From: Paul E. McKenney <paulmck@kernel.org>
Date: Mon, 23 Dec 2019 03:55:50 +0000 (-0800)
Subject: rcu: Fix exp_funnel_lock()/rcu_exp_wait_wake() datarace
X-Git-Tag: v5.7-rc1~183^2^2^2~26
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=24bb9eccf7ff335c16c2970ac7cd5c32a92821a6;p=thirdparty%2Flinux.git

rcu: Fix exp_funnel_lock()/rcu_exp_wait_wake() datarace

The rcu_node structure's ->exp_seq_rq field is accessed locklessly, so
updates must use WRITE_ONCE().  This commit therefore adds the needed
WRITE_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@kernel.org>
---

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index dcbd75791f392..d7e04849c7ab7 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -589,7 +589,7 @@ static void rcu_exp_wait_wake(unsigned long s)
 			spin_lock(&rnp->exp_lock);
 			/* Recheck, avoid hang in case someone just arrived. */
 			if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
-				rnp->exp_seq_rq = s;
+				WRITE_ONCE(rnp->exp_seq_rq, s);
 			spin_unlock(&rnp->exp_lock);
 		}
 		smp_mb(); /* All above changes before wakeup. */