netdev - Re: [PATCH net 1/2] netpoll: Use rcu_access_pointer() in __netpoll

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Message-ID: <Zzswim8DI85fYlRR@localhost.localdomain>
Date: Mon, 18 Nov 2024 13:18:18 +0100
From: Michal Kubiak <michal.kubiak@...el.com>
To: Breno Leitao <leitao@...ian.org>
CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Herbert Xu
	<herbert@...dor.apana.org.au>, Stephen Hemminger
	<stephen@...workplumber.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <paulmck@...nel.org>
Subject: Re: [PATCH net 1/2] netpoll: Use rcu_access_pointer() in
 __netpoll_setup

On Mon, Nov 18, 2024 at 03:15:17AM -0800, Breno Leitao wrote:
> The ndev->npinfo pointer in __netpoll_setup() is RCU-protected but is being
> accessed directly for a NULL check. While no RCU read lock is held in this
> context, we should still use proper RCU primitives for consistency and
> correctness.
> 
> Replace the direct NULL check with rcu_access_pointer(), which is the
> appropriate primitive when only checking for NULL without dereferencing
> the pointer. This function provides the necessary ordering guarantees
> without requiring RCU read-side protection.
> 
> Signed-off-by: Breno Leitao <leitao@...ian.org>
> Fixes: 8fdd95ec162a ("netpoll: Allow netpoll_setup/cleanup recursion")

nitpick: Shouldn't the "Signed-off-by" tag go as the last one?

Thanks,
Reviewed-by: Michal Kubiak <michal.kubiak@...el.com>

> ---
>  net/core/netpoll.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> index aa49b92e9194babab17b2e039daf092a524c5b88..45fb60bc4803958eb07d4038028269fc0c19622e 100644
> --- a/net/core/netpoll.c
> +++ b/net/core/netpoll.c
> @@ -626,7 +626,7 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
>  		goto out;
>  	}
>  
> -	if (!ndev->npinfo) {
> +	if (!rcu_access_pointer(ndev->npinfo)) {
>  		npinfo = kmalloc(sizeof(*npinfo), GFP_KERNEL);
>  		if (!npinfo) {
>  			err = -ENOMEM;
> 
> -- 
> 2.43.5
> 
>