linux-kernel - Re: [PATCH v2 4/5] kscan: Use preemption model accessors

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Date:   Thu, 11 Nov 2021 10:39:49 +0100
From:   Marco Elver <elver@...gle.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kbuild@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Mike Galbraith <efault@....de>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 4/5] kscan: Use preemption model accessors

On Thu, 11 Nov 2021 at 10:11, Marco Elver <elver@...gle.com> wrote:
>
> Subject s/kscan/kcsan/
>
> On Wed, Nov 10, 2021 at 08:24PM +0000, Valentin Schneider wrote:
> > Per PREEMPT_DYNAMIC, checking CONFIG_PREEMPT doesn't tell you the actual
> > preemption model of the live kernel. Use the newly-introduced accessors
> > instead.
> >
> > Signed-off-by: Valentin Schneider <valentin.schneider@....com>
>
> Reviewed-by: Marco Elver <elver@...gle.com>
>
> Though it currently doesn't compile as a module due to missing
> EXPORT_SYMBOL of is_preempt*().
>
> > ---
> >  kernel/kcsan/kcsan_test.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/kcsan/kcsan_test.c b/kernel/kcsan/kcsan_test.c
> > index dc55fd5a36fc..14d811eb9a21 100644
> > --- a/kernel/kcsan/kcsan_test.c
> > +++ b/kernel/kcsan/kcsan_test.c
> > @@ -1005,13 +1005,13 @@ static const void *nthreads_gen_params(const void *prev, char *desc)
> >       else
> >               nthreads *= 2;
> >
> > -     if (!IS_ENABLED(CONFIG_PREEMPT) || !IS_ENABLED(CONFIG_KCSAN_INTERRUPT_WATCHER)) {
> > +     if (!is_preempt_full() || !IS_ENABLED(CONFIG_KCSAN_INTERRUPT_WATCHER)) {

In case you introduce the 5th helper I suggested
(is_preempt_full_or_rt() or whatever you'll call it), this one can be
switched, because this check really does want to know if "at least
full preemption" and not "precisely full preemption".

Thanks,
-- Marco