[PATCH v2] mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock

[Waiman Long <longman@xxxxxxxxxx>]

A circular locking problem is reported by lockdep due to the following
circular locking dependency.

  +--> cpu_hotplug_lock --> slab_mutex --> kn->active --+
  |                                                     |
  +-----------------------------------------------------+

The forward cpu_hotplug_lock ==> slab_mutex ==> kn->active dependency
happens in

  kmem_cache_destroy():	cpus_read_lock(); mutex_lock(&slab_mutex);
  ==> sysfs_slab_unlink()
      ==> kobject_del()
          ==> kernfs_remove()
	      ==> __kernfs_remove()
	          ==> kernfs_drain(): rwsem_acquire(&kn->dep_map, ...);

The backward kn->active ==> cpu_hotplug_lock dependency happens in

  kernfs_fop_write_iter(): kernfs_get_active();
  ==> slab_attr_store()
      ==> cpu_partial_store()
          ==> flush_all(): cpus_read_lock()

One way to break this circular locking chain is to avoid holding
cpu_hotplug_lock and slab_mutex while deleting the kobject in
sysfs_slab_unlink() which should be equivalent to doing a write_lock
and write_unlock pair of the kn->active virtual lock.

Since the kobject structures are not protected by slab_mutex or the
cpu_hotplug_lock, we can certainly release those locks before doing
the delete operation.

Move sysfs_slab_unlink() and sysfs_slab_release() to the newly
created kmem_cache_release() and call it outside the slab_mutex &
cpu_hotplug_lock critical sections.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 [v2] Break kmem_cache_release() helper into 2 separate ones.

 mm/slab_common.c | 54 +++++++++++++++++++++++++++++++++---------------
 1 file changed, 37 insertions(+), 17 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 17996649cfe3..7742d0446d8b 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -392,6 +392,36 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
 }
 EXPORT_SYMBOL(kmem_cache_create);
 
+#ifdef SLAB_SUPPORTS_SYSFS
+static void kmem_cache_workfn_release(struct kmem_cache *s)
+{
+	sysfs_slab_release(s);
+}
+#else
+static void kmem_cache_workfn_release(struct kmem_cache *s)
+{
+	slab_kmem_cache_release(s);
+}
+#endif
+
+/*
+ * For a given kmem_cache, kmem_cache_destroy() should only be called
+ * once or there will be a use-after-free problem. The actual deletion
+ * and release of the kobject does not need slab_mutex or cpu_hotplug_lock
+ * protection. So they are now done without holding those locks.
+ */
+static void kmem_cache_release(struct kmem_cache *s)
+{
+#ifdef SLAB_SUPPORTS_SYSFS
+	sysfs_slab_unlink(s);
+#endif
+
+	if (s->flags & SLAB_TYPESAFE_BY_RCU)
+		schedule_work(&slab_caches_to_rcu_destroy_work);
+	else
+		kmem_cache_workfn_release(s);
+}
+
 static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
 {
 	LIST_HEAD(to_destroy);
@@ -418,11 +448,7 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
 	list_for_each_entry_safe(s, s2, &to_destroy, list) {
 		debugfs_slab_release(s);
 		kfence_shutdown_cache(s);
-#ifdef SLAB_SUPPORTS_SYSFS
-		sysfs_slab_release(s);
-#else
-		slab_kmem_cache_release(s);
-#endif
+		kmem_cache_workfn_release(s);
 	}
 }
 
@@ -437,20 +463,10 @@ static int shutdown_cache(struct kmem_cache *s)
 	list_del(&s->list);
 
 	if (s->flags & SLAB_TYPESAFE_BY_RCU) {
-#ifdef SLAB_SUPPORTS_SYSFS
-		sysfs_slab_unlink(s);
-#endif
 		list_add_tail(&s->list, &slab_caches_to_rcu_destroy);
-		schedule_work(&slab_caches_to_rcu_destroy_work);
 	} else {
 		kfence_shutdown_cache(s);
 		debugfs_slab_release(s);
-#ifdef SLAB_SUPPORTS_SYSFS
-		sysfs_slab_unlink(s);
-		sysfs_slab_release(s);
-#else
-		slab_kmem_cache_release(s);
-#endif
 	}
 
 	return 0;
@@ -465,14 +481,16 @@ void slab_kmem_cache_release(struct kmem_cache *s)
 
 void kmem_cache_destroy(struct kmem_cache *s)
 {
+	int refcnt;
+
 	if (unlikely(!s) || !kasan_check_byte(s))
 		return;
 
 	cpus_read_lock();
 	mutex_lock(&slab_mutex);
 
-	s->refcount--;
-	if (s->refcount)
+	refcnt = --s->refcount;
+	if (refcnt)
 		goto out_unlock;
 
 	WARN(shutdown_cache(s),
@@ -481,6 +499,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
 out_unlock:
 	mutex_unlock(&slab_mutex);
 	cpus_read_unlock();
+	if (!refcnt)
+		kmem_cache_release(s);
 }
 EXPORT_SYMBOL(kmem_cache_destroy);
 
-- 
2.31.1