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

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Date:   Tue, 9 Nov 2021 17:15:24 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        Leon Romanovsky <leon@...nel.org>,
        Ido Schimmel <idosch@...sch.org>,
        "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

Tue, Nov 09, 2021 at 04:07:02PM CET, kuba@...nel.org wrote:
>On Tue, 9 Nov 2021 10:43:58 -0400 Jason Gunthorpe wrote:
>> This becomes all entangled in the aux device stuff we did before.
>
>So entangled in fact that neither of you is willing to elucidate 
>the exact need ;)
>
>> devlink reload is defined, for reasons unrelated to netns, to do a
>> complete restart of the aux devices below the devlink. This happens
>> necessarily during actual reconfiguration operations, for instance.
>> 
>> So we have a situation, which seems like bad design, where reload is
>> also triggered by net namespace change that has nothing to do with
>> reconfiguring.
>
>Agreed, it is somewhat uncomfortable that the same callback achieves
>two things. As clear as the need for reload-for-reset is (reconfig,
>recovery etc.) I'm not as clear on reload for netns.
>
>The main use case for reload for netns is placing a VF in a namespace,
>for a container to use. Is that right? I've not seen use cases
>requiring the PF to be moved, are there any?
>
>devlink now lives in a networking namespace yet it spans such
>namespaces (thru global notifiers). I think we need to define what it
>means for devlink to live in a namespace. Is it just about the
>configuration / notification channel? Or do we expect proper isolation?
>
>Jiri?

Well honestly the primary motivation was to be able to run smoothly with
syzkaller for which the "configuration / notification channel" is
enough.

By "proper isolation" you mean what exactly?


>
>> In this case the per-net-ns becomes a BKL that gets
>> held across way too much stuff as it recuses down the reload path,
>> through aux devices, into the driver core and beyond.
>> 
>> When I looked at trying to fix this from the RDMA side I could not
>> find any remedy that didn't involve some kind of change in netdev
>> land. The drivers must be able to register/unregister notifiers in
>> their struct device_driver probe/remove functions.
>> 
>> I once sketched out fixing this by removing the need to hold the
>> per_net_rwsem just for list iteration, which in turn avoids holding it
>> over the devlink reload paths. It seemed like a reasonable step toward
>> finer grained locking.
>
>Seems to me the locking is just a symptom.