Commit 4ffdd22e authored Oct 09, 2019 by Eric Dumazet Committed by Jakub Kicinski Oct 09, 2019

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: 83b1bc12 ("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>

parent e37542ba

drivers/net/tun.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -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);

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