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

[Peter Zijlstra]

On Fri, Sep 08, 2023 at 10:15:07AM -0700, Linus Torvalds wrote:
> On Fri, 8 Sept 2023 at 00:03, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > Which on a PREEMPT_COUNT=n build will cause preemption while holding the
> > spinlock. I think something like the below will cause sufficient
> > warnings to avoid growing patterns like that.
> 
> Hmm. I don't think that warning is valid.
> 
> Disabling preemption is actually fine if it's done in an interrupt,
> iow if we have
> 
>         allow_resched();
>            -> irq happens
>                 spin_lock();  // Ok and should *not* complain
>                 ...
>                 spin_unlock();
>             <- irq return (and preemption)

Indeed.

> 
> 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 (and preemption)
> 
> ie the allow_resched() needs to still honor the irq count, and a
> nested irq return obviously must not cause any preemption.

I think we killed nested interrupts a fair number of years ago, but I'll
recheck -- but not today, sleep is imminent.