linux-kernel - [PATCH tip/core/rcu 19/30] rcu: Add WRITE_ONCE() to rcu_state ->gp

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Date:   Fri, 14 Feb 2020 15:55:56 -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 19/30] rcu: Add WRITE_ONCE() to rcu_state ->gp_start

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

The rcu_state structure's ->gp_start 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@...nel.org>
---
 kernel/rcu/tree_stall.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 3275f27..56df88e 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -102,7 +102,7 @@ static void record_gp_stall_check_time(void)
 	unsigned long j = jiffies;
 	unsigned long j1;
 
-	rcu_state.gp_start = j;
+	WRITE_ONCE(rcu_state.gp_start, j);
 	j1 = rcu_jiffies_till_stall_check();
 	/* Record ->gp_start before ->jiffies_stall. */
 	smp_store_release(&rcu_state.jiffies_stall, j + j1); /* ^^^ */
-- 
2.9.5