Re: [PATCH] tracing: Fix lockdep warning in global_clock()

Li Zefan
2009-12-22T05:18:10Z

Steven Rostedt wrote:
> On Mon, 2009-12-21 at 15:40 +0100, Peter Zijlstra wrote:
>> On Mon, 2009-12-21 at 15:36 +0800, Li Zefan wrote:
>>
>>> +++ b/kernel/sched_clock.c
>>> @@ -250,9 +250,9 @@ unsigned long long cpu_clock(int cpu)
>>> unsigned long long clock;
>>> unsigned long flags;
>>>
>>> - raw_local_irq_save(flags);
>>> + local_irq_save(flags);
>>> clock = sched_clock_cpu(cpu);
>>> - raw_local_irq_restore(flags);
>>> + local_irq_restore(flags);
>>>
>>> return clock;
>>> }
>>> ===============================================
>>>
>>> I guess it's still true that lower level functions can take locks?
>> No, I removed the locks from cpu_clock a while back.
>>
>
> The problem isn't with locks, it's with disabling interrupts. Lockdep
> keeps track of every time interrupts are disabled. If something disables
> interrupts with raw_* but then calls something that disables interrupts
> the normal way, lockdep will detect that interrupts are being disabled
> but they already are disabled. The internal irq disabled variable in
> lockdep wont match reality, and this will blow up (well, disable
> lockdep).
>
> So if cpu_clock disables interrupts with local_irq_* then so must
> trace_clock_global.
>

Looking into sched_clock.c, seems the only place that can trigger lockdep
is the local_irq_* in cpu_clock(), though I'm not sure for architecture-
specific sched_clock().

If so, I think we can use raw_local_irq_* in cpu_clock(). Actually I
tried it and the warning is gone.