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

On 2020-10-20 10:13, Sumit Garg wrote:

On Mon, 19 Oct 2020 at 17:50, Marc Zyngier [off-list ref] wrote:

quoted

On 2020-10-14 12:12, Sumit Garg wrote:

quoted

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 <redacted>
---
 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...

Unfortunately, NMIs doesn't seem to support shared mode [1].

You are totally missing the point: shared interrupts *cannot* work
reliably with edge signalling. What I am saying here is that you need
implement the sharing yourself in this function.

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

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