linux-kernel - [PATCH rcu 4/8] rcu: Use READ_ONCE() for lockless read of rnp->qsmask

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Message-Id: <20221019224659.2499511-4-paulmck@kernel.org>
Date:   Wed, 19 Oct 2022 15:46:55 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 4/8] rcu: Use READ_ONCE() for lockless read of rnp->qsmask

From: "Joel Fernandes (Google)" <joel@...lfernandes.org>

The rnp->qsmask is locklessly accessed from rcutree_dying_cpu(). This
may help avoid load tearing due to concurrent access, KCSAN
issues, and preserve sanity of people reading the mask in tracing.

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0ca21ac0f0648..5ec97e3f7468f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2106,7 +2106,7 @@ int rcutree_dying_cpu(unsigned int cpu)
 	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
 		return 0;
 
-	blkd = !!(rnp->qsmask & rdp->grpmask);
+	blkd = !!(READ_ONCE(rnp->qsmask) & rdp->grpmask);
 	trace_rcu_grace_period(rcu_state.name, READ_ONCE(rnp->gp_seq),
 			       blkd ? TPS("cpuofl-bgp") : TPS("cpuofl"));
 	return 0;
-- 
2.31.1.189.g2e36527f23