slqb: fix deadlock

Fix the lockdep error reported by Fengguang where down_read slqb_lock
is taken twice when reading /proc/slabinfo, with the possibility for a
deadlock.
Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Tested-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

mm/slqb.c

@@ -3079,7 +3079,9 @@ static void __gather_stats(void *arg)
 	spin_unlock(&gather->lock);
 }
 
-static void gather_stats(struct kmem_cache *s, struct stats_gather *stats)
+/* must be called with slqb_lock held */
+static void gather_stats_locked(struct kmem_cache *s,
+				struct stats_gather *stats)
 {
 #ifdef CONFIG_NUMA
 	int node;
@@ -3089,8 +3091,6 @@ static void gather_stats(struct kmem_cache *s, struct stats_gather *stats)
 	stats->s = s;
 	spin_lock_init(&stats->lock);
 
-	down_read(&slqb_lock); /* hold off hotplug */
-
 	on_each_cpu(__gather_stats, stats, 1);
 
 #ifdef CONFIG_NUMA
@@ -3119,10 +3119,15 @@ static void gather_stats(struct kmem_cache *s, struct stats_gather *stats)
 	}
 #endif
 
-	up_read(&slqb_lock);
-
 	stats->nr_objects = stats->nr_slabs * s->objects;
 }
+
+static void gather_stats(struct kmem_cache *s, struct stats_gather *stats)
+{
+	down_read(&slqb_lock); /* hold off hotplug */
+	gather_stats_locked(s, stats);
+	up_read(&slqb_lock);
+}
 #endif
 
 /*
@@ -3175,7 +3180,7 @@ static int s_show(struct seq_file *m, void *p)
 
 	s = list_entry(p, struct kmem_cache, list);
 
-	gather_stats(s, &stats);
+	gather_stats_locked(s, &stats);
 
 	seq_printf(m, "%-17s %6lu %6lu %6u %4u %4d", s->name, stats.nr_inuse,
 			stats.nr_objects, s->size, s->objects, (1 << s->order));