[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.