linux-kernel - Re: lockdep recursive locking detected (rcu_kthread / __cache

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Date:	Tue, 4 Oct 2011 09:28:54 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Sitsofe Wheeler <sitsofe@...oo.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	penberg@...nel.org, mpm@...enic.com, linux-mm@...ck.org
Subject: Re: lockdep recursive locking detected (rcu_kthread /
 __cache_free)

On Mon, 3 Oct 2011, Paul E. McKenney wrote:

> 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.

We could give the kmalloc array a different class from created slab
caches. That should have the desired effect.

But that seems to be already the case (looking at init_node_lock_keys).
Non OFF_SLAB caches seem to be getting a different lock class? Why is this
not working?

static void init_node_lock_keys(int q)
{
        struct cache_sizes *s = malloc_sizes;

        if (g_cpucache_up != FULL)
                return;

        for (s = malloc_sizes; s->cs_size != ULONG_MAX; s++) {
                struct kmem_list3 *l3;

                l3 = s->cs_cachep->nodelists[q];
                if (!l3 || OFF_SLAB(s->cs_cachep))
                        continue;

                slab_set_lock_classes(s->cs_cachep, &on_slab_l3_key,
                                &on_slab_alc_key, q);
        }
}




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