linux-kernel - Re: [PATCH v2 3/5] powerpc: Use preemption model accessors

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Date:   Mon, 15 Nov 2021 15:29:53 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kbuild@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Mike Galbraith <efault@....de>, Marco Elver <elver@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        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 3/5] powerpc: Use preemption model accessors


Doh, thought I had sent this one out already...

On 11/11/21 15:55, Michael Ellerman wrote:
> Valentin Schneider <valentin.schneider@....com> writes:
>> Per PREEMPT_DYNAMIC, checking CONFIG_PREEMPT doesn't tell you the actual
>> preemption model of the live kernel. Use the newly-introduced accessors
>> instead.
>>
>> sched_init() -> preempt_dynamic_init() happens way before IRQs are set up,
>> so this should be fine.
>
> Despite the name interrupt_exit_kernel_prepare() is called before IRQs
> are setup, traps and page faults are "interrupts" here.
>
> So I'm not sure about adding that call there, because it will trigger a
> WARN if called early in boot, which will trigger a trap and depending on
> the context we may not survive.
>
> I'd be happier if we can make it a build-time check.
>

This can't be done at build-time for PREEMPT_DYNAMIC, but that can be
punted off to whoever will implement ppc support for that :-) AFAICT if
this can't use preempt_dynamic_mode (due to how "late" it is setup), the
preempt_schedule_irq() needs to go and ppc needs to use irqentry_exit() /
irqentry_exit_cond_resched().

I dropped that for v2.

> cheers
>