Commit 0495e337 authored by Waiman Long's avatar Waiman Long Committed by Vlastimil Babka
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mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding... 


mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock

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. There will be a slight delay
in the deletion of sysfs files if kmem_cache_release() is called
indirectly from a work function.

Fixes: 5a836bf6 ("mm: slub: move flush_cpu_slab() invocations __free_slab() invocations out of IRQ context")
Signed-off-by: default avatarWaiman Long <longman@redhat.com>
Reviewed-by: default avatarHyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: default avatarRoman Gushchin <roman.gushchin@linux.dev>
Acked-by: default avatarDavid Rientjes <rientjes@google.com>
Link: https://lore.kernel.org/all/YwOImVd+nRUsSAga@hyeyoo/


Signed-off-by: default avatarVlastimil Babka <vbabka@suse.cz>
parent 1c23f9e6

mm/slab_common.c

+29 −16
Original line number Diff line number Diff line
@@ -392,6 +392,28 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align, 
}

EXPORT_SYMBOL(kmem_cache_create);



#ifdef SLAB_SUPPORTS_SYSFS

/*

 * 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.

 *

 * Note that there will be a slight delay in the deletion of sysfs files

 * if kmem_cache_release() is called indrectly from a work function.

 */

static void kmem_cache_release(struct kmem_cache *s)

{

	sysfs_slab_unlink(s);

	sysfs_slab_release(s);

}

#else

static void kmem_cache_release(struct kmem_cache *s)

{

	slab_kmem_cache_release(s);

}

#endif



static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)

{

	LIST_HEAD(to_destroy);
@@ -418,11 +440,7 @@ 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_release(s);

	}

}
@@ -437,20 +455,11 @@ 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 +474,16 @@ 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 +492,8 @@ void kmem_cache_destroy(struct kmem_cache *s) 
out_unlock:

	mutex_unlock(&slab_mutex);

	cpus_read_unlock();

	if (!refcnt && !(s->flags & SLAB_TYPESAFE_BY_RCU))

		kmem_cache_release(s);

}

EXPORT_SYMBOL(kmem_cache_destroy);