netdev - Re: [PATCH net v4 2/2] arp: switch to dev_getbyhwaddr() in arp_req_set

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Message-ID: <20250218-garrulous-pristine-nautilus-fcc676@leitao>
Date: Tue, 18 Feb 2025 08:27:44 -0800
From: Breno Leitao <leitao@...ian.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	David Ahern <dsahern@...nel.org>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>,
	kuniyu@...zon.co.jp, ushankar@...estorage.com,
	Kuniyuki Iwashima <kuniyu@...zon.com>
Subject: Re: [PATCH net v4 2/2] arp: switch to dev_getbyhwaddr() in
 arp_req_set_public()

Hello Jakub,

On Tue, Feb 18, 2025 at 06:29:20AM -0800, Jakub Kicinski wrote:
> On Tue, 18 Feb 2025 01:36:30 -0800 Breno Leitao wrote:
> > On Mon, Feb 17, 2025 at 04:33:44PM -0800, Jakub Kicinski wrote:
> > > On Thu, 13 Feb 2025 04:42:38 -0800 Breno Leitao wrote:  
> > > > The arp_req_set_public() function is called with the rtnl lock held,
> > > > which provides enough synchronization protection. This makes the RCU
> > > > variant of dev_getbyhwaddr() unnecessary. Switch to using the simpler
> > > > dev_getbyhwaddr() function since we already have the required rtnl
> > > > locking.
> > > > 
> > > > This change helps maintain consistency in the networking code by using
> > > > the appropriate helper function for the existing locking context.  
> > > 
> > > I think you should make it clearer whether this fixes a splat with
> > > PROVE_RCU_LIST=y  
> > 
> > This one doesn't fix the splat in fact, since rtnl lock was held, and it
> > is moving from dev_getbyhwaddr_rcu() to dev_getbyhwaddr(), since rtnl
> > lock was held.
> 
> Are you sure? I don't see the RCU lock being taken on the path that
> ends up here. arp_ioctl() -> arp_req_set() -> arp_req_set_public()

Ack, this will fix the PROVE_RCU_LIST issue.