linux-kernel - Re: [PATCH v2 19/24] xtensa/cpu: Make sure cpu

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Message-ID: <20230214182322.r5tyeowxzloiuh72@treble>
Date:   Tue, 14 Feb 2023 10:23:22 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Philippe Mathieu-Daudé <philmd@...aro.org>
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Subject: Re: [PATCH v2 19/24] xtensa/cpu: Make sure cpu_die() doesn't return

On Tue, Feb 14, 2023 at 08:55:32AM +0100, Philippe Mathieu-Daudé wrote:
> Hi Josh,
> 
> On 14/2/23 08:05, Josh Poimboeuf wrote:
> > cpu_die() doesn't return.  Make that more explicit with a BUG().
> > 
> > BUG() is preferable to unreachable() because BUG() is a more explicit
> > failure mode and avoids undefined behavior like falling off the edge of
> > the function into whatever code happens to be next.
> > 
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> > ---
> >   arch/xtensa/kernel/smp.c | 2 ++
> >   1 file changed, 2 insertions(+)
> > 
> > diff --git a/arch/xtensa/kernel/smp.c b/arch/xtensa/kernel/smp.c
> > index 4dc109dd6214..7bad78495536 100644
> > --- a/arch/xtensa/kernel/smp.c
> > +++ b/arch/xtensa/kernel/smp.c
> 
> Can you update the documentation along? Currently we have:
> 
>   /*
>    * Called from the idle thread for the CPU which has been shutdown.
>    *
>    * Note that we disable IRQs here, but do not re-enable them
>    * before returning to the caller. This is also the behaviour
>    * of the other hotplug-cpu capable cores, so presumably coming
>    * out of idle fixes this.
>    */

void __ref cpu_die(void)
{
	idle_task_exit();
	local_irq_disable();
	__asm__ __volatile__(
			"	movi	a2, cpu_restart\n"
			"	jx	a2\n");

	BUG();
}

Hm, not only is the comment wrong, but it seems to be branching to
cpu_restart?  That doesn't seem right at all.

Max/Chris?

-- 
Josh