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

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Message-ID: <20241105133955.GF311159@unreal>
Date: Tue, 5 Nov 2024 15:39:55 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Wenjia Zhang <wenjia@...ux.ibm.com>
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 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

> 
> Thanks,
> Wenjia