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

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Message-ID: <e88d6049-6be1-4967-b88d-94d437900c3d@linux.ibm.com>
Date: Tue, 5 Nov 2024 13:30:24 +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 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.
>   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.

Thanks,
Wenjia