Subject: Re: [PATCH v5 5/5] arm64: ipi_nmi: Add support for NMI backtrace

From: Marc Zyngier <[email-masked]>
Date: Mon, 19 Oct 2020 13:20:41 +0100

On 2020-10-14 12:12, Sumit Garg wrote:
> Enable NMI backtrace support on arm64 using IPI turned as an NMI
> leveraging pseudo NMIs support. It is now possible for users to get a
> backtrace of a CPU stuck in hard-lockup using magic SYSRQ.
> 
> Signed-off-by: Sumit Garg <[email-masked]>
> ---
>  arch/arm64/include/asm/irq.h |  6 ++++++
>  arch/arm64/kernel/ipi_nmi.c  | 12 +++++++++++-
>  2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/irq.h 
> b/arch/arm64/include/asm/irq.h
> index b2b0c64..e840bf1 100644
> --- a/arch/arm64/include/asm/irq.h
> +++ b/arch/arm64/include/asm/irq.h
> @@ -6,6 +6,12 @@
> 
>  #include <asm-generic/irq.h>
> 
> +#ifdef CONFIG_SMP
> +extern void arch_trigger_cpumask_backtrace(const cpumask_t *mask,
> +					   bool exclude_self);
> +#define arch_trigger_cpumask_backtrace arch_trigger_cpumask_backtrace
> +#endif
> +
>  struct pt_regs;
> 
>  static inline int nr_legacy_irqs(void)
> diff --git a/arch/arm64/kernel/ipi_nmi.c b/arch/arm64/kernel/ipi_nmi.c
> index e0a9e03..e1dc19d 100644
> --- a/arch/arm64/kernel/ipi_nmi.c
> +++ b/arch/arm64/kernel/ipi_nmi.c
> @@ -9,6 +9,7 @@
>  #include <linux/interrupt.h>
>  #include <linux/irq.h>
>  #include <linux/kgdb.h>
> +#include <linux/nmi.h>
>  #include <linux/smp.h>
> 
>  #include <asm/nmi.h>
> @@ -25,12 +26,21 @@ void arch_send_call_nmi_func_ipi_mask(cpumask_t 
> *mask)
>  	__ipi_send_mask(ipi_desc, mask);
>  }
> 
> +void arch_trigger_cpumask_backtrace(const cpumask_t *mask, bool 
> exclude_self)
> +{
> +	nmi_trigger_cpumask_backtrace(mask, exclude_self,
> +				      arch_send_call_nmi_func_ipi_mask);
> +}
> +
>  static irqreturn_t ipi_nmi_handler(int irq, void *data)
>  {
>  	unsigned int cpu = smp_processor_id();
> 
> -	ipi_kgdb_nmicallback(cpu, get_irq_regs());
> +	if (nmi_cpu_backtrace(get_irq_regs()))
> +		goto out;
> 
> +	ipi_kgdb_nmicallback(cpu, get_irq_regs());
> +out:
>  	return IRQ_HANDLED;
>  }

Can't you have *both* a request for a backtrace and a KGDB call?
It really shouldn't be either/or. It also outlines how well shared
interrupts work with edge triggered signalling...

         M.
-- 
Jazz is not dead. It just smells funny...

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