On Thu, Apr 23 2026 at 07:53, Dmitry Vyukov wrote:

On Thu, 23 Apr 2026 at 03:48, Jinjie Ruan [off-list ref] wrote:

This part of the rseq man page needs to be fixed as well I think. The
kernel no longer reliably provides clearing of rseq_cs on preemption,
right?

https://git.kernel.org/pub/scm/libs/librseq/librseq.git/tree/doc/man/rseq.2#n241

"and set to NULL by the kernel when it restarts an assembly
instruction sequence block,
as well as when the kernel detects that it is preempting or delivering
a signal outside of the range targeted by the rseq_cs."

The kernel clears rseq_cs reliably when user space was interrupted and:

    the task was preempted
or
    the return from interrupt delivers a signal

If the task invoked a syscall then there is absolutely no reason to do
either of this because syscalls from within a critical section are a
bug and catched when enabling rseq debugging.

The original code did this along with unconditionally updating CPU/MMCID
which resulted in ~15% performance regression on a syscall heavy
database benchmark once glibc started to register rseq.

Thanks,

        tglx