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Date: Tue, 16 Sep 2025 07:57:21 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Qingfang Deng <dqfext@gmail.com>
Cc: Andrew Lunn <andrew+netdev@lunn.ch>, "David S. Miller"
 <davem@davemloft.net>, Eric Dumazet <edumazet@google.com>, Paolo Abeni
 <pabeni@redhat.com>, linux-ppp@vger.kernel.org, netdev@vger.kernel.org,
 linux-kernel@vger.kernel.org, Felix Fietkau <nbd@nbd.name>
Subject: Re: [PATCH net-next] ppp: enable TX scatter-gather
Message-ID: <20250916075721.273ea979@kernel.org>
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	<20250915181015.67588ec2@kernel.org>
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On Tue, 16 Sep 2025 10:57:49 +0800 Qingfang Deng wrote:
> Hi Jakub,
>=20
> On Tue, Sep 16, 2025 at 9:10=E2=80=AFAM Jakub Kicinski <kuba@kernel.org> =
wrote:
> > Seems a bit racy. We can't netdev_update_features() under the spin lock
> > so there's going to be a window of time where datapath will see new
> > state but netdev flags won't be cleared, yet?
> >
> > We either need to add a explicit linearization check in the xmit path,
> > or always reset the flags to disabled before we start tweaking the
> > config and re-enable after config (tho the latter feels like a bit of
> > a hack). =20
>=20
> Can I modify dev->features directly under the spin lock (without
> .ndo_fix_features) ?

Hm, I'm not aware of a reason not to. You definitely need to hold
rtnl_lock, and call netdev_update_features() after.