Comment by Tomte
2 days ago
> rust folks should fix the rust binding when C changes breaks the binding
I have never understood how that could work long-time. How do you release a kernel, where some parts are broken? Either you wait for Rust people to fix their side or you drop the C changes. Or your users suddenly find their driver doesn‘t work anymore after a kernel update.
As a preliminary measure when there isn‘t a substantial amount of Rust code, yet, sure. But the fears of some maintainers that the policy will change to "you either learn Rust and fix things or your code can be held up until someone else helps you out" are well-founded, IMO.
Are you familiar with Linux kernel development process? Features can be merged only in two weeks long merge window. After the merge window closes, only fixes are merged for eight weeks. Rust binding can be fixed in that time. I don't see any problems.
That's a gross simplificaftion of the development process. Yes, new features are mostly merged in that two-weeks window -- but you're now talking about the Linux release management process more than its development.
Before features are merged to Linus' release branch, pretty much all changes are published and merged to linux-next first. It is exactly here that build issues and conflicts are first detected and worked out, giving maintainers early visibility into changes that are happening outside their subsystem. Problems with the rust bindings will probably show up here, and the Rust developers will have ample time to fix/realign their code before the merge window even starts. And it's not uncommon for larger features (e.g. when they require coordination across subsystems) to remain in linux-next for more than one cycle.
And if no Rust developer has time or interest in those eight weeks? I don‘t claim that it can never work (or it cannot work in the common case), but as a hard rule it seems untenable.
> And if no Rust developer has time or interest in those eight weeks?
What if Linus decided to go on a two month long vacation in the middle of the merge window?
> I don‘t claim that it can never work (or it cannot work in the common case), but as a hard rule it seems untenable.
There are quite a few rust developers already involved, if they cannot coordinate that at least some are available during a release critical two month period then none of them should be part of any professional project.
I'm not familiar with kernel development, but what's the difference anyway with C code? If you change the interface of some part, any users of it will be broken Rust or not. It will require coordination anyway.
It is customary for maintainers to fix _all_ usage of their code themselves? That doesn't seem scalable.
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If so kernel is released with broken Rust. That is the policy, and I am flabbergasted why everyone is going "that policy must not be literal".
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I think the way you do this is set things up so that no bits that are written in Rust are built by default, and make sure that the build system is set up such that Rust bindings for C code are only built when there's Rust code that's enabled that requires them.
Then sure, some people who download a kernel release might enable a Rust driver, and that makes the build fail. But until Rust is considered a first-class, fully-supported language in the kernel, that's fine.
In practice, though, I would expect that the Rust maintainers would fix those sorts of things up before an actual release is cut, after the 2-week merge window, during the period when only fixes are accepted. Maybe not every single time, but most of the time; if no one is available to fix a particular bit of breakage, then it's broken for that release. And that's fine too, even if it might be annoying to some users.
> I think the way you do this is set things up so that no bits that are written in Rust are built by default, and make sure that the build system is set up such that Rust bindings for C code are only built when there's Rust code that's enabled that requires them.
Which is currently the only way possible and it will stay that way for a long time because remember that clang support less targets than gcc and gcc cannot compile Rust.
Once gcc can /reliably/ compile Rust, then and only then Rust could be "upgraded" to a first class citizen in Linux. The "C-maintainers don't want to learn Rust" issue, will still be here of course, but there will already be many years of having a mixed code base..
I agree with all you say, but with longterm I really mean when we've arrived here
> But until Rust is considered a first-class, fully-supported language in the kernel, that's fine
A first-class language whose kernel parts may always break does seem unreasonable. I still think policy will have to change by that point.
Because nothing is forcing a distro to adopt a kernel that has items that are broken. Not a lot of folks out there are manually compiling and deploying standalone kernels to production systems.
C can break rust, and Debian/Ubuntu/Redhat/suse/etc can wait for it to be fixed before pushing a new kernel to end users.
You can merge it into your branch as e.g. the DMA maintainer, then the rust folk can pull your changes and fix the bindings. Maybe you as maintainer could give them a heads up and a quick consideration of the error.
Yes, Rust as something optional doesn't really make sense long term. Either it will continue to only be used in nieche drivers (in which case why bother?) or eventually you need to build Rust code to have a usable kernel for common hardware. Any promises to the contrary need to be backed up with more than "trus me bro".