On 2022/7/29 11:01, Jakub Kicinski wrote:
On Wed, 27 Jul 2022 16:06:09 +0800 Hangyu Hua wrote:
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
This linie should say "not full"?
dccp_qpolicy_full only call dccp_qpolicy_drop when queue is full. You
can check out qpolicy_prio_full. qpolicy_prio_full will drop a skb to
make suer there is enough space for the next data. So I think it should
be "full" here.
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")
This code was added in b1308dc015eb0, AFAICT. Please double check.
My fault. I will fix this.
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;
+ }
Shouldn't this be earlier, right after relocking? Why copy the data etc.
if we know the queue is full?
You are right. The queue should be checked first after relocking. I will
send a v2 later.
Thanks,
Hangyu.
dccp_qpolicy_push(sk, skb);
/*
* The xmit_timer is set if the TX CCID is rate-based and will expire
