linux-kernel - Re: [PATCH RFC 01/14] rcu: Add WARN_ON_ONCE for blocked flag invariant in exit

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Message-ID: <73d2561b-a36a-4578-b68e-bb8e7a7585be@nvidia.com>
Date: Mon, 5 Jan 2026 10:44:17 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, "Paul E . McKenney" <paulmck@...nel.org>,
 Frederic Weisbecker <frederic@...nel.org>,
 Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
 Josh Triplett <josh@...htriplett.org>, Boqun Feng <boqun.feng@...il.com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang@...ux.dev>,
 Uladzislau Rezki <urezki@...il.com>, joel@...lfernandes.org,
 rcu@...r.kernel.org
Subject: Re: [PATCH RFC 01/14] rcu: Add WARN_ON_ONCE for blocked flag
 invariant in exit_rcu()

On 1/5/2026 10:31 AM, Steven Rostedt wrote:
> On Fri,  2 Jan 2026 19:23:30 -0500
> Joel Fernandes <joelagnelf@...dia.com> wrote:
> 
>> Add a WARN_ON_ONCE to detect this invariant violation. If this
>> warning ever fires, it indicates a bug where a task was added to
>> a blocked list without properly setting the blocked flag first.
>>
>> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
>> ---
>>  kernel/rcu/tree_plugin.h | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index dbe2d02be824..73ba5f4a968d 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -846,6 +846,7 @@ void exit_rcu(void)
>>  	if (unlikely(!list_empty(&current->rcu_node_entry))) {
>>  		rcu_preempt_depth_set(1);
>>  		barrier();
>> +		WARN_ON_ONCE(!t->rcu_read_unlock_special.b.blocked);
>>  		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);

Right, so it is ever not set then we are trying to "right" a "wrong", so..

> 
> If we warn when it is not set, could we just remove setting it?
> Or do:
> 
> 		if (WARN_ON_ONCE(!t->rcu_read_unlock_special.b.blocked))
> 	 		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);

.. we could just do this. I'll apply the suggestion, thanks!

 - Joel