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

[Linus Torvalds]

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)

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've lost sight of the original patch series, and I assume / hope that
the above isn't actually an issue, but exactly because I've lost sight
of the original patches and only have this one in my mailbox I wanted
to check.

            Linus