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

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Date:   Mon, 6 Feb 2023 11:11:53 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     linux-kernel@...r.kernel.org, jgross@...e.com,
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Subject: Re: [PATCH 05/22] csky/cpu: Make sure arch_cpu_idle_dead() doesn't return

On Sat, Feb 4, 2023 at 10:29 AM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> 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.
Thx for the clarification.

Acked-by: Guo Ren <guoren@...nel.org>

>
> --
> Josh



-- 
Best Regards
 Guo Ren