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

[Marc Zyngier]

On 2020-10-20 10:13, Sumit Garg wrote:
> On Mon, 19 Oct 2020 at 17:50, Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > 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...
>
> 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...