Re: lockdep recursive locking detected (rcu_kthread / __cache_free)

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Oct 03, 2011 at 03:46:11PM -0500, Christoph Lameter wrote:
> On Mon, 3 Oct 2011, Paul E. McKenney wrote:
> 
> > The first lock was acquired here in an RCU callback.  The later lock that
> > lockdep complained about appears to have been acquired from a recursive
> > call to __cache_free(), with no help from RCU.  This looks to me like
> > one of the issues that arise from the slab allocator using itself to
> > allocate slab metadata.
> 
> Right. However, this is a false positive since the slab cache with
> the metadata is different from the slab caches with the slab data. The slab
> cache with the metadata does not use itself any metadata slab caches.

Wouldn't it be possible to pass a new flag to the metadata slab caches
upon creation so that their locks could be placed in a separate lock
class?  Just allocate a separate lock_class_key structure for each such
lock in that case, and then use lockdep_set_class_and_name to associate
that structure with the corresponding lock.  I do this in kernel/rcutree.c
in order to allow the rcu_node tree's locks to nest properly.

							Thanx, Paul