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 <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Cc: <[email protected]>

Author: paulmckrcu

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 */