linux-kernel - [PATCH RFC 09/14] rcu: Promote blocked tasks before QS report in rcutree_report_cpu

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Message-Id: <20260103002343.6599-10-joelagnelf@nvidia.com>
Date: Fri,  2 Jan 2026 19:23:38 -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 09/14] rcu: Promote blocked tasks before QS report in rcutree_report_cpu_dead()

When a CPU dies and reports QS via rcutree_report_cpu_dead(), any tasks
blocked on that CPU's per-CPU blocked list must first be promoted to
the rcu_node's blkd_tasks list.

Without this promotion, blocked tasks on the dying CPU's per-CPU list
won't have gp_tasks point to them, so the GP machinery won't wait for
them. This can cause "Wrong-GP reads" errors where a GP completes while
readers are still in their critical sections.

Therefore, call rcu_promote_blocked_tasks_rdp() before reporting QS.

Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
---
 kernel/rcu/tree.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 19fd13c1e6be..5e73ebb260e3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4460,6 +4460,11 @@ void rcutree_report_cpu_dead(void)
 	rdp->rcu_ofl_gp_seq = READ_ONCE(rcu_state.gp_seq);
 	rdp->rcu_ofl_gp_state = READ_ONCE(rcu_state.gp_state);
 	if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */
+		/*
+		 * Promote blocked tasks from dying CPU's per-CPU list before
+		 * reporting QS. Otherwise those tasks won't block the GP.
+		 */
+		rcu_promote_blocked_tasks_rdp(rdp, rnp);
 		/* Report quiescent state -before- changing ->qsmaskinitnext! */
 		rcu_disable_urgency_upon_qs(rdp);
 		rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
-- 
2.34.1