linux-kernel - Re: [PATCH v2 04/24] arm64/cpu: Mark cpu_die() _

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Date:   Wed, 15 Feb 2023 13:09:21 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Philippe Mathieu-Daudé <philmd@...aro.org>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v2 04/24] arm64/cpu: Mark cpu_die() __noreturn

On Tue, Feb 14, 2023 at 09:13:08AM +0100, Philippe Mathieu-Daudé wrote:
> On 14/2/23 08:05, Josh Poimboeuf wrote:
> > cpu_die() doesn't return.  Annotate it as such.  By extension this also
> > makes arch_cpu_idle_dead() noreturn.
> > 
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> > ---
> >   arch/arm64/include/asm/smp.h | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
> > index fc55f5a57a06..5733a31bab08 100644
> > --- a/arch/arm64/include/asm/smp.h
> > +++ b/arch/arm64/include/asm/smp.h
> > @@ -100,7 +100,7 @@ static inline void arch_send_wakeup_ipi_mask(const struct cpumask *mask)
> >   extern int __cpu_disable(void);
> >   extern void __cpu_die(unsigned int cpu);
> > -extern void cpu_die(void);
> > +extern void __noreturn cpu_die(void);
> >   extern void cpu_die_early(void);
> 
> Shouldn't cpu_operations::cpu_die() be declared noreturn first?

The cpu_die() function ends with a BUG(), and so does not return, even if a
cpu_operations::cpu_die() function that it calls erroneously returned.

We *could* mark cpu_operations::cpu_die() as noreturn, but I'd prefer that we
did not so that the compiler doesn't optimize away the BUG() which is there to
catch such erroneous returns.

That said, could we please add __noreturn to the implementation of cpu_die() in
arch/arm64/kernel/smp.c? i.e. the fixup below.

With that fixup:

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.

---->8----
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index ffc5d76cf695..a98a76f7c1c6 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -361,7 +361,7 @@ void __cpu_die(unsigned int cpu)
  * Called from the idle thread for the CPU which has been shutdown.
  *
  */
-void cpu_die(void)
+void __noreturn cpu_die(void)
 {
        unsigned int cpu = smp_processor_id();
        const struct cpu_operations *ops = get_cpu_ops(cpu);