Re: [PATCH net] net/smc: Fix lookup of netdev by using ib_device_get_netdev()

[Leon Romanovsky <leon@kernel.org>]

On Sun, Oct 27, 2024 at 10:18:57PM +0200, 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")

Honestly, this patch in Fixes line doesn't look right to me. It pokes inside
of ib_device to get netdev index. For example call to smc_ib_ndev_change()
will return completely unpredictable results, due to races.

It is bad that RDMA ML wasn't even CCed back then, we would say NAK to
this patch.
https://lore.kernel.org/netdev/20201201192049.53517-6-kgraul@linux.ibm.com/

Thanks