From: Paul E. McKenney <paulmck@kernel.org>
Date: Fri, 5 Apr 2024 19:02:11 +0000 (-0700)
Subject: rcutorture: Fix rcu_torture_fwd_cb_cr() data race
X-Git-Tag: v6.11-rc1~225^2^6~2
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=6040072f4774a575fa67b912efe7722874be337b;p=thirdparty%2Fkernel%2Flinux.git

rcutorture: Fix rcu_torture_fwd_cb_cr() data race

On powerpc systems, spinlock acquisition does not order prior stores
against later loads.  This means that this statement:

	rfcp->rfc_next = NULL;

Can be reordered to follow this statement:

	WRITE_ONCE(*rfcpp, rfcp);

Which is then a data race with rcu_torture_fwd_prog_cr(), specifically,
this statement:

	rfcpn = READ_ONCE(rfcp->rfc_next)

KCSAN located this data race, which represents a real failure on powerpc.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Marco Elver <elver@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: <kasan-dev@googlegroups.com>
---

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 44cc455e1b615..cafe047d046e8 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -2630,7 +2630,7 @@ static void rcu_torture_fwd_cb_cr(struct rcu_head *rhp)
 	spin_lock_irqsave(&rfp->rcu_fwd_lock, flags);
 	rfcpp = rfp->rcu_fwd_cb_tail;
 	rfp->rcu_fwd_cb_tail = &rfcp->rfc_next;
-	WRITE_ONCE(*rfcpp, rfcp);
+	smp_store_release(rfcpp, rfcp);
 	WRITE_ONCE(rfp->n_launders_cb, rfp->n_launders_cb + 1);
 	i = ((jiffies - rfp->rcu_fwd_startat) / (HZ / FWD_CBS_HIST_DIV));
 	if (i >= ARRAY_SIZE(rfp->n_launders_hist))