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

Li Zefan
2009-12-21T07:36:58Z

Ingo Molnar wrote:
> * Li Zefan <lizf@cn.fujitsu.com> wrote:
>
>> # echo 1 > events/enable
>> # echo global > trace_clock
>>
>> ------------[ cut here ]------------
>> WARNING: at kernel/lockdep.c:3162 check_flags+0xb2/0x190()
>> ...
>> ---[ end trace 3f86734a89416623 ]---
>> possible reason: unannotated irqs-on.
>> ...
>>
>> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
>> ---
>> kernel/trace/trace_clock.c | 4 ++--
>> 1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/trace/trace_clock.c b/kernel/trace/trace_clock.c
>> index 84a3a7b..11563c9 100644
>> --- a/kernel/trace/trace_clock.c
>> +++ b/kernel/trace/trace_clock.c
>> @@ -83,7 +83,7 @@ u64 notrace trace_clock_global(void)
>> int this_cpu;
>> u64 now;
>>
>> - raw_local_irq_save(flags);
>> + local_irq_save(flags);
>
> Hm, wont this cause problems when we trace inside lockdep? Have you tried the
> lockdep events - do they still work?
>

Yes, they still work.

trace_clock_global() calls cpu_clock() which calls local_irq_save(),
which causes this lockdep warning.

And I noticed this commit:

===============================================
commit 2d452c9b10caeec455eb5e56a0ef4ed485178213
Author: Ingo Molnar <mingo@elte.hu>
Date: Sun Jun 29 15:01:59 2008 +0200

sched: sched_clock_cpu() based cpu_clock(), lockdep fix

Vegard Nossum reported:

> WARNING: at kernel/lockdep.c:2738 check_flags+0x142/0x160()

which happens due to:

unsigned long long cpu_clock(int cpu)
{
unsigned long long clock;
unsigned long flags;

raw_local_irq_save(flags);

as lower level functions can take locks, we must not do that, use
proper lockdep-annotated irq save/restore.

Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index ed5a8c4..60094e2 100644
--- a/kernel/sched_clock.c
+++ 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?