linux-kernel - Re: [breakage] panic() does not halt arm64 systems under certain conditions

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Message-ID: <20190930135306.p5r4sy2bbmq5zxgm@willie-the-truck>
Date:   Mon, 30 Sep 2019 14:53:07 +0100
From:   Will Deacon <will@...nel.org>
To:     Jookia <166291@...il.com>
Cc:     Xogium <contact@...ium.me>, linux-arch@...r.kernel.org,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        linux@...linux.org.uk, mingo@...hat.com, bp@...en8.de,
        tglx@...utronix.de, linux-arm-kernel@...ts.infradead.org
Subject: Re: [breakage] panic() does not halt arm64 systems under certain
 conditions

On Fri, Sep 20, 2019 at 02:25:01PM +1000, Jookia wrote:
> On Tue, Sep 17, 2019 at 11:45:19AM +0100, Will Deacon wrote:
> > A straightforward fix is to disable preemption explicitly on the panic()
> > path (diff below), but I've expanded the cc list to see both what others
> > think, but also in case smp_send_stop() is supposed to have the side-effect
> > of disabling interrupt delivery for the local CPU.
> > 
> > diff --git a/kernel/panic.c b/kernel/panic.c
> > index 057540b6eee9..02d0de31c42d 100644
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -179,6 +179,7 @@ void panic(const char *fmt, ...)
> > 	 * after setting panic_cpu) from invoking panic() again.
> > 	 */
> > 	local_irq_disable();
> > +	preempt_disable_notrace();
> >  
> > 	/*
> > 	 * It's possible to come here directly from a panic-assertion and
> > 
> When you run with panic=... it will send you to a loop earlier in the
> panic code before local_irq_disable() is hit, working around the bug.
> A patch like this would make the behaviour the same:
> 
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 4d9f55bf7d38..92abbb5f8d38 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -331,7 +331,6 @@ void panic(const char *fmt, ...)
> 
>         /* Do not scroll important messages printed above */
>         suppress_printk = 1;
> -       local_irq_enable();
>         for (i = 0; ; i += PANIC_TIMER_STEP) {
>                 touch_softlockup_watchdog();
>                 if (i >= i_next) {

The reason I kept irqs enabled is because I figured they might be useful
for magic sysrq keyboard interrupts (e.g. if you wanted to reboot the box).

With 'panic=', the reboot happens automatically, so there's no issue there
afaict.

Will