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

[Peter Zijlstra]

On Wed, Apr 22, 2026 at 11:50:26AM +0200, Mathias Stearn wrote:

> Additionally, it breaks tcmalloc specifically by failing to overwrite
> the cpu_id_start field at points where it was relied on for
> correctness.

This specific behaviour was documented as being wrong and running with
DEBUG_RSEQ would have flagged it.

The tcmalloc issue has been contentious for a long time. The tcmalloc
folks relied on something that was documented to be wrong. It has been
reported to the tcmalloc people many years ago and if you were to run
tcmalloc on most any kernel (very much including 6.19) with
DEBUG_RSEQ=y, it would have yelled.

The tcmalloc people didn't care. There was a proposal for an RSEQ
extension for what they need, and they didn't care. All this should be
in their bugzilla or whatever.

The RSEQ rework improved performance significantly for everyone, and
kept all the documented behaviour (+- arm64 bug). Tcmalloc got screwed
over because they relied on implementation behaviour that was
specifically documented to be broken. And they didn't care. Google was
very much aware of this. And hasn't lifted a finger to remedy it.