On 8/10/22 14:10, Roman Gushchin wrote:
On Wed, Aug 10, 2022 at 12:49:46PM -0400, Waiman Long wrote:
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);
Hi Waiman!
This version is much more readable, thank you!
But can we, please, leave this schedule_work(&slab_caches_to_rcu_destroy_work)
call here? I don't see a good reason to move it, do I miss something?
It's nice to have list_add_tail() and schedule_work() calls nearby, so
it's obvious we can't miss the latter.
The reason that I need to move out schedule_work() as well is to make
sure that sysfs_slab_unlink() is called before sysfs_slab_release(). I
can't guarantee that if I do schedule_work() first. On the other hand,
moving sysfs_slab_unlink() into kmem_cache_workfn_release() introduces
unknown delay of when the sysfs file will be removed. I can add some
comment to make it more clear.
Please let me know if you have a better idea of dealing with this issue.
Thanks,
Longman
