slab: Explain why SLAB_DESTROY_BY_RCU reference before locking
It is not obvious to the casual user why it is absolutely necessary to
acquire a reference to a SLAB_DESTROY_BY_RCU structure before acquiring
a lock in that structure. Therefore, add a comment explaining this point.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: <linux-mm@kvack.org>
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 0fefdf5..78c9b97 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -74,6 +74,12 @@
* rcu_read_lock before reading the address, then rcu_read_unlock after
* taking the spinlock within the structure expected at that address.
*
+ * Note that it is not possible to acquire a lock within a structure
+ * allocated with SLAB_DESTROY_BY_RCU without first acquiring a reference
+ * as described above. The reason is that SLAB_DESTROY_BY_RCU pages are
+ * not zeroed before being given to the slab, which means that any locks
+ * must be initialized after each and every kmem_struct_alloc().
+ *
* Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU.
*/
/* Defer freeing slabs to RCU */
