netdev - [PATCH net] tun: remove possible false sharing in tun_flow

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Date:   Wed,  9 Oct 2019 09:20:02 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Zhang Yu <zhangyu31@...du.com>, Wang Li <wangli39@...du.com>,
        Li RongQing <lirongqing@...du.com>,
        Jason Wang <jasowang@...hat.com>
Subject: [PATCH net] tun: remove possible false sharing in tun_flow_update()

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>
Cc: Jason Wang <jasowang@...hat.com>
---
 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 812dc3a65efbb9d1ee2724e73978dbc4803ec171..a8d3141582a53caf407dc9aff61c452998de068f 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);
-- 
2.23.0.581.g78d2f28ef7-goog