Re: SIGSEGVs after 39a167560a61 ("rseq: Optimize event setting")

[Dmitry Vyukov]

On Tue, 27 Jan 2026 at 21:34, Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> +CC Dmitry and Marco.
>
> On 2026-01-26 17:35, Mathieu Desnoyers wrote:
> > On 2026-01-26 17:27, David Matlack wrote:
> >> On Mon, Jan 26, 2026 at 1:51 PM Thomas Gleixner <tglx@xxxxxxxxxx> wrote:
> > [...]
> >>>> Perhaps this is the nudge Google needs to go fix this.
> >>>
> >>> The real question is whether the segfault is triggered from the rseq
> >>> sanity checks or if the application segfaults becauses it relies on
> >>> something something which is not guaranteed by the ABI. As this is
> >>> secret sauce, I can't tell.
> >>
> >> I tried enabling /debug/rseq/debug but many of the daemons on my host
> >> started crash-looping so much that I wasn't able to even run my test.
> >>
> >> Next I tried disabling CONFIG_RSEQ and as expected the issue went
> >> away. I will use that for now to unblock my VFIO testing.
> >>
> >> I have reported the tcmalloc regression internally within Google to
> >> figure out what next step they want to take.
> >
> > Note that I've proposed to help out the tcmalloc people a few
> > times in the past years to fix this, but I've been told that
> > it was not a priority on their end, and that they would not be
> > able to even test whatever I would come up with.

I see the patch sets t->rseq.event.ids_changed when cid changes.
David, perhaps we miss the same update vcpu changes b/c logically vcpu
is the same as cid?
Otherwise I don't see why this change would interfere with tcmalloc's
[ab]use of rseq. Fences (membarrier) should continue to update rseq
unconditionally for tcmalloc, but that's not the scheduler fast paths.