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

Yong Zhang
2009-12-22T06:16:18Z

[snip]
>> 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.
>

I think the reason is on local_irq_restore():

#define local_irq_restore(flags) \
do { \
typecheck(unsigned long, flags); \
if (raw_irqs_disabled_flags(flags)) { \
raw_local_irq_restore(flags); \
trace_hardirqs_off(); \ <======
} else { \
trace_hardirqs_on(); \
raw_local_irq_restore(flags); \
} \
} while (0)

If we have a call trace like below:
raw_local_irq_save();
...
local_irq_save();
...
local_irq_restore();
...
raw_locla_irq_save();

Then when we leave from local_irq_restore(), current->hardirqs_enabled will
changed by trace_hardirqs_off() to 0, this will break the all consistent.

Thanks,
Yong