netdev - Re: [PATCH net] net/smc: Fix lookup of netdev by using ib_device_get

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Message-ID: <67fb5917-a30d-4b01-9659-72c76ce53c1a@linux.ibm.com>
Date: Tue, 5 Nov 2024 15:14:26 +0100
From: Wenjia Zhang <wenjia@...ux.ibm.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Wen Gu <guwen@...ux.alibaba.com>, "D. Wythe" <alibuda@...ux.alibaba.com>,
        Tony Lu <tonylu@...ux.alibaba.com>, David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-s390@...r.kernel.org,
        Heiko Carstens <hca@...ux.ibm.com>, Jan Karcher <jaka@...ux.ibm.com>,
        Gerd Bayer <gbayer@...ux.ibm.com>,
        Alexandra Winter <wintera@...ux.ibm.com>,
        Halil Pasic <pasic@...ux.ibm.com>, Nils Hoppmann <niho@...ux.ibm.com>,
        Niklas Schnell <schnelle@...ux.ibm.com>,
        Thorsten Winkler <twinkler@...ux.ibm.com>,
        Karsten Graul <kgraul@...ux.ibm.com>,
        Stefan Raspl <raspl@...ux.ibm.com>, Aswin K <aswin@...ux.ibm.com>
Subject: Re: [PATCH net] net/smc: Fix lookup of netdev by using
 ib_device_get_netdev()



On 05.11.24 14:39, Leon Romanovsky wrote:
> On Tue, Nov 05, 2024 at 01:30:24PM +0100, Wenjia Zhang wrote:
>>
>>
>> On 05.11.24 12:23, Leon Romanovsky wrote:
>>> On Tue, Nov 05, 2024 at 10:50:45AM +0100, Wenjia Zhang wrote:
>>>>
>>>>
>>>> On 27.10.24 21:18, Leon Romanovsky wrote:
>>>>> On Fri, Oct 25, 2024 at 09:23:55AM +0200, Wenjia Zhang wrote:
>>>>>> Commit c2261dd76b54 ("RDMA/device: Add ib_device_set_netdev() as an
>>>>>> alternative to get_netdev") introduced an API ib_device_get_netdev.
>>>>>> The SMC-R variant of the SMC protocol continued to use the old API
>>>>>> ib_device_ops.get_netdev() to lookup netdev.
>>>>>
>>>>> I would say that calls to ibdev ops from ULPs was never been right
>>>>> thing to do. The ib_device_set_netdev() was introduced for the drivers.
>>>>>
>>>>> So the whole commit message is not accurate and better to be rewritten.
>>>>>
>>>>>> As this commit 8d159eb2117b
>>>>>> ("RDMA/mlx5: Use IB set_netdev and get_netdev functions") removed the
>>>>>> get_netdev callback from mlx5_ib_dev_common_roce_ops, calling
>>>>>> ib_device_ops.get_netdev didn't work any more at least by using a mlx5
>>>>>> device driver.
>>>>>
>>>>> It is not a correct statement too. All modern drivers (for last 5 years)
>>>>> don't have that .get_netdev() ops, so it is not mlx5 specific, but another
>>>>> justification to say that SMC-R was doing it wrong.
>>>>>
>>>>>> Thus, using ib_device_set_netdev() now became mandatory.
>>>>>
>>>>> ib_device_set_netdev() is mandatory for the drivers, it is nothing to do
>>>>> with ULPs.
>>>>>
>>>>>>
>>>>>> Replace ib_device_ops.get_netdev() with ib_device_get_netdev().
>>>>>
>>>>> It is too late for me to do proper review for today, but I would say
>>>>> that it is worth to pay attention to multiple dev_put() calls in the
>>>>> functions around the ib_device_get_netdev().
>>>>>
>>>>>>
>>>>>> Fixes: 54903572c23c ("net/smc: allow pnetid-less configuration")
>>>>>> Fixes: 8d159eb2117b ("RDMA/mlx5: Use IB set_netdev and get_netdev functions")
>>>>>
>>>>> It is not related to this change Fixes line.
>>>>>
>>>>
>>>> Hi Leon,
>>>>
>>>> Thank you for the review! I agree that SMC could do better. However, we
>>>> should fix it and give enough information and reference on the changes,
>>>> since the code has already existed and didn't work with the old way.
>>>
>>> The code which you change worked by chance and was wrong from day one.
>>>
>>>> I can rewrite the commit message.
>>>>
>>>> What about:
>>>> "
>>>> The SMC-R variant of the SMC protocol still called
>>>> ib_device_ops.get_netdev() to lookup netdev. As we used mlx5 device driver
>>>> to run SMC-R, it failed to find a device, because in mlx5_ib the internal
>>>> net device management for retrieving net devices was replaced by a common
>>>> interface ib_device_get_netdev() in commit 8d159eb2117b ("RDMA/mlx5: Use IB
>>>> set_netdev and get_netdev functions"). Thus, replace
>>>> ib_device_ops.get_netdev() with ib_device_get_netdev() in SMC.
>>>> "
>>>
>>>    The SMC-R variant of the SMC protocol used direct call to ib_device_ops.get_netdev()
>>>    function to lookup netdev. Such direct accesses are not correct for any
>>>    usage outside of RDMA core code.
>>>
>> Is such an absolute statement documented somewhere? If not, I don't think
>> it's convenient that I use it. Maybe you guys as RDMA core maintainer can,
>> not I.
> 
> You can too as it is very clear. All functions which can be used have
> EXPORT_SYMBOL near them, ops.get_netdev() has nothing like that.
> 
>>>    RDMA subsystem provides ib_device_get_netdev() function that works on
>>>    all RDMA drivers returns valid netdev with proper locking an reference
>>>    counting. The commit 8d159eb2117b ("RDMA/mlx5: Use IB set_netdev and get_netdev
>>>    functions") exposed that SMC-R didn't use that function.
>>>
>>>    So update the SMC-R to use proper API,
>>>
>>> Thanks
>>>
>> mhhh, I'd like to stick to my version, which sounds more neutral IMO. I
>> think the purpose is the same.
> 
> I don't want to argue about the words, my point is that get_netdev() was
> never been the right interface.
> 
> Thanks
> 
Ok, I got you. I'll send v2 with a proper commit message.

Thanks,
Wenjia