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

[Peter Zijlstra]

On Wed, Apr 22, 2026 at 02:56:47PM +0200, Peter Zijlstra wrote:
> 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.

Also: https://lore.kernel.org/all/874io5andc.ffs@tglx/