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

[Linus Torvalds]

On Sat, 9 Sept 2023 at 13:16, Ankur Arora <ankur.a.arora@xxxxxxxxxx> wrote:
>
> > +     if (WARN_ON(resched_allowed()))
> > +             return;
>
> And, maybe something like this to guard against __this_cpu_read()
> etc:
>
> +++ b/lib/smp_processor_id.c
> @@ -13,6 +13,9 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2)
>  {
>         int this_cpu = raw_smp_processor_id();
>
> +       if (unlikely(resched_allowed()))
> +               goto out_error;

Again, both of those checks are WRONG.

They'll error out even in exceptions / interrupts, when we have a
preempt count already from the exception itself.

So testing "resched_allowed()" that only tests the TIF_RESCHED_ALLOW
bit is wrong, wrong, wrong.

These situations aren't errors if we already had a preemption count
for other reasons. Only trying to disable preemption when in process
context (while TIF_RESCHED_ALLOW) is a problem. Your patch is missing
the check for "are we in a process context" part.

                Linus