Re: [PATCH bpf-next v1 00/22] Resilient Queued Spin Lock

[Linus Torvalds]

On Wed, 8 Jan 2025 at 12:13, Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote:
>
> Yes, we also noticed during development that try_cmpxchg_tail (in
> patch 9) couldn't rely on 16-bit cmpxchg being available everywhere

I think that's purely a "we have had no use for it" issue.

A 16-bit cmpxchg can always be written using a larger size, and we did
that for 8-bit ones for RCU.

See commit d4e287d7caff ("rcu-tasks: Remove open-coded one-byte
cmpxchg() emulation") which switched RCU over to use a "native" 8-bit
cmpxchg, because Paul had added the capability to all architectures,
sometimes using a bigger size and "emulating" it: a88d970c8bb5 ("lib:
Add one-byte emulation function").

In fact, I think that series added a couple of 16-bit cases too, but I
actually went "if we have no users, don't bother".

              Linus