linux-kernel - Re: [PATCH 05/22] csky/cpu: Make sure arch_cpu_idle

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Message-ID: <20230204022932.k24laszjs3v4bc3v@treble>
Date:   Fri, 3 Feb 2023 18:29:32 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Guo Ren <guoren@...nel.org>
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Subject: Re: [PATCH 05/22] csky/cpu: Make sure arch_cpu_idle_dead() doesn't
 return

On Sat, Feb 04, 2023 at 09:12:31AM +0800, Guo Ren wrote:
> On Sat, Feb 4, 2023 at 6:05 AM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> >
> > arch_cpu_idle_dead() 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/csky/kernel/smp.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/arch/csky/kernel/smp.c b/arch/csky/kernel/smp.c
> > index b45d1073307f..0ec20efaf5fd 100644
> > --- a/arch/csky/kernel/smp.c
> > +++ b/arch/csky/kernel/smp.c
> > @@ -317,5 +317,7 @@ void arch_cpu_idle_dead(void)
> >                 "jmpi   csky_start_secondary"
> >                 :
> >                 : "r" (secondary_stack));
> > +
> > +       BUG();
> Why not:
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index f26ab2675f7d..1d3bf903add2 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -285,6 +285,7 @@ static void do_idle(void)
>                         tick_nohz_idle_stop_tick();
>                         cpuhp_report_idle_dead();
>                         arch_cpu_idle_dead();
> +                       BUG();

Without the BUG() in csky arch_cpu_idle_dead(), the compiler will warn
about arch_cpu_idle_dead() returning, because it's marked __noreturn but
doesn't clearly return (as far as the compiler knows).

And we want it marked __noreturn so we'll be more likely to catch such
bugs at build time.

And as a bonus we get better code generation and clearer code semantics
which helps both humans and tooling understand the intent of the code.

-- 
Josh