Re: [PATCH RESEND] arm64: arch_timer: fix the infinite recursion when enable ftrace and erratum workaround

[Ding Tianhong]

On 2017/7/10 19:22, Mark Rutland wrote:
> On Sun, Jul 09, 2017 at 04:30:54PM +0800, Ding Tianhong wrote:
>> When enable preempt and debug ftrace, and perform the following steps, the
>> system will hang:
>> mount -t debugfs nodev /sys/kernel/debug/
>> cd /sys/kernel/debug/tracing/
>> echo function_graph > current_tracer
>>
>> This is because tracing the preempt_disable/enable calls would cause
>> trace_clock() which would get local timer to go into infinite recursion
>> when enable the arch timer erratum workaround for some chips, so Prevent
>> tracing of preempt_disable/enable() in arch_timer_reg_read_stable().
>>
>> This problem is similar to the fixed by upstream commit 96b3d28bf4
>> ("sched/clock: Prevent tracing recursion in sched_clock_cpu()").
> 
> As I mentioned before, the patch itself looks fine to me, but the commit
> message is somewhat difficult to read.
> 
> Can we please change this to:
> 
>   arm64: arch_timer: avoid infinite recursion when ftrace is enabled
> 
>   On platforms with an arch timer erratum workaround, it's possible for
>   arch_timer_reg_read_stable() to recurse into itself when certain
>   tracing options are enabled, leading to stack overflows and related
>   problems.
>  
>   For example, when PREEMPT_TRACER and FUNCTION_GRAPH_TRACER are
>   selected, it's possible to trigger this with:
>   
>   $ mount -t debugfs nodev /sys/kernel/debug/
>   $ echo function_graph > /sys/kernel/debug/tracing/current_tracer
> 
>   The problem is that in such cases, preempt_disable() instrumentation
>   attempts to acquire a timestamp via trace_clock(), resulting in a call
>   back to arch_timer_reg_read_stable(), and hence recursion.
> 
>   This patch changes arch_timer_reg_read_stable() to use
>   preempt_{disable,enable}_notrace(), which avoids this.
> 
> With that commit message:
> 
> Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
> 
> Daniel, Thomas, would you be happy to fold that in when picking this? Or
> would you prefer that I fix this up and resend?
> 

Hi Danial, Thomas:

It looks didn't merge to the mainline tree yet, should I update the commit and
resend this patch again?

Thanks
Ding

> Thanks,
> Mark.
> 
>> Fixes: 6acc71ccac71 ("arm64: arch_timer: Allows a CPU-specific erratum to only affect a subset of CPUs")
>> Signed-off-by: Ding Tianhong <dingtianhong@xxxxxxxxxx>
>> ---
>>  arch/arm64/include/asm/arch_timer.h | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
>> index 74d08e4..67bb7a4 100644
>> --- a/arch/arm64/include/asm/arch_timer.h
>> +++ b/arch/arm64/include/asm/arch_timer.h
>> @@ -65,13 +65,13 @@ struct arch_timer_erratum_workaround {
>>  	u64 _val;							\
>>  	if (needs_unstable_timer_counter_workaround()) {		\
>>  		const struct arch_timer_erratum_workaround *wa;		\
>> -		preempt_disable();					\
>> +		preempt_disable_notrace();				\
>>  		wa = __this_cpu_read(timer_unstable_counter_workaround); \
>>  		if (wa && wa->read_##reg)				\
>>  			_val = wa->read_##reg();			\
>>  		else							\
>>  			_val = read_sysreg(reg);			\
>> -		preempt_enable();					\
>> +		preempt_enable_notrace();				\
>>  	} else {							\
>>  		_val = read_sysreg(reg);				\
>>  	}								\
>> -- 
>> 1.9.0
>>
>>
> 
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