From: "Paul E. McKenney" <paulmck@kernel.org>
Date: Fri, 3 Jan 2020 15:17:12 -0800
Subject: rcu: Add WRITE_ONCE() to rcu_state ->gp_req_activity
Patch-mainline: v5.7-rc1
Git-commit: 2906d2154cd6ad6e4452cc25315d3cea7bb7f2d7
References: bsc#1171828

The rcu_state structure's ->gp_req_activity field is read locklessly,
so this commit adds the WRITE_ONCE() to an update in order to provide
proper documentation and READ_ONCE()/WRITE_ONCE() pairing.

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

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Daniel Wagner <dwagner@suse.de>
---
 kernel/rcu/tree.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1173,7 +1173,7 @@ static bool rcu_start_this_gp(struct rcu
 	}
 	trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedroot"));
 	WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_INIT);
-	rcu_state.gp_req_activity = jiffies;
+	WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
 	if (!rcu_state.gp_kthread) {
 		trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("NoGPkthread"));
 		goto unlock_out;
@@ -1724,7 +1724,7 @@ static void rcu_gp_cleanup(void)
 	/* Advance CBs to reduce false positives below. */
 	if (!rcu_accelerate_cbs(rnp, rdp) && needgp) {
 		WRITE_ONCE(rcu_state.gp_flags, RCU_GP_FLAG_INIT);
-		rcu_state.gp_req_activity = jiffies;
+		WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
 		trace_rcu_grace_period(rcu_state.name,
 				       READ_ONCE(rcu_state.gp_seq),
 				       TPS("newreq"));