[tip: core/rcu] rcutorture: Fix stray access to rcu_fwd_cb_nodelay

[tip-bot2 for Paul E. McKenney]

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

Commit-ID:     102c14d2f87976e8390d2cb892ccd14e3532e020
Gitweb:        https://git.kernel.org/tip/102c14d2f87976e8390d2cb892ccd14e3532e020
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Sat, 21 Dec 2019 11:23:50 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Thu, 20 Feb 2020 16:03:31 -08:00

rcutorture: Fix stray access to rcu_fwd_cb_nodelay

The rcu_fwd_cb_nodelay variable suppresses excessively long read-side
delays while carrying out an rcutorture forward-progress test.  As such,
it is accessed both by readers and updaters, and most of the accesses
therefore use *_ONCE().  Except for one in rcu_read_delay(), which this
commit fixes.

This data race was reported by KCSAN.  Not appropriate for backporting
due to this being rcutorture.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/rcutorture.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index edd9746..124160a 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -339,7 +339,7 @@ rcu_read_delay(struct torture_random_state *rrsp, struct rt_read_seg *rtrsp)
 	 * period, and we want a long delay occasionally to trigger
 	 * force_quiescent_state. */
 
-	if (!rcu_fwd_cb_nodelay &&
+	if (!READ_ONCE(rcu_fwd_cb_nodelay) &&
 	    !(torture_random(rrsp) % (nrealreaders * 2000 * longdelay_ms))) {
 		started = cur_ops->get_gp_seq();
 		ts = rcu_trace_clock_local();