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Date:   Fri, 14 Feb 2020 15:55:52 -0800
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 15/30] rcu: Add *_ONCE() to rcu_data ->rcu_forced_tick

From: "Paul E. McKenney" <paulmck@...nel.org>

The rcu_data structure's ->rcu_forced_tick field is read locklessly, so
this commit adds WRITE_ONCE() to all updates and READ_ONCE() to all
lockless reads.

This data race was reported by KCSAN.  Not appropriate for backporting
due to failure being unlikely.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e851a12..be59a5d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -818,11 +818,12 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
 		incby = 1;
 	} else if (tick_nohz_full_cpu(rdp->cpu) &&
 		   rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE &&
-		   READ_ONCE(rdp->rcu_urgent_qs) && !rdp->rcu_forced_tick) {
+		   READ_ONCE(rdp->rcu_urgent_qs) &&
+		   !READ_ONCE(rdp->rcu_forced_tick)) {
 		raw_spin_lock_rcu_node(rdp->mynode);
 		// Recheck under lock.
 		if (rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
-			rdp->rcu_forced_tick = true;
+			WRITE_ONCE(rdp->rcu_forced_tick, true);
 			tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
 		}
 		raw_spin_unlock_rcu_node(rdp->mynode);
@@ -899,7 +900,7 @@ static void rcu_disable_urgency_upon_qs(struct rcu_data *rdp)
 	WRITE_ONCE(rdp->rcu_need_heavy_qs, false);
 	if (tick_nohz_full_cpu(rdp->cpu) && rdp->rcu_forced_tick) {
 		tick_dep_clear_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
-		rdp->rcu_forced_tick = false;
+		WRITE_ONCE(rdp->rcu_forced_tick, false);
 	}
 }
 
-- 
2.9.5

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