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

[Jookia <166291@gmail.com>]

On Tue, Sep 17, 2019 at 11:45:19AM +0100, Will Deacon wrote:
> Hi,
> 
> [Expanding CC list; original message is here:
>  https://lore.kernel.org/linux-arm-kernel/BX1W47JXPMR8.58IYW53H6M5N@dragonstone/]
> 
> On Mon, Sep 16, 2019 at 09:35:36PM -0400, Xogium wrote:
> > On arm64 in some situations userspace will continue running even after a
> > panic. This means any userspace watchdog daemon will continue pinging,
> > that service managers will keep running and displaying messages in certain
> > cases, and that it is possible to enter via ssh in the now unstable system
> > and to do almost anything except reboot/power off and etc. If
> > CONFIG_PREEMPT=n is set in the kernel's configuration, the issue is fixed.
> > I have reproduced the very same behavior with linux 4.19, 5.2 and 5.3. On
> > x86/x86_64 the issue does not seem to be present at all.
> 
> I've managed to reproduce this under both 32-bit and 64-bit ARM kernels.
> The issue is that the infinite loop at the end of panic() can run with
> preemption enabled (particularly when invoking by echoing 'c' to
> /proc/sysrq-trigger), so we end up rescheduling user tasks. On x86, this
> doesn't happen because smp_send_stop() disables the local APIC in
> native_stop_other_cpus() and so interrupts are effectively masked while
> spinning.
> 
> 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.
> 
> Will
> 
> --->8
> 
> 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
> 
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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) {

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