Re: [PATCH] arm: Fix backtrace generation when IPI is masked

[Daniel Thompson]

On 15/09/15 07:58, Hillf Danton wrote:
> > Currently on ARM when <SysRq-L> is triggered from an interrupt handler
> > (e.g. a SysRq issued using UART or kbd) the main CPU will wedge for ten
> > seconds with interrupts masked before issuing a backtrace for every CPU
> > except itself.
> >
> > The new backtrace code introduced by commit 96f0e00378d4 ("ARM: add
> > basic support for on-demand backtrace of other CPUs") does not work
> > correctly when run from an interrupt handler because IPI_CPU_BACKTRACE
> > is used to generate the backtrace on all CPUs but cannot preempt the
> > current calling context.
> >
> > This can be fixed by detecting that the calling context cannot be
> > preempted and issuing the backtrace directly in this case. Some small
> > changes to the generic code are required to support this.
> >
> > Signed-off-by: Daniel Thompson <daniel.thompson at linaro.org>
> > ---
> >   arch/arm/kernel/smp.c | 7 +++++++
> >   lib/nmi_backtrace.c   | 5 ++++-
> >   2 files changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> > index 48185a773852..4d8a80328c74 100644
> > --- a/arch/arm/kernel/smp.c
> > +++ b/arch/arm/kernel/smp.c
> > @@ -748,6 +748,13 @@ core_initcall(register_cpufreq_notifier);
> >
> >   static void raise_nmi(cpumask_t *mask)
> >   {
> > +	/*
> > +	 * Generate the backtrace directly if we are running in a
> > +	 * calling context that is not preemptible by the backtrace IPI.
> > +	 */
> > +	if (cpumask_test_cpu(smp_processor_id(), mask) && irqs_disabled())
> > +		nmi_cpu_backtrace(NULL);
> > +
> >   	smp_cross_call(mask, IPI_CPU_BACKTRACE);
> >   }
> >
> > diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c
> > index 88d3d32e5923..be0466a80d0b 100644
> > --- a/lib/nmi_backtrace.c
> > +++ b/lib/nmi_backtrace.c
> > @@ -149,7 +149,10 @@ bool nmi_cpu_backtrace(struct pt_regs *regs)
> >   		/* Replace printk to write into the NMI seq */
> >   		this_cpu_write(printk_func, nmi_vprintk);
> >   		pr_warn("NMI backtrace for cpu %d\n", cpu);
> > -		show_regs(regs);
> > +		if (regs)
> > +			show_regs(regs);
> > +		else
> > +			dump_stack();
>
> Better if dump_stack() is added in a separate patch, given that
> it is not mentioned in commit message.

Adding dump_stack() is mentioned in passing ("Some small changes to the 
generic code are required to support this.") but you're right that the 
reason for the change is not explicitly called out.

I can certainly respin as two patches but perhaps its better just to 
improve the commit message. Something like:

> This can be fixed by detecting that the calling context cannot be
> preempted and issuing the backtrace directly in this case. Issuing
> directly leaves us without any pt_regs to pass to nmi_cpu_backtrace().
> Modify the generic code to call dump_stack() when its argument is
> NULL.

Which do you prefer?


Daniel.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/