Re: [PATCH] locking/lockdep: Fix expected depth value in __lock_release()

[Jerome Glisse]

On Tue, Oct 20, 2015 at 02:18:53PM +0200, Peter Zijlstra wrote:
> On Mon, Oct 19, 2015 at 03:30:28PM -0400, j.glisse@xxxxxxxxx wrote:
> > From: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > 
> > In __lock_release() we are removing one entry from the stack and
> > rebuilding the hash chain by re-adding entry above the entry we
> > just removed. If the entry removed was between 2 entry of same
> > class then this 2 entry might be coalesced into one single entry
> > which in turns means that the lockdep_depth value will not be
> > incremented and thus the expected lockdep_depth value after this
> > operation will be wrong triggering an unjustified WARN_ONCE() at
> > the end of __lock_release().
> 
> This is the nest_lock stuff, right? Where it checks:
> 
>   if (hlock->class_idx == class_idx && nest_lock) {
> 	...
> 	return 1;
>   }

Yes this code.

> 
> What code did you find that triggered this? That is, what code is taking
> nested locks with other locks in the middle? (Not wrong per-se, just
> curious how that would come about).

Well i am not able to reproduce myself but it happens as part of
mm_drop_all_locks() as to which lock does trigger i am unsure as
all the i_mmap_rwsem are taken one after the other and same for
anon_vma rwsem so they should already coalesce properly. My guess
is that code calling all lock also have a mutex and once all vma
lock are drop the mutex coalesce with mm_all_locks_mutex.

> 
> > This patch adjust the expect depth value by decrementing it if
> > what was previously 2 entry inside the stack are coalesced into
> > only one entry.
> 
> Would it not make more sense to scan the entire hlock stack on
> __lock_acquire() and avoid getting collapsible entries in the first
> place?
> 
> Something like so...

It would work too, probably more compute intensive than my solution
but this is lockdep code so i guess it is fine. Also dunno if we loose
any valuable information by not keeping the stack ordered so one
can check order in whick lock are taken.

> 
> ---
>  kernel/locking/lockdep.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 4e49cc4c9952..6fcd98b86e7b 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -3125,15 +3125,21 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>  
>  	class_idx = class - lock_classes + 1;
>  
> -	if (depth) {
> -		hlock = curr->held_locks + depth - 1;
> -		if (hlock->class_idx == class_idx && nest_lock) {
> -			if (hlock->references)
> -				hlock->references++;
> -			else
> -				hlock->references = 2;
> +	if (depth && nest_lock) {
> +		int i;
>  
> -			return 1;
> +		for (i = depth; i; --i) {
> +			hlock = curr->held_locks + i - 1;
> +			if (hlock->class_idx == class_idx &&
> +			    hlock->nest_lock == nest_lock) {
> +
> +				if (hlock->references)
> +					hlock->references++;
> +				else
> +					hlock->references = 2;
> +
> +				return 1;
> +			}
>  		}
>  	}
>  
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