linux-kernel - Re: [PATCH] dccp: put dccp_qpolicy_full() and dccp_qpolicy

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Date:   Fri, 29 Jul 2022 18:34:39 +0800
From:   Hangyu Hua <hbh25y@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
        kuniyu@...zon.co.jp, richard_siegfried@...temli.org,
        joannelkoong@...il.com, socketcan@...tkopp.net,
        gerrit@....abdn.ac.uk, tomasz@...belny.oswiecenia.net,
        dccp@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dccp: put dccp_qpolicy_full() and dccp_qpolicy_push() in
 the same lock

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@...il.com>
>> ---
>>   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
>