[tip: core/rcu] srcu: Fix process_srcu()/srcu_batches_completed() datarace

[tip-bot2 for Paul E. McKenney]

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

Commit-ID:     39f91504a03a7a2abdb52205106942fa4a548d0d
Gitweb:        https://git.kernel.org/tip/39f91504a03a7a2abdb52205106942fa4a548d0d
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Sun, 22 Dec 2019 19:39:35 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Thu, 20 Feb 2020 16:01:11 -08:00

srcu: Fix process_srcu()/srcu_batches_completed() datarace

The srcu_struct structure's ->srcu_idx field is accessed locklessly,
so reads must use READ_ONCE().  This commit therefore adds the needed
READ_ONCE() invocation where it was missed.

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/srcutree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 79848f7..119a373 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1079,7 +1079,7 @@ EXPORT_SYMBOL_GPL(srcu_barrier);
  */
 unsigned long srcu_batches_completed(struct srcu_struct *ssp)
 {
-	return ssp->srcu_idx;
+	return READ_ONCE(ssp->srcu_idx);
 }
 EXPORT_SYMBOL_GPL(srcu_batches_completed);