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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Wed, 17 Mar 2021 13:55:04 +0100
Message-ID: <CAK8P3a2qqWV3RGvb4ooiz4LS5GAKL0OHEiVtdgnHAgtmiRDSNA@mail.gmail.com>
Subject: Re: [PATCH] sched: replace if (cond) BUG() with BUG_ON()
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Jiapeng Chong <jiapeng.chong@linux.alibaba.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Wed, Mar 17, 2021 at 9:45 AM Ingo Molnar <mingo@kernel.org> wrote:
> * Ingo Molnar <mingo@kernel.org> wrote:
> > * Jiapeng Chong <jiapeng.chong@linux.alibaba.com> wrote:
> >
> > > Fix the following coccicheck warnings:
> > >
> > > ./kernel/sched/core.c:8039:2-5: WARNING: Use BUG_ON instead of if
> > > condition followed by BUG.
> > >
> > > Reported-by: Abaci Robot <abaci@linux.alibaba.com>
> > > Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
> > > ---
> > >  kernel/sched/core.c | 3 +--
> > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 9819121..7392bc0 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -8035,8 +8035,7 @@ void __init sched_init_smp(void)
> > >     mutex_unlock(&sched_domains_mutex);
> > >
> > >     /* Move init over to a non-isolated CPU */
> > > -   if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0)
> > > -           BUG();
> > > +   BUG(set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0);
> >
> > The patch doesn't quite do what the title & changelog claims...
>
> More importantly, we use this pattern when we don't want !CONFIG_BUG
> to remove the 'condition'.
>
> I.e. the "side effect" here is important scheduler logic. It must
> never be optimized out.

This behavior for !CONFIG_BUG has changed a while ago, it is now safe
to rely on the side-effect of the BUG_ON() condition regardless of
CONFIG_BUG. When that option is disabled, running into the condition
just ends up in a "do {} while (1)" loop.

        Arnd