://lore.kernel.org/all/20250213071426.01490615@kernel.org/

Thanks for the review!
--breno