linux-kernel - [PATCH AUTOSEL 5.3 50/99] tun: remove possible false sharing in tun_flow

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Date:   Sat, 26 Oct 2019 09:15:11 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Eric Dumazet <edumazet@...gle.com>, Zhang Yu <zhangyu31@...du.com>,
        Wang Li <wangli39@...du.com>,
        Li RongQing <lirongqing@...du.com>,
        Jason Wang <jasowang@...hat.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Sasha Levin <sashal@...nel.org>, netdev@...r.kernel.org
Subject: [PATCH AUTOSEL 5.3 50/99] tun: remove possible false sharing in tun_flow_update()

From: Eric Dumazet <edumazet@...gle.com>

[ Upstream commit 4ffdd22e49f47db543906d75453a0048a53071ab ]

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@...gle.com>
Cc: Zhang Yu <zhangyu31@...du.com>
Cc: Wang Li <wangli39@...du.com>
Cc: Li RongQing <lirongqing@...du.com>
Acked-by: Jason Wang <jasowang@...hat.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 aab0be40d4430..dd1d7dc361f1c 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);
-- 
2.20.1