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

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Message-Id: <20260103002343.6599-2-joelagnelf@nvidia.com>
Date: Fri,  2 Jan 2026 19:23:30 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: linux-kernel@...r.kernel.org
Cc: "Paul E . McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	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: [PATCH RFC 01/14] rcu: Add WARN_ON_ONCE for blocked flag invariant in exit_rcu()

If a task is on the rcu_node_entry list, its blocked flag should
already be set (it's set before adding to any list in
rcu_note_context_switch()). The current code silently re-sets it,
which could mask bugs.

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);
 	} else if (unlikely(rcu_preempt_depth())) {
 		rcu_preempt_depth_set(1);
-- 
2.34.1