Re: [PATCH] lockdep: make print_lock_name() robust against non-existing lock_class

[Sebastian Andrzej Siewior]

On 04/15/2015 04:14 PM, Peter Zijlstra wrote:
> On Wed, Apr 15, 2015 at 03:24:36PM +0200, Sebastian Andrzej Siewior wrote:
>> During sysrq's show-held-locks command it is possible that hlock_class()
>> returns NULL for a given lock. The result is then (after the warning):
>>
>> |BUG: unable to handle kernel NULL pointer dereference at 0000001c
>> |IP: [<c1088145>] get_usage_chars+0x5/0x100
>> |Call Trace:
>> | [<c1088263>] print_lock_name+0x23/0x60
>> | [<c1576b57>] print_lock+0x5d/0x7e
>> | [<c1088314>] lockdep_print_held_locks+0x74/0xe0
>> | [<c1088652>] debug_show_all_locks+0x132/0x1b0
>> | [<c1315c48>] sysrq_handle_showlocks+0x8/0x10
>>
>> This *might* happen because the thread on the other CPU drops the lock
>> after we are looking ->lockdep_depth and ->held_locks points no longer
>> to a lock that is held.
>> The fix here is to simply ignore it and continue.
> 
> Hmm, but in that case we might equally run into the hlock_class() debug
> check which would kill all of lockdep.
> 
> Note that lock_release_nested() with CONFIG_DEBUG_LOCKDEP will actually
> clear those fields.
> 
> Would something like the below work for you?

Andreas confirmed that it works for him on v3.18 with minor adjustment.

<---
+       struct held_lock lock = READ_ONCE(*hlock);
+       unsigned int class_idx = lock.class_idx;
--->

So, yes, thanks.

> ---
>  kernel/locking/lockdep.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index ba77ab5f64dd..0ef89f830ff4 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -551,7 +551,18 @@ static void print_lockdep_cache(struct lockdep_map *lock)
>  
>  static void print_lock(struct held_lock *hlock)
>  {
> -	print_lock_name(hlock_class(hlock));
> +	/*
> +	 * We can be called locklessly through debug_show_all_locks() so be
> +	 * extra careful, the hlock might have been released and cleared.
> +	 */
> +	unsigned int class_idx = READ_ONCE(hlock->class_idx);
> +
> +	if (!class_idx || (class_idx - 1) >= MAX_LOCKDEP_KEYS) {
> +		printk("<RELEASED>\n");
> +		return;
> +	}
> +
> +	print_lock_name(lock_classes + class_idx - 1);
>  	printk(", at: ");
>  	print_ip_sym(hlock->acquire_ip);
>  }
> 
Sebastian
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