[patch for-3.14] mm, slub: list_lock may not be held in somecircumstances

[David Rientjes]

Commit c65c1877bd68 ("slub: use lockdep_assert_held") incorrectly required 
that add_full() and remove_full() hold n->list_lock.  The lock is only 
taken when kmem_cache_debug(s), since that's the only time it actually 
does anything.

Require that the lock only be taken under such a condition.

Reported-by: Larry Finger <Larry.Finger@xxxxxxxxxxxx>
Tested-by: Larry Finger <Larry.Finger@xxxxxxxxxxxx>
Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Acked-by: Christoph Lameter <cl@xxxxxxxxx>
Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
---
 mm/slub.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1004,21 +1004,19 @@ static inline void slab_free_hook(struct kmem_cache *s, void *x)
 static void add_full(struct kmem_cache *s,
 	struct kmem_cache_node *n, struct page *page)
 {
-	lockdep_assert_held(&n->list_lock);
-
 	if (!(s->flags & SLAB_STORE_USER))
 		return;
 
+	lockdep_assert_held(&n->list_lock);
 	list_add(&page->lru, &n->full);
 }
 
 static void remove_full(struct kmem_cache *s, struct kmem_cache_node *n, struct page *page)
 {
-	lockdep_assert_held(&n->list_lock);
-
 	if (!(s->flags & SLAB_STORE_USER))
 		return;
 
+	lockdep_assert_held(&n->list_lock);
 	list_del(&page->lru);
 }
 
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