netdev - Re: [PATCH net-next 09/10] mptcp: preserve const qualifier in mptcp

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Message-ID: <618e9ca8-29fd-abf2-9f0b-eca253b6d2f4@tessares.net>
Date:   Fri, 17 Mar 2023 18:59:04 +0100
From:   Matthieu Baerts <matthieu.baerts@...sares.net>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        David Ahern <dsahern@...nel.org>,
        Simon Horman <simon.horman@...igine.com>,
        Willem de Bruijn <willemb@...gle.com>,
        eric.dumazet@...il.com, MPTCP Upstream <mptcp@...ts.linux.dev>
Subject: Re: [PATCH net-next 09/10] mptcp: preserve const qualifier in
 mptcp_sk()

Hi Eric,

Thank you for your quick reply!

On 17/03/2023 18:47, Eric Dumazet wrote:
> On Fri, Mar 17, 2023 at 10:32 AM Matthieu Baerts
> <matthieu.baerts@...sares.net> wrote:
>> On 17/03/2023 16:55, Eric Dumazet wrote:
>>
>> (...)
>>
>>> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
>>> index 61fd8eabfca2028680e04558b4baca9f48bbaaaa..4ed8ffffb1ca473179217e640a23bc268742628d 100644
>>> --- a/net/mptcp/protocol.h
>>> +++ b/net/mptcp/protocol.h
>>
>> (...)
>>
>>> @@ -381,7 +378,7 @@ static inline struct mptcp_data_frag *mptcp_pending_tail(const struct sock *sk)
>>>       return list_last_entry(&msk->rtx_queue, struct mptcp_data_frag, list);
>>>  }
>>>
>>> -static inline struct mptcp_data_frag *mptcp_rtx_head(const struct sock *sk)
>>> +static inline struct mptcp_data_frag *mptcp_rtx_head(struct sock *sk)
>>
>> It was not clear to me why you had to remove the "const" qualifier here
>> and not just have to add one when assigning the msk just below. But then
>> I looked at what was behind the list_first_entry_or_null() macro used in
>> this function and understood what was the issue.
>>
>>
>> My naive approach would be to modify this macro but I guess we don't
>> want to go down that road, right?
>>
>> -------------------- 8< --------------------
>> diff --git a/include/linux/list.h b/include/linux/list.h
>> index f10344dbad4d..cd770766f451 100644
>> --- a/include/linux/list.h
>> +++ b/include/linux/list.h
>> @@ -550,7 +550,7 @@ static inline void list_splice_tail_init(struct
>> list_head *list,
>>   * Note that if the list is empty, it returns NULL.
>>   */
>>  #define list_first_entry_or_null(ptr, type, member) ({ \
>> -       struct list_head *head__ = (ptr); \
>> +       const struct list_head *head__ = (ptr); \
>>         struct list_head *pos__ = READ_ONCE(head__->next); \
>>         pos__ != head__ ? list_entry(pos__, type, member) : NULL; \
>>  })
>> -------------------- 8< --------------------
> 
> This could work, but it is a bit awkward.
> 
> mptcp_rtx_head() is used  in a context where we are changing the
> socket, not during a readonly lookup ?

Indeed, you are right. It is currently only used in a context where we
are changing the socket.

I can see cases where a new packets scheduler might just need to check
if the rtx queue is empty but then we should probably add a new helper
using list_empty() instead.

So yes, no need to change anything!

Cheers,
Matt
-- 
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