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From:   Jason Wang <jasowang@redhat.com>
To:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     yuehaibing@huawei.com, xiyou.wangcong@gmail.com,
        weiyongjun1@huawei.com, eric.dumazet@gmail.com,
        Jason Wang <jasowang@redhat.com>
Subject: [PATCH net V3 2/2] tuntap: synchronize through tfiles array instead of tun->numqueues
Date:   Wed,  8 May 2019 23:20:18 -0400
Message-Id: <1557372018-18544-2-git-send-email-jasowang@redhat.com>
In-Reply-To: <1557372018-18544-1-git-send-email-jasowang@redhat.com>
References: <1557372018-18544-1-git-send-email-jasowang@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

When a queue(tfile) is detached through __tun_detach(), we move the
last enabled tfile to the position where detached one sit but don't
NULL out last position. We expect to synchronize the datapath through
tun->numqueues. Unfortunately, this won't work since we're lacking
sufficient mechanism to order or synchronize the access to
tun->numqueues.

To fix this, NULL out the last position during detaching and check
RCU protected tfile against NULL instead of checking tun->numqueues in
datapath.

Cc: YueHaibing <yuehaibing@huawei.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: weiyongjun (A) <weiyongjun1@huawei.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Fixes: c8d68e6be1c3b ("tuntap: multiqueue support")
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
Changes from V2:
- resample during detach in tun_xdp_xmit()
Changes from V1:
- keep the check in tun_xdp_xmit()
---
 drivers/net/tun.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index dc62fc3..f4c933a 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -705,6 +705,8 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
 				   tun->tfiles[tun->numqueues - 1]);
 		ntfile = rtnl_dereference(tun->tfiles[index]);
 		ntfile->queue_index = index;
+		rcu_assign_pointer(tun->tfiles[tun->numqueues - 1],
+				   NULL);
 
 		--tun->numqueues;
 		if (clean) {
@@ -1087,7 +1089,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 	tfile = rcu_dereference(tun->tfiles[txq]);
 
 	/* Drop packet if interface is not attached */
-	if (txq >= tun->numqueues)
+	if (!tfile)
 		goto drop;
 
 	if (!rcu_dereference(tun->steering_prog))
@@ -1310,6 +1312,7 @@ static int tun_xdp_xmit(struct net_device *dev, int n,
 
 	rcu_read_lock();
 
+resample:
 	numqueues = READ_ONCE(tun->numqueues);
 	if (!numqueues) {
 		rcu_read_unlock();
@@ -1318,6 +1321,8 @@ static int tun_xdp_xmit(struct net_device *dev, int n,
 
 	tfile = rcu_dereference(tun->tfiles[smp_processor_id() %
 					    numqueues]);
+	if (unlikely(!tfile))
+		goto resample;
 
 	spin_lock(&tfile->tx_ring.producer_lock);
 	for (i = 0; i < n; i++) {
-- 
1.8.3.1