Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

[Peter Zijlstra]

On Fri, Sep 08, 2023 at 10:30:57PM -0700, Ankur Arora wrote:

> > which actually makes me worry about the nested irq case, because this
> > would *not* be ok:
> >
> >         allow_resched();
> >            -> irq happens
> >                 -> *nested* irq happens
> >                 <- nested irq return (andapreemption)
> >
> > ie the allow_resched() needs to still honor the irq count, and a
> > nested irq return obviously must not cause any preemption.
> 
> IIUC, this should be equivalent to:
> 
> 01         allow_resched();
> 02            -> irq happens
> 03               preempt_count_add(HARDIRQ_OFFSET);
> 04                -> nested irq happens
> 05                   preempt_count_add(HARDIRQ_OFFSET);
> 06
> 07                   preempt_count_sub(HARDIRQ_OFFSET);
> 08                 <- nested irq return
> 09               preempt_count_sub(HARDIRQ_OFFSET);
> 
> So, even if there were nested interrrupts, then the !preempt_count()
> check in raw_irqentry_exit_cond_resched() should ensure that no
> preemption happens until after line 09.

Yes, this.