[PATCH rcu 5/8] slab: Explain why SLAB_DESTROY_BY_RCU reference before locking

[Paul E. McKenney]

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@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
Cc: <linux-mm@xxxxxxxxx>
---
 include/linux/slab.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 90877fcde70bd..446303e385265 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -76,6 +76,12 @@
  * 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 */
-- 
2.31.1.189.g2e36527f23