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

From: Jakub Kicinski <kuba@kernel.org>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leon@kernel.org>,
	Ido Schimmel <idosch@idosch.org>, Jiri Pirko <jiri@resnulli.us>,
	"David S . Miller" <davem@davemloft.net>,
	Jiri Pirko <jiri@nvidia.com>,
	linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	edwin.peer@broadcom.com
Subject: Re: [PATCH net-next] devlink: Require devlink lock during device reload
Date: Tue, 9 Nov 2021 07:07:02 -0800	[thread overview]
Message-ID: <20211109070702.17364ec7@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> (raw)
In-Reply-To: <20211109144358.GA1824154@nvidia.com>

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.