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

[Hangyu Hua]

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