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From:   yanfei.xu@windriver.com
To:     paulmck@kernel.org, josh@joshtriplett.org, rostedt@goodmis.org,
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Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH] rcu: fix a deadlock caused by not release rcu_node->lock
Date:   Sun, 16 May 2021 17:28:37 +0800
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From: Yanfei Xu <yanfei.xu@windriver.com>

rcu_node->lock isn't released in rcu_print_task_stall() if the rcu_node
don't contain tasks which blocking the GP. However this rcu_node->lock
will be used again in rcu_dump_cpu_stacks() soon while the ndetected is
non-zero. As a result the cpu will hung by this deadlock.

Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
---
 kernel/rcu/tree_stall.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index b72311d24a9f..4e97c9977d1c 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -267,8 +267,10 @@ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
 	struct task_struct *ts[8];
 
 	lockdep_assert_irqs_disabled();
-	if (!rcu_preempt_blocked_readers_cgp(rnp))
+	if (!rcu_preempt_blocked_readers_cgp(rnp)) {
+		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 		return 0;
+	}
 	pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
 	       rnp->level, rnp->grplo, rnp->grphi);
 	t = list_entry(rnp->gp_tasks->prev,
-- 
2.27.0