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

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Message-ID: <20260105103145.5b3eaaf5@gandalf.local.home>
Date: Mon, 5 Jan 2026 10:31:45 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Joel Fernandes <joelagnelf@...dia.com>
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 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);

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);

-- Steve

>  	} else if (unlikely(rcu_preempt_depth())) {
>  		rcu_preempt_depth_set(1);
> --