Re: [patch V4 26/36] rseq: Optimize event setting

Thomas Gleixner
2025-09-11T16:06:16Z

On Thu, Sep 11 2025 at 10:03, Mathieu Desnoyers wrote:
> On 2025-09-08 17:32, Thomas Gleixner wrote:
>> static inline void rseq_handle_notify_resume(struct pt_regs *regs)
>> {
>> - if (current->rseq.event.has_rseq)
>> + /* '&' is intentional to spare one conditional branch */
>> + if (current->rseq.event.sched_switch & current->rseq.event.has_rseq)
>
> I wonder.. except for the corner case of rseq unregistration,
> when can we have sched_switch set but not has_rseq ?
>
> We could remove a load from the fast path and the AND if we
> clear the sched_switch flag on rseq unregistration.

We probably could. Though I doubt it matters much and I opted for
correctness instead of premature optimization.

Thanks,

tglx