[PATCH net v2 2/2] tuntap: correctly add the missing xdp flush

From: Jason Wang <jasowang@redhat.com>
To: netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Cc: mst@redhat.com, christoffer.dall@linaro.org,
	Jason Wang <jasowang@redhat.com>
Subject: [PATCH net v2 2/2] tuntap: correctly add the missing xdp flush
Date: Thu, 22 Feb 2018 14:24:18 +0800	[thread overview]
Message-ID: <1519280658-4918-2-git-send-email-jasowang@redhat.com> (raw)
In-Reply-To: <1519280658-4918-1-git-send-email-jasowang@redhat.com>

Commit 762c330d670e ("tuntap: add missing xdp flush") tries to fix the
devmap stall caused by missed xdp flush by counting the pending xdp
redirected packets and flush when it exceeds NAPI_POLL_WEIGHT or
MSG_MORE is clear. This may lead BUG() since xdp_do_flush() was
called under process context with preemption enabled. Simply disable
preemption may silent the warning but be not enough since process may
move between different CPUS during a batch which cause xdp_do_flush()
misses some CPU where the process run previously. Consider the several
fallouts, that commit was reverted. To fix the issue correctly, we can
simply calling xdp_do_flush() immediately after xdp_do_redirect(),
a side effect is that this removes any possibility of batching which
could be addressed in the future.

Reported-by: Christoffer Dall <christoffer.dall@linaro.org>
Fixes: 762c330d670e ("tuntap: add missing xdp flush")
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 drivers/net/tun.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 2823a4a..a363ea2 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1662,6 +1662,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 			get_page(alloc_frag->page);
 			alloc_frag->offset += buflen;
 			err = xdp_do_redirect(tun->dev, &xdp, xdp_prog);
+			xdp_do_flush_map();
 			if (err)
 				goto err_redirect;
 			rcu_read_unlock();
-- 
2.7.4
