Re: [PATCH net] mptcp: fix soft lockup in mptcp_recvmsg()

[Li Xiasong]

Hi Paolo,

On 3/4/2026 5:07 PM, Paolo Abeni wrote:
> On 3/2/26 6:26 AM, Li Xiasong wrote:
>> @@ -2343,7 +2347,7 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>>  
>>  		pr_debug("block timeout %ld\n", timeo);
>>  		mptcp_cleanup_rbuf(msk, copied);
>> -		err = sk_wait_data(sk, &timeo, NULL);
>> +		err = sk_wait_data(sk, &timeo, last);
> 
> Isn't
> 		err = sk_wait_data(sk, &timeo,  skb_peek_tail(&sk->sk_receive_queue));
> 
> Enough?

Thanks for the review! In the general case, letting sk_wait_data block is
correct. The exception is within __mptcp_recvmsg_mskq, where a failure in
skb_copy_datagram_msg will cause a break in the consumption loop. This
leaves the actual processing position inconsistent with
skb_peek_tail(&sk->sk_receive_queue), and at that point, we must not wait.

> 
> Also, could you please add a paired tests-case? Thanks!
> 

Further to our discussion, a minimal reproduction case for this scenario is
available here:
https://lore.kernel.org/netdev/e91bf909-bf4d-4f90-a370-688a9424478b@xxxxxxxxxx/

When running the reproduced program, you can observe the server's sy
(system) CPU usage hitting 100%.

Thanks,
Li Xiasong