Re: Rust kernel policy

Lyude Paul
2025-02-20T20:49:14Z

On Wed, 2025-02-19 at 17:05 +0200, Laurent Pinchart wrote:
>
> In reality, in order to provide API that are possible to use correctly,
> we have many areas deep in kernel code that will require a complete
> redesign (similar in effort to the introduction of the device model),
> affecting all the drivers using them. I understand that the development
> of rust bindings has already helped improving some in-kernel C APIs, but
> I have only seen such improvements of a relatively small scale compared
> to what would be needed to fix life time management of objects in V4L2.
> I would be very surprised if I was working in the only area in the
> kernel that is considered broken beyond repair by many people related to
> life time management, so I think this kind of maintainer nightmare is
> not an isolated case.
>
> The theory is that rust bindings would come with C API improvements and
> fixes. However, when I expressed the fact that rust bindings for V4L2
> would first require a complete rewrite of object life time management in
> the subsystem, I was told this was way too much yak shaving. As a
> maintainer facing the horrendous prospect of fixing this one day, I just
> can't agree to rust bindings being built on top of such a bad
> foundation, as it would very significantly increase the amount of work
> needed to fix the problem.

I don't know that this is really specific to rust though. While I'm somewhat
aware of the V4L2 bindings you're referring to and have the same reservations
(they came up in some of the panthor related discussions), I don't think the
issue of a contributor wanting to rush something is exclusive to rust.
Remember we're selling rust as a tool for making API design a lot easier and
enforcing it much more easily, but just like with anything that only works if
the rust code goes in is held to a high standard. I think that's an inevitable
trait of pretty much any tool, the difference with rust is that when we do
merge well reviewed and thought out bindings the job of reviewing usages of
those bindings can be a lot less work than in C - and can also point out
issues to contributors before their patches even reach the mailing list.

>
> If we want real maintainer buy-in for rust in the kernel, I believe this
> is the kind of problem space we should start looking into. Helping
> maintainers solve these issues will help decreasing their work load and
> stress level significantly in the long term, regardless of other
> benefits rust as a language may provide. I believe that cooperation
> between the C and rust camps on such issues would really improve mutual
> understanding, and ultimately create a lot of trust that seems to be
> missing. If someone were to be interested in rust bindings for V4L2 and
> willing to put significant time and effort in fixing the underlying
> issue, I would be very happy to welcome them, and willing to learn
> enough rust to review the rust API.

I certainly can't argue that upstream in most cases it's been small wins
rather than very big wins. At the same time though, I don't think that's a
symptom of rust but a symptom of the huge hurdle of getting rust patches
upstream through in the first place since so much of the work we've been
dealing with is just convincing maintainers to consider bindings at all. And
it's usually dealing with the exact same set of arguments each time, just
different maintainers. In that regard, I'd say that we don't really have a
reasonable way of accomplishing big gains with rust yet simply because the
opportunity hasn't really been available. Especially when you look at what
projects like Asahi have been able to accomplish - shockingly few bugs
happening there are actually coming from the rust code!

I wish I could see this sort of thing in the actual mainline kernel right now
and point to examples there, but with the pace that things have been going I'm
not sure how that would be possible. To see big gains, a willingness to
actually try rust and allow it to prove itself needs to be present and more
widespread in the community. Otherwise, the only gains we'll get are whatever
handful of patches we do manage to get upstream. It's a catch 22.

I do want to mention too: having worked on the kernel for almost a decade I'm
well aware that kernel submissions take time - and I don't think that's a bad
thing at all! In fact, I think the review process is integral to where the
kernel has gotten today. But there's a difference when a lot of the time with
the average kernel submission is spent on constructive iterative design,
whereas a pretty large chunk of the time I've seen spent trying to upstream
rust code has been dedicated to trying to convince upstream to allow any kind
of rust code in the first place. Historically, that's where a lot of rust work
has gotten stuck well before anyone actually reaches the phase of iterative
design. Even though a lot of these repeated arguments aren't necessarily
unreasonable, it's more difficult to treat them as such when they get resolved
in one area of the kernel only to come back up again in another area. There's
a cycle here too - the widespread resistance to rust submissions at face value
sets a tone for rust contributors that leaks into the actually productive
iterative discussions that do happen. As these contributors get more burned
out, this can work to train contributors to see the whole process as just
another form of gate keeping.

I also feel like that one of the other obstacles I've observed with this is
that often in the upstreaming process, some of these arguments revolve around
maintainer workload - but at the same time aren't actually dissuaded when the
submitter actually offers their own time to reduce the workload or asks about
working to find solutions to make this easier on the maintainer. I wouldn't
dream of arguing that being a maintainer isn't a tough job that burns people
out, it certainly is, but I'd really like to see more areas of the kernel be
open to delegation since we know how prevalent of an issue this burnout is.
Otherwise, we're apt to always say no to solutions that don't prioritize the
short-term gains. And generally, it's just not sustainable in the long term.

IMO: Further contributing to these challenges is the social impact of burnout
from maintainers being so widespread, which I think hits a lot different for
new contributors than it does regular contributors or maintainers. We're
fairly used to the way things work, and aren't strangers to the tone that
comes along with a burned out maintainer. Regulars like us have tools to work
with that and can tell the difference between someone who's burned out and
someone who's just arguing in bad faith. But that's a skill we've acquired
from our experience here, not a given. I think that this can even extend
further: over the last decade I've worked on the kernel, burnout seems so
regular that I've observed that the responses and tones in discussion that
come with being burned out almost seem to get normalized with a lot of
maintainers and regular contributors. And I don't think this is intentionally,
but rather often happens without people realizing it. To me, that certainly
helps explain why so many discussions where a contributor blows up can seem
outlandish a new contributor, but up until that point seem fairly normal to
maintainers or regular contributors. Especially when those of us who have had
the privilege to speak with these maintainers in real life to try to solve
these issues generally get a wildly different perspective on their viewpoints
then we get from the mailing list.

The other side-effect of all of this too is that in the cases where we -do-
actually deal with bad-faith arguments in trying to upstream rust work, which
certainly do happen, it is much more difficult for a contributor to tell that
apart from a well-meaning maintainer who is just burned out. I can certainly
say myself that if I wasn't familiar with the tone of burnout I probably would
have given up upstreaming any of the work that I've done and just left. It's
also a huge reason I've leaned into the maintainership aspect of rust when
advocating for it in the kernel - because I hope that addressing this burnout
could be a step forward in making the community more welcoming to everyone.

--
Cheers,
Lyude Paul (she/her)
Software Engineer at Red Hat

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