[PATCH 1/1] tap: RCU usage and comment fixes

[PATCH 1/1] tap: RCU usage and comment fixes

[Wang Jian]

The tap_queue and the 'tap_dev' are loosely coupled, not 'macvlan_dev'.

Taking rcu_read_lock a little later seems can slightly reduce rcu read critical section.

Signed-off-by: Wang Jian <jianjian.wang1@gmail.com>
---
 drivers/net/tap.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index f0f7cd977667..e5e5a8e4a60d 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -125,7 +125,7 @@ static struct tap_dev *tap_dev_get_rcu(const struct net_device *dev)
 
 /*
  * RCU usage:
- * The tap_queue and the macvlan_dev are loosely coupled, the
+ * The tap_queue and the tap_dev are loosely coupled, the
  * pointers from one to the other can only be read while rcu_read_lock
  * or rtnl is held.
  *
@@ -720,8 +720,6 @@ static ssize_t tap_get_user(struct tap_queue *q, struct msghdr *m,
 	    __vlan_get_protocol(skb, skb->protocol, &depth) != 0)
 		skb_set_network_header(skb, depth);
 
-	rcu_read_lock();
-	tap = rcu_dereference(q->tap);
 	/* copy skb_ubuf_info for callback when skb has no error */
 	if (zerocopy) {
 		skb_shinfo(skb)->destructor_arg = m->msg_control;
@@ -732,6 +730,8 @@ static ssize_t tap_get_user(struct tap_queue *q, struct msghdr *m,
 		uarg->callback(uarg, false);
 	}
 
+	rcu_read_lock();
+	tap = rcu_dereference(q->tap);
 	if (tap) {
 		skb->dev = tap->dev;
 		dev_queue_xmit(skb);
-- 
2.17.1

Re: [PATCH 1/1] tap: RCU usage and comment fixes

[David Miller]

From: Wang Jian <jianjian.wang1@gmail.com>
Date: Fri, 17 Aug 2018 08:22:53 +0000

> The tap_queue and the 'tap_dev' are loosely coupled, not 'macvlan_dev'.

There is another reference to macvlan_dev in that comment, which is therefore
also similarly inaccurate.  You should add an appropriate Fixes: line for
where this inaccuracy was introduced, which is:

Fixes: 6fe3faf86757 ("tap: Abstract type of virtual interface from tap implementation")

> Taking rcu_read_lock a little later seems can slightly reduce rcu read critical section.

This is a separate change from fixing up a comment.