From 4ffdd22e49f47db543906d75453a0048a53071ab Mon Sep 17 00:00:00 2001
From: Eric Dumazet <edumazet@google.com>
Date: Wed, 9 Oct 2019 09:20:02 -0700
Subject: [PATCH] tun: remove possible false sharing in tun_flow_update()
Git-commit: 4ffdd22e49f47db543906d75453a0048a53071ab
Patch-mainline: v5.4-rc4
References: git-fixes

As mentioned in https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE#it-may-improve-performance
a C compiler can legally transform

if (e->queue_index != queue_index)
	e->queue_index = queue_index;

to :

	e->queue_index = queue_index;

Note that the code using jiffies has no issue, since jiffies
has volatile attribute.

if (e->updated != jiffies)
    e->updated = jiffies;

Fixes: 83b1bc122cab ("tun: align write-heavy flow entry members to a cache line")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Zhang Yu <zhangyu31@baidu.com>
Cc: Wang Li <wangli39@baidu.com>
Cc: Li RongQing <lirongqing@baidu.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/net/tun.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 812dc3a65efb..a8d3141582a5 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -526,8 +526,8 @@ static void tun_flow_update(struct tun_struct *tun, u32 rxhash,
 	e = tun_flow_find(head, rxhash);
 	if (likely(e)) {
 		/* TODO: keep queueing to old queue until it's empty? */
-		if (e->queue_index != queue_index)
-			e->queue_index = queue_index;
+		if (READ_ONCE(e->queue_index) != queue_index)
+			WRITE_ONCE(e->queue_index, queue_index);
 		if (e->updated != jiffies)
 			e->updated = jiffies;
 		sock_rps_record_flow_hash(e->rps_rxhash);
-- 
2.16.4