[REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64 and tcmalloc everywhere

[Mark Rutland]

On Thu, Apr 23, 2026 at 10:41:02AM -0700, Linus Torvalds wrote:
> On Thu, 23 Apr 2026 at 10:19, Thomas Gleixner <tglx at kernel.org> wrote:
> I see some patches flying around, but am not clear on whether there
> was an actual patch that make this work again?

There's not a patch yet.

The diffs sent so far were options for fixing the arm64-specific issue
(missing aborts on preemption), NOT the generic issue (missing
clobbering of cpu_id_start that tcmalloc was depending upon).

For the arm64 issue, I think we can have a fix tomorrow (as it's end of
day here in the UK). Now that I've pored the entry code and the rseq
code, I think a variant of one of Thomas's proposed fixes will work, but
I'd like to make the naming/layering crystal clear so that it's harder
to break this by accident in future.

For the generic issue, hopefully the option Mathias proposed (clearing
cpu_id_start when rseq_cs is cleared) is sufficient. I'll work with
Mathias and Thomas for that.

I've also poked folk to make sure that CI systems run the rseq selftests
(which they evidently weren't), so that we catch this sort of thing
earlier.

Mark.