[Ksummit-discuss] [MAINTAINER TOPIC] ABI feature gates?
Andy Lutomirski
luto at kernel.org
Fri Aug 4 01:16:44 UTC 2017
[Note: I'm not entirely sure I can make it to the kernel summit this
year, due to having a tiny person and tons of travel]
This may be highly controversial, but: there seems to be a weakness in
the kernel development model in the way that new ABI features become
stable. The current model is, roughly:
1. Someone writes the code. Maybe they cc linux-abi, maybe they don't.
2. People hopefully review the code.
3. A subsystem maintainer merges the code. They hope the ABI is right.
4. Linus gets a pull request. Linus probably doesn't review the ABI
for sanity, style, blatant bugs, etc. If Linus did, then he'd never
get anything else done.
5. The new ABI lands in -rc1.
6. If someone finds a problem or objects, it had better get fixed
before the next real release.
There's a few problems here. One is that the people who would really
review the ABI might not even notice until step 5 or 6 or so. Another
is that it takes some time for userspace to get experience with a new
ABI.
I'm wondering if there are other models that could work. I think it
would be nice for us to be able to land a kernel in Linus tree and
still wait a while before stabilizing it. Rust, for example, has a
strict policy for this that seems to work quite well.
Maybe we could pull something off where big new features hide behind a
named feature gate for a while. That feature gate can only be enabled
under some circumstances that make it very hard to mistake it for true
stability. (For example, maybe you *can't* enable feature gates on a
final kernel unless you manually patch something.)
Here are a few examples that come to mind for where this would have helped:
- Whatever that new RDMA socket type was that was deemed totally
broken but only just after it hit a real release.
- O_TMPFILE. I discovered that it corrupted filesystems in -rc6 or
-rc7. That got fixed, the the API is still a steaming pile of crap.
- Some cgroup+bpf stuff that got cleaned up in a -rc7 or so a few releases ago.
I'm sure there are tons more.
Is this too crazy, or is it worth discussing?
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