Comment list behavior wrt lock-freedom

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/rculfhash.c b/rculfhash.c
index f1e6674bd7f1cc63f6eb09a0946904fc1dfc4f10..4abf27a9a5dd5e9f56f97d669de88e58705e741d 100644 (file)
--- a/rculfhash.c
+++ b/rculfhash.c
@@ -225,7 +225,10 @@ void _ht_add(struct rcu_ht *ht, struct rcu_table *t, struct rcu_ht_node *node)
                        iter_prev = iter;
                        check_resize(ht, t, ++chain_len);
                }
-               /* add in iter_prev->next */
+               /*
+                * add in iter_prev->next: TODO: check for helping
+                * delete, for lock-freedom...
+                */
                if (is_removed(iter))
                        continue;
                assert(node != iter);
@@ -286,9 +289,13 @@ retry:
                flagged = 1;
        }
        /*
-        * Remove the element from the list. Retry if there has been a
-        * concurrent add (there cannot be a concurrent delete, because
-        * we won the deletion flag cmpxchg).
+        * Remove the element from the list.
+        * Retry if there has been a concurrent add before us.
+        * Retry if the prev node has been deleted.
+        * There cannot be a concurrent delete for our position, because
+        * we won the deletion flag cmpxchg.
+        * If there is a concurrent add after us, our deletion flag
+        * makes it busy-loop (FIXME: not lock-free).
         */
        if (uatomic_cmpxchg(&iter_prev->next, iter, clear_flag(next)) != iter)
                goto retry;