rcu: Mark writes to rcu_sync ->gp_count field

The rcu_sync structure's ->gp_count field is updated under the protection
of ->rss_lock, but read locklessly, and KCSAN noted the data race.
This commit therefore uses WRITE_ONCE() to do this update to clearly
document its racy nature.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

kernel/rcu/sync.c

@@ -122,7 +122,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
 		 * we are called at early boot time but this shouldn't happen.
 		 */
 	}
-	rsp->gp_count++;
+	WRITE_ONCE(rsp->gp_count, rsp->gp_count + 1);
 	spin_unlock_irq(&rsp->rss_lock);
 
 	if (gp_state == GP_IDLE) {
@@ -151,11 +151,15 @@ void rcu_sync_enter(struct rcu_sync *rsp)
  */
 void rcu_sync_exit(struct rcu_sync *rsp)
 {
+	int gpc;
+
 	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
 	WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
 
 	spin_lock_irq(&rsp->rss_lock);
-	if (!--rsp->gp_count) {
+	gpc = rsp->gp_count - 1;
+	WRITE_ONCE(rsp->gp_count, gpc);
+	if (!gpc) {
 		if (rsp->gp_state == GP_PASSED) {
 			WRITE_ONCE(rsp->gp_state, GP_EXIT);
 			rcu_sync_call(rsp);