linux-kernel - Re: [PATCH -next 1/8] rcu: Fix rcu_read

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Message-ID: <252063db-ec72-42df-b9e0-b8dc0aa6bef9@nvidia.com>
Date: Fri, 2 Jan 2026 19:41:38 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: paulmck@...nel.org, Steven Rostedt <rostedt@...dmis.org>
Cc: Boqun Feng <boqun.feng@...il.com>, rcu@...r.kernel.org,
 Frederic Weisbecker <frederic@...nel.org>,
 Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
 Josh Triplett <josh@...htriplett.org>, Uladzislau Rezki <urezki@...il.com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang@...ux.dev>,
 Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org, Yao Kai <yaokai34@...wei.com>,
 Tengda Wu <wutengda2@...wei.com>
Subject: Re: [PATCH -next 1/8] rcu: Fix rcu_read_unlock() deadloop due to
 softirq



On 1/2/2026 2:51 PM, Paul E. McKenney wrote:
> On Fri, Jan 02, 2026 at 12:30:09PM -0500, Steven Rostedt wrote:
>> On Fri, 2 Jan 2026 12:28:07 -0500
>> Steven Rostedt <rostedt@...dmis.org> wrote:
>>
>>> Stacktrace should have recursion protection too.
>>>
>>> Can you try this patch to see if it would have fixed the problem too?
>>
>> As I believe the recursion protection should be in the tracing
>> infrastructure more than in RCU. As RCU is used as an active participant in
>> the kernel whereas tracing is supposed to be only an observer.
>>
>> If tracing is the culprit, it should be the one that is fixed.
> 
> Makes sense to me!  But then it would...  ;-)
> 
Could we fix it in both? (RCU and tracing). The patch just adds 3 more net lines
to RCU code. It'd be good to have a guard rail against softirq recursion in RCU
read unlock path, as much as the existing guard rail we already have with
irq_work? After all, both paths attempt to do deferred work when it is safer to
do so.

Yao, if you could test Steve's patch and reply whether it fixes it too?

thanks,

 - Joel