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

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Message-ID: <20211109070702.17364ec7@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 9 Nov 2021 07:07:02 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Leon Romanovsky <leon@...nel.org>,
        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 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?

> 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.