[tip: core/rcu] rcu: Add WRITE_ONCE() to rcu_state ->gp_start

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     59881bcd85a06565c53fd13ce3b0ad7fa55e560c
Gitweb:        https://git.kernel.org/tip/59881bcd85a06565c53fd13ce3b0ad7fa55e560c
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Mon, 20 Jan 2020 15:29:04 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Thu, 20 Feb 2020 15:58:23 -08:00

rcu: Add WRITE_ONCE() to rcu_state ->gp_start

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@xxxxxxxxxx>
---
 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 bca637b..488b71d 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); /* ^^^ */