linux-kernel - Re: [PATCH] tracing: Fix lockdep warning in global

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Date:	Tue, 22 Dec 2009 13:17:52 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	rostedt@...dmis.org
CC:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: Fix lockdep warning in global_clock()

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.

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