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"?
> 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.
> 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?
> dccp_qpolicy_push(sk, skb);
> /*
> * The xmit_timer is set if the TX CCID is rate-based and will expire
