[PATCH v2] docs/RCU: Bring back smp_wmb()

[Alan Huang <mmpgouride@gmail.com>]

The objects are allocated with SLAB_TYPESAFE_BY_RCU, and there is
n->next = first within hlist_add_head_rcu() before rcu_assign_pointer(),
which modifies obj->obj_node.next. There may be readers holding the
reference of obj in lockless_lookup, and when updater modifies ->next,
readers can see the change immediately because of SLAB_TYPESAFE_BY_RCU.

There are two memory ordering required in the insertion algorithm,
we need to make sure obj->key is updated before obj->obj_node.next
and obj->refcnt, atomic_set_release is not enough to provide the
required memory barrier.

Signed-off-by: Alan Huang <mmpgouride@gmail.com>
---
Changelog:
  v1 -> v2: Use _ONCE to protect obj->key.

 Documentation/RCU/rculist_nulls.rst | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/Documentation/RCU/rculist_nulls.rst b/Documentation/RCU/rculist_nulls.rst
index 21e40fcc08de..2a9f5a63d334 100644
--- a/Documentation/RCU/rculist_nulls.rst
+++ b/Documentation/RCU/rculist_nulls.rst
@@ -47,7 +47,7 @@ objects, which is having below type.
     * reuse these object before the RCU grace period, we
     * must check key after getting the reference on object
     */
-    if (obj->key != key) { // not the object we expected
+    if (READ_ONCE(obj->key) != key) { // not the object we expected
       put_ref(obj);
       rcu_read_unlock();
       goto begin;
@@ -64,10 +64,10 @@ but a version with an additional memory barrier (smp_rmb())
   {
     struct hlist_node *node, *next;
     for (pos = rcu_dereference((head)->first);
-         pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) &&
+         pos && ({ next = READ_ONCE(pos->next); smp_rmb(); prefetch(next); 1; }) &&
          ({ obj = hlist_entry(pos, typeof(*obj), obj_node); 1; });
          pos = rcu_dereference(next))
-      if (obj->key == key)
+      if (READ_ONCE(obj->key) == key)
         return obj;
     return NULL;
   }
@@ -111,8 +111,13 @@ detect the fact that it missed following items in original chain.
    */
   obj = kmem_cache_alloc(...);
   lock_chain(); // typically a spin_lock()
-  obj->key = key;
-  atomic_set_release(&obj->refcnt, 1); // key before refcnt
+  WRITE_ONCE(obj->key, key);
+  /*
+   * We need to make sure obj->key is updated before obj->obj_node.next
+   * and obj->refcnt.
+   */
+  smp_wmb();
+  atomic_set(&obj->refcnt, 1);
   hlist_add_head_rcu(&obj->obj_node, list);
   unlock_chain(); // typically a spin_unlock()
 
@@ -165,12 +170,12 @@ Note that using hlist_nulls means the type of 'obj_node' field of
   begin:
   rcu_read_lock();
   hlist_nulls_for_each_entry_rcu(obj, node, head, obj_node) {
-    if (obj->key == key) {
+    if (READ_ONCE(obj->key) == key) {
       if (!try_get_ref(obj)) { // might fail for free objects
 	rcu_read_unlock();
         goto begin;
       }
-      if (obj->key != key) { // not the object we expected
+      if (READ_ONCE(obj->key) != key) { // not the object we expected
         put_ref(obj);
 	rcu_read_unlock();
         goto begin;
@@ -206,7 +211,7 @@ hlist_add_head_rcu().
    */
   obj = kmem_cache_alloc(cachep);
   lock_chain(); // typically a spin_lock()
-  obj->key = key;
+  WRITE_ONCE(obj->key, key);
   atomic_set_release(&obj->refcnt, 1); // key before refcnt
   /*
    * insert obj in RCU way (readers might be traversing chain)
-- 
2.34.1