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

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Date:   Thu, 11 Nov 2021 15:55:27 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Valentin Schneider <valentin.schneider@....com>,
        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

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.

cheers

> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
> index de10a2697258..c56c10b59be3 100644
> --- a/arch/powerpc/kernel/interrupt.c
> +++ b/arch/powerpc/kernel/interrupt.c
> @@ -552,7 +552,7 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
>  		/* Returning to a kernel context with local irqs enabled. */
>  		WARN_ON_ONCE(!(regs->msr & MSR_EE));
>  again:
> -		if (IS_ENABLED(CONFIG_PREEMPT)) {
> +		if (is_preempt_full()) {
>  			/* Return to preemptible kernel context */
>  			if (unlikely(current_thread_info()->flags & _TIF_NEED_RESCHED)) {
>  				if (preempt_count() == 0)
> diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
> index aac8c0412ff9..1cb31bbdc925 100644
> --- a/arch/powerpc/kernel/traps.c
> +++ b/arch/powerpc/kernel/traps.c
> @@ -265,7 +265,7 @@ static int __die(const char *str, struct pt_regs *regs, long err)
>  	printk("%s PAGE_SIZE=%luK%s%s%s%s%s%s %s\n",
>  	       IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN) ? "LE" : "BE",
>  	       PAGE_SIZE / 1024, get_mmu_str(),
> -	       IS_ENABLED(CONFIG_PREEMPT) ? " PREEMPT" : "",
> +	       is_preempt_full() ? " PREEMPT" : "",
>  	       IS_ENABLED(CONFIG_SMP) ? " SMP" : "",
>  	       IS_ENABLED(CONFIG_SMP) ? (" NR_CPUS=" __stringify(NR_CPUS)) : "",
>  	       debug_pagealloc_enabled() ? " DEBUG_PAGEALLOC" : "",
> -- 
> 2.25.1