Re: [PATCH v3] 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. Issuing
> directly leaves us without any pt_regs to pass to nmi_cpu_backtrace()
> so we also modify the generic code to call dump_stack() when its
> argument is NULL.
> 
> Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> ---

Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>
> 
> Notes:
>     Changes in v3:
> 
>     * Added comments to describe how raise_nmi() and nmi_cpu_backtrace()
>       interact with backtrace_mask (Russell King).
> 
>     Changes in v2:
> 
>     * Improved commit message to better describe the changes to the generic
>       code (Hillf Danton).
> 
>  arch/arm/kernel/smp.c |  9 +++++++++
>  lib/nmi_backtrace.c   | 11 ++++++++++-
>  2 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 48185a773852..0c4e7fdb9636 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -748,6 +748,15 @@ 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. Note
> +	 * that nmi_cpu_backtrace() automatically removes the current cpu
> +	 * from mask.
> +	 */
> +	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..6019c53c669e 100644
> --- a/lib/nmi_backtrace.c
> +++ b/lib/nmi_backtrace.c
> @@ -43,6 +43,12 @@ static void print_seq_line(struct nmi_seq_buf *s, int start, int end)
>  	printk("%.*s", (end - start) + 1, buf);
>  }
> 
> +/*
> + * When raise() is called it will be is passed a pointer to the
> + * backtrace_mask. Architectures that call nmi_cpu_backtrace()
> + * directly from their raise() functions may rely on the mask
> + * they are passed being updated as a side effect of this call.
> + */
>  void nmi_trigger_all_cpu_backtrace(bool include_self,
>  				   void (*raise)(cpumask_t *mask))
>  {
> @@ -149,7 +155,10 @@ 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();
>  		this_cpu_write(printk_func, printk_func_save);
> 
>  		cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
> --
> 2.4.3

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