On 8/9/22 18:25, Roman Gushchin wrote:
On Tue, Aug 09, 2022 at 04:59:01PM -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#126 --+
| |
+---------------------------------------------------------+
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>
---
mm/slab_common.c | 48 +++++++++++++++++++++++++++++++-----------------
1 file changed, 31 insertions(+), 17 deletions(-)
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 17996649cfe3..9274fb03563e 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -392,6 +392,30 @@ 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.
+ */
+static void kmem_cache_release(struct kmem_cache *s, bool workfn)
+{
+ if (!workfn)
+ sysfs_slab_unlink(s);
+
+ if (workfn || !(s->flags & SLAB_TYPESAFE_BY_RCU))
+ sysfs_slab_release(s);
+ else
+ schedule_work(&slab_caches_to_rcu_destroy_work);
+}
+#else
+static inline void kmem_cache_release(struct kmem_cache *s, bool workfn)
+{
+ slab_kmem_cache_release(s);
+}
+#endif
+
static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
{
LIST_HEAD(to_destroy);
@@ -418,11 +442,7 @@ 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, true);
Hi Waiman!
As I understand, with SLAB_SUPPORTS_SYSFS kmem_cache_release() can effectively call
into itself: first it's called with workfn == false from shutdown_cache() and
then optionally it's scheduled to call itself from a work context with
workfn == true just to call sysfs_slab_release(). Is it right?
If !SLAB_SUPPORTS_SYSFS, shutdown_cache() optionally adds kmem_cache to the
slab_caches_to_rcu_destroy list and calls kmem_cache_release(s, false) ==
slab_kmem_cache_release(). How it's then removed from the list?
Overall the patch is a bit hard to follow (not like this code was easy to read
before, so can't blame the patch). But I wonder if it will make things simpler
to decouple kmem_cache_release(workfn == true) and kmem_cache_release(workfn == false)
into 2 different helpers? Or at least add a bold comment on how things are supposed
to work.
Thanks!
You are right. I agree that it can be hard to read. Simpler is always
better. Will post a v2 with the change suggested.
Thanks,
Longman
