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

[Hillf Danton]

> 
> 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.

>  		this_cpu_write(printk_func, printk_func_save);
> 
>  		cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
> --
> 2.4.3