[PATCH] dccp: put dccp_qpolicy_full() and dccp_qpolicy_push() in the same lock

[Hangyu Hua]

In the case of sk->dccps_qpolicy == DCCPQ_POLICY_PRIO, dccp_qpolicy_full
will drop a skb when qpolicy is full. And the lock in dccp_sendmsg is
released before sock_alloc_send_skb and then relocked after
sock_alloc_send_skb. The following conditions may lead dccp_qpolicy_push
to add skb to an already full sk_write_queue:

thread1--->lock
thread1--->dccp_qpolicy_full: queue is full. drop a skb
thread1--->unlock
thread2--->lock
thread2--->dccp_qpolicy_full: queue is not full. no need to drop.
thread2--->unlock
thread1--->lock
thread1--->dccp_qpolicy_push: add a skb. queue is full.
thread1--->unlock
thread2--->lock
thread2--->dccp_qpolicy_push: add a skb!
thread2--->unlock

Fix this by moving dccp_qpolicy_full.

Fixes: 871a2c16c21b ("dccp: Policy-based packet dequeueing infrastructure")
Signed-off-by: Hangyu Hua <hbh25y@xxxxxxxxx>
---
 net/dccp/proto.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/net/dccp/proto.c b/net/dccp/proto.c
index eb8e128e43e8..1a0193823c82 100644
--- a/net/dccp/proto.c
+++ b/net/dccp/proto.c
@@ -736,11 +736,6 @@ int dccp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 
 	lock_sock(sk);
 
-	if (dccp_qpolicy_full(sk)) {
-		rc = -EAGAIN;
-		goto out_release;
-	}
-
 	timeo = sock_sndtimeo(sk, noblock);
 
 	/*
@@ -773,6 +768,11 @@ int dccp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	if (rc != 0)
 		goto out_discard;
 
+	if (dccp_qpolicy_full(sk)) {
+		rc = -EAGAIN;
+		goto out_discard;
+	}
+
 	dccp_qpolicy_push(sk, skb);
 	/*
 	 * The xmit_timer is set if the TX CCID is rate-based and will expire
-- 
2.25.1