From mboxrd@z Thu Jan 1 00:00:00 1970 From: Josh Poimboeuf Date: Sat, 04 Feb 2023 02:29:32 +0000 Subject: Re: [PATCH 05/22] csky/cpu: Make sure arch_cpu_idle_dead() doesn't return Message-Id: <20230204022932.k24laszjs3v4bc3v@treble> List-Id: References: In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Guo Ren Cc: linux-kernel@vger.kernel.org, jgross@suse.com, richard.henderson@linaro.org, ink@jurassic.park.msu.ru, mattst88@gmail.com, linux-alpha@vger.kernel.org, linux@armlinux.org.uk, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, will@kernel.org, linux-csky@vger.kernel.org, linux-ia64@vger.kernel.org, chenhuacai@kernel.org, kernel@xen0n.name, loongarch@lists.linux.dev, f.fainelli@gmail.com, bcm-kernel-feedback-list@broadcom.com, tsbogend@alpha.franken.de, linux-mips@vger.kernel.org, jiaxun.yang@flygoat.com, mpe@ellerman.id.au, npiggin@gmail.com, christophe.leroy@csgroup.eu, linuxppc-dev@lists.ozlabs.org, ysato@users.sourceforge.jp, dalias@libc.org, linux-sh@vger.kernel.org, davem@davemloft.net, sparclinux@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, chris@zankel.net, jcmvbkbc@gmail.com, linux-xtensa@linux-xtensa.org, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, paulmck@kernel.org On Sat, Feb 04, 2023 at 09:12:31AM +0800, Guo Ren wrote: > On Sat, Feb 4, 2023 at 6:05 AM Josh Poimboeuf 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 > > --- > > 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