Travis (travis.downs.delete@this.gmail.com) on August 23, 2018 3:11 pm wrote:
>
> If the op_vec stuff gets merged, you could implement the lock-friendly context switches by using a
> variant of the op_vec mechanism: have the kernel itself either "roll back" or "roll forward" the lock
> body when it wants to schedule a thread that is in the body. This only works for small bodies, but maybe
> the compiler could decide that. We all know the compiler always makes the right decisions...

So I think it's much more reasonable to have that kind of "we guarantee atomic execution" behavior - if you can just describe what you want done to the kernel in a form that is statically verifiable, the kernel can do it for you, and then give some (percpu) atomicity guarantees.

But the reason the op_vec stuff didn't get merged was that unlike the really core rseq stuff, it wasn't obvious that it was the right thing, or that people would really use it.

Also, honestly, I'd personally rather do it with something like very restricted eBPF or something.

So instead of just aborting the rseq thing and jumping to an abort address, the kernel could do some fixup. Whether op_vec or ebpf or something else. But without known use cases that are truly important and generic, it simply isn't clear that it's a good idea.

The "bare" rseq part that was merged had a few known sequences that were agreed upon to be generic and important enough to be worth it, and that was the simplest part of the patches too. Anything more complex and fragile would need a much stronger argument for it too. Which doesn't really mean it won't be done, but it really needs a very concrete example that matters for some generic real case that can't be done some other way.

So we ended up only merging the part that was pretty non-controversial. We've really had a horrible track record with clever interfaces before (ie splice, clone, a lot of stuff that is clever and gave interfaces that are more powerful than the standard ones, but that really doesn't get a lot of use).

Linus