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

From: Marc Zyngier
Date: Mon Oct 19 2020 - 08:20:46 EST


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 <sumit.garg@xxxxxxxxxx>
---
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...