netdev - Re: [RFC mptcp-next] tcp: mptcp: use mptcp receive buffer space to select rcv window

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Date:   Wed, 18 Mar 2020 11:14:38 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Florian Westphal <fw@...len.de>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, mptcp@...ts.01.org,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [RFC mptcp-next] tcp: mptcp: use mptcp receive buffer space to
 select rcv window



On 3/18/20 11:05 AM, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@...il.com> wrote:
>>> @@ -2771,6 +2771,11 @@ u32 __tcp_select_window(struct sock *sk)
>>>  	int full_space = min_t(int, tp->window_clamp, allowed_space);
>>>  	int window;
>>>  
>>> +	if (sk_is_mptcp(sk)) {
>>> +		mptcp_space(sk, &free_space, &allowed_space);
>>> +		full_space = min_t(int, tp->window_clamp, allowed_space);
>>> +	}
>>
>> You could move the full_space = min_t(int, tp->window_clamp, allowed_space);
>> after this block factorize it.
> 
> Indeed, will do.
> 
>>> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
>>> index 40ad7995b13b..aefcbb8bb737 100644
>>> --- a/net/mptcp/subflow.c
>>> +++ b/net/mptcp/subflow.c
>>> @@ -745,6 +745,23 @@ bool mptcp_subflow_data_available(struct sock *sk)
>>>  	return subflow->data_avail;
>>>  }
>>>  
>>> +/* If ssk has an mptcp parent socket, use the mptcp rcvbuf occupancy,
>>> + * not the ssk one.
>>> + *
>>> + * In mptcp, rwin is about the mptcp-level connection data.
>>> + *
>>> + * Data that is still on the ssk rx queue can thus be ignored,
>>> + * as far as mptcp peer is concerened that data is still inflight.
>>> + */
>>> +void mptcp_space(const struct sock *ssk, int *space, int *full_space)
>>> +{
>>> +	const struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
>>> +	const struct sock *sk = READ_ONCE(subflow->conn);
>>
>> What are the rules protecting subflow->conn lifetime ?
>>
>> Why dereferencing sk after this line is safe ?
> 
> Subflow sockets hold a reference on the master/parent mptcp-socket.
> 

Presence of READ_ONCE() tells something might happen on
this pointer after you read it.

Can this pointer be set while this thread is owning the socket lock ?

If not, then you do not need READ_ONCE(), this is confusing.

If yes, then it means that whatever changes the pointer might also release the reference
on the old object.

Thanks.