linux-kernel - Re: [PATCH net v2] ipv6: fix a BUG in rt6_get_pcpu_route() under PREEMPT

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Message-ID: <CANn89iLeASUZyonYSLX0AG5mbC=gxux0efehkBc_j1bbj6xrvA@mail.gmail.com>
Date: Mon, 22 Dec 2025 09:50:58 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>
Cc: netdev@...r.kernel.org, 
	syzbot+9b35e9bc0951140d13e6@...kaller.appspotmail.com, 
	"David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 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 v2] ipv6: fix a BUG in rt6_get_pcpu_route() under PREEMPT_RT

On Fri, Dec 19, 2025 at 3:52 AM Jiayuan Chen <jiayuan.chen@...ux.dev> wrote:
>
> On PREEMPT_RT kernels, after rt6_get_pcpu_route() returns NULL, the
> current task can be preempted. Another task running on the same CPU
> may then execute rt6_make_pcpu_route() and successfully install a
> pcpu_rt entry. When the first task resumes execution, its cmpxchg()
> in rt6_make_pcpu_route() will fail because rt6i_pcpu is no longer
> NULL, triggering the BUG_ON(prev). It's easy to reproduce it by adding
> mdelay() after rt6_get_pcpu_route().
>
> Using preempt_disable/enable is not appropriate here because
> ip6_rt_pcpu_alloc() may sleep.
>
> Fix this by:
> 1. Removing the BUG_ON and instead handling the race gracefully by
>    freeing our allocation and returning the existing pcpu_rt when
>    cmpxchg() fails.
> 2. Keeping the BUG_ON for non-PREEMPT_RT kernels, since preemption
>    should not occur in this context and a cmpxchg failure would
>    indicate a real bug.
>
> Link: https://syzkaller.appspot.com/bug?extid=9b35e9bc0951140d13e6
> Fixes: 951f788a80ff ("ipv6: fix a BUG in rt6_get_pcpu_route()")

I would rather find when PREEMPT_RT was added/enabled, there is no
point blaming such an old commit
which was correct at the time, and forcing pointless backports to old
linux kernels.