rcu: documents rculist_nulls

Adds Documentation/RCU/rculist_nulls.txt file to describe how 'nulls'
end-of-list can help in some RCU algos.
Signed-off-by: NEric Dumazet <dada1@cosmosbay.com>
Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/RCU/rculist_nulls.txt

+Using hlist_nulls to protect read-mostly linked lists and
+objects using SLAB_DESTROY_BY_RCU allocations.
+
+Please read the basics in Documentation/RCU/listRCU.txt
+
+Using special makers (called 'nulls') is a convenient way
+to solve following problem :
+
+A typical RCU linked list managing objects which are
+allocated with SLAB_DESTROY_BY_RCU kmem_cache can
+use following algos :
+
+1) Lookup algo
+--------------
+rcu_read_lock()
+begin:
+obj = lockless_lookup(key);
+if (obj) {
+  if (!try_get_ref(obj)) // might fail for free objects
+    goto begin;
+  /*
+   * Because a writer could delete object, and a writer could
+   * reuse these object before the RCU grace period, we
+   * must check key after geting the reference on object
+   */
+  if (obj->key != key) { // not the object we expected
+     put_ref(obj);
+     goto begin;
+   }
+}
+rcu_read_unlock();
+
+Beware that lockless_lookup(key) cannot use traditional hlist_for_each_entry_rcu()
+but a version with an additional memory barrier (smp_rmb())
+
+lockless_lookup(key)
+{
+   struct hlist_node *node, *next;
+   for (pos = rcu_dereference((head)->first);
+          pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) &&
+          ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
+          pos = rcu_dereference(next))
+      if (obj->key == key)
+         return obj;
+   return NULL;
+
+And note the traditional hlist_for_each_entry_rcu() misses this smp_rmb() :
+
+   struct hlist_node *node;
+   for (pos = rcu_dereference((head)->first);
+		pos && ({ prefetch(pos->next); 1; }) &&
+		({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
+		pos = rcu_dereference(pos->next))
+      if (obj->key == key)
+         return obj;
+   return NULL;
+}
+
+Quoting Corey Minyard :
+
+"If the object is moved from one list to another list in-between the
+ time the hash is calculated and the next field is accessed, and the
+ object has moved to the end of a new list, the traversal will not
+ complete properly on the list it should have, since the object will
+ be on the end of the new list and there's not a way to tell it's on a
+ new list and restart the list traversal.  I think that this can be
+ solved by pre-fetching the "next" field (with proper barriers) before
+ checking the key."
+
+2) Insert algo :
+----------------
+
+We need to make sure a reader cannot read the new 'obj->obj_next' value
+and previous value of 'obj->key'. Or else, an item could be deleted
+from a chain, and inserted into another chain. If new chain was empty
+before the move, 'next' pointer is NULL, and lockless reader can
+not detect it missed following items in original chain.
+
+/*
+ * Please note that new inserts are done at the head of list,
+ * not in the middle or end.
+ */
+obj = kmem_cache_alloc(...);
+lock_chain(); // typically a spin_lock()
+obj->key = key;
+atomic_inc(&obj->refcnt);
+/*
+ * we need to make sure obj->key is updated before obj->next
+ */
+smp_wmb();
+hlist_add_head_rcu(&obj->obj_node, list);
+unlock_chain(); // typically a spin_unlock()
+
+
+3) Remove algo
+--------------
+Nothing special here, we can use a standard RCU hlist deletion.
+But thanks to SLAB_DESTROY_BY_RCU, beware a deleted object can be reused
+very very fast (before the end of RCU grace period)
+
+if (put_last_reference_on(obj) {
+   lock_chain(); // typically a spin_lock()
+   hlist_del_init_rcu(&obj->obj_node);
+   unlock_chain(); // typically a spin_unlock()
+   kmem_cache_free(cachep, obj);
+}
+
+
+
+--------------------------------------------------------------------------
+With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup()
+and extra smp_wmb() in insert function.
+
+For example, if we choose to store the slot number as the 'nulls'
+end-of-list marker for each slot of the hash table, we can detect
+a race (some writer did a delete and/or a move of an object
+to another chain) checking the final 'nulls' value if
+the lookup met the end of chain. If final 'nulls' value
+is not the slot number, then we must restart the lookup at
+the begining. If the object was moved to same chain,
+then the reader doesnt care : It might eventually
+scan the list again without harm.
+
+
+1) lookup algo
+
+ head = &table[slot];
+ rcu_read_lock();
+begin:
+ hlist_nulls_for_each_entry_rcu(obj, node, head, member) {
+   if (obj->key == key) {
+      if (!try_get_ref(obj)) // might fail for free objects
+         goto begin;
+      if (obj->key != key) { // not the object we expected
+         put_ref(obj);
+         goto begin;
+      }
+  goto out;
+ }
+/*
+ * if the nulls value we got at the end of this lookup is
+ * not the expected one, we must restart lookup.
+ * We probably met an item that was moved to another chain.
+ */
+ if (get_nulls_value(node) != slot)
+   goto begin;
+ obj = NULL;
+
+out:
+ rcu_read_unlock();
+
+2) Insert function :
+--------------------
+
+/*
+ * Please note that new inserts are done at the head of list,
+ * not in the middle or end.
+ */
+obj = kmem_cache_alloc(cachep);
+lock_chain(); // typically a spin_lock()
+obj->key = key;
+atomic_set(&obj->refcnt, 1);
+/*
+ * insert obj in RCU way (readers might be traversing chain)
+ */
+hlist_nulls_add_head_rcu(&obj->obj_node, list);
+unlock_chain(); // typically a spin_unlock()