Re: [debug patch] printk: Add a printk killswitch to robustify NMIwatchdog messages

[Arne Jansen]

On 06.06.2011 11:01, Peter Zijlstra wrote:
> On Sun, 2011-06-05 at 22:15 +0200, Arne Jansen wrote:
>>
>> Can lockdep just get confused by the lockdep_off/on calls in printk
>> while scheduling is allowed? There aren't many users of lockdep_off().
> 
> Yes!, in that case lock_is_held() returns false, triggering the warning.
> I guess there's an argument to be made in favour of the below..


Two questions... is there any protection between the lockdep_recursion
check and the set to one? I guess in our case it is, because it's the
scheduler that calls it, but in general?
And why is lockdep needed to check if a lock is help? Isn't it reflected
in the lock structure itself?

-Arne

> 
> ---
>  kernel/lockdep.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index 53a6895..e4129cf 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -3242,7 +3242,7 @@ int lock_is_held(struct lockdep_map *lock)
>  	int ret = 0;
>  
>  	if (unlikely(current->lockdep_recursion))
> -		return ret;
> +		return 1; /* avoid false negative lockdep_assert_held */
>  
>  	raw_local_irq_save(flags);
>  	check_flags(flags);
> 
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