netdev - Re: [PATCH net-next v1] ipv6: fix a BUG in rt6_get_pcpu_route() under PREEMPT

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Message-ID: <280b18c6-2348-426f-a1b1-8c17d229c21c@redhat.com>
Date: Thu, 18 Dec 2025 13:25:31 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>, netdev@...r.kernel.org
Cc: syzbot+9b35e9bc0951140d13e6@...kaller.appspotmail.com,
 "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Simon Horman <horms@...nel.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
 linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH net-next v1] ipv6: fix a BUG in rt6_get_pcpu_route() under
 PREEMPT_RT

On 12/9/25 1:48 PM, Jiayuan Chen wrote:
> @@ -2299,11 +2308,13 @@ struct rt6_info *ip6_pol_route(struct net *net, struct fib6_table *table,
>  	} else {
>  		/* Get a percpu copy */
>  		local_bh_disable();
> +		migrate_disable();
>  		rt = rt6_get_pcpu_route(&res);
>  
>  		if (!rt)
>  			rt = rt6_make_pcpu_route(net, &res);
>  
> +		migrate_enable();

AFAICS, this part is not needed: local_bh_disable() ensures migrating is
already disabled, if !CONFIG_PREEMPT_RT_NEEDS_BH_LOCK or preemption is
disabled, when CONFIG_PREEMPT_RT_NEEDS_BH_LOCK==y

Side note: this patch looks suitable for the 'net' tree, please change
the subj prefix accordingly in the next revision.

Cheers,

Paolo