netdev - Re: [PATCH net-next] devlink: Require devlink lock during device reload

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Date:   Tue, 9 Nov 2021 16:12:33 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Ido Schimmel <idosch@...sch.org>, Jiri Pirko <jiri@...nulli.us>,
        "David S . Miller" <davem@...emloft.net>,
        Jiri Pirko <jiri@...dia.com>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, edwin.peer@...adcom.com
Subject: Re: [PATCH net-next] devlink: Require devlink lock during device
 reload

On Mon, Nov 08, 2021 at 03:31:26PM -0800, Jakub Kicinski wrote:
> On Mon, 8 Nov 2021 21:58:36 +0200 Leon Romanovsky wrote:
> > > > > nfp will benefit from the simplified locking as well, and so will bnxt,
> > > > > although I'm not sure the maintainers will opt for using devlink framework
> > > > > due to the downstream requirements.    
> > > > 
> > > > Exactly why devlink should be fixed first.  
> > > 
> > > If by "fixed first" you mean it needs 5 locks to be added and to remove
> > > any guarantees on sub-object lifetime then no thanks.  
> > 
> > How do you plan to fix pernet_ops_rwsem lock? By exposing devlink state
> > to the drivers? By providing unlocked version of unregister_netdevice_notifier?
> > 
> > This simple scenario has deadlocks:
> > sudo ip netns add n1
> > sudo devlink dev reload pci/0000:00:09.0 netns n1
> > sudo ip netns del n1
> 
> Okay - I'm not sure why you're asking me this. This is not related to
> devlink locking as far as I can tell. Neither are you fixing this
> problem in your own RFC.

I asked you because you clearly showed to me that things that makes
sense for me, doesn't make sense for you and vice versa.

I don't want to do work that will be thrown away.

> 
> You'd need to tell me more about what the notifier is used for (I see
> RoCE in the call trace). I don't understand why you need to re-register 
> a global (i.e. not per netns) notifier when devlink is switching name
> spaces.

RDMA subsystem supports two net namespace aware scenarios.

We need global netdev_notifier for shared mode. This is legacy mode where
we listen to all namespaces. We must support this mode otherwise we break
whole RDMA world.

See commit below:
de641d74fb00 ("Revert "RDMA/mlx5: Fix devlink deadlock on net namespace deletion"")

Thanks