rcuja: lower equal: handle concurrent removal with retry

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

diff --git a/rcuja/rcuja.c b/rcuja/rcuja.c
index 0171f2d80ff5db9ef7f7588d3d7b586d46896ac8..e6c6d5b51493e8b5bbc58d3044c04ae2a84bb183 100644 (file)
--- a/rcuja/rcuja.c
+++ b/rcuja/rcuja.c
@@ -1738,6 +1738,10 @@ struct cds_hlist_head cds_ja_lookup(struct cds_ja *ja, uint64_t key)
        return head;
 }
 
+/*
+ * cds_ja_lookup_lower_equal() may need to retry if a concurrent removal
+ * causes failure to find the previous node.
+ */
 struct cds_hlist_head cds_ja_lookup_lower_equal(struct cds_ja *ja, uint64_t key)
 {
        int tree_depth, level;
@@ -1747,6 +1751,7 @@ struct cds_hlist_head cds_ja_lookup_lower_equal(struct cds_ja *ja, uint64_t key)
        if (caa_unlikely(key > ja->key_max || !key))
                return head;
 
+retry:
        memset(cur_node_depth, 0, sizeof(cur_node_depth));
        tree_depth = ja->tree_depth;
        node_flag = rcu_dereference(ja->root);
@@ -1799,6 +1804,18 @@ struct cds_hlist_head cds_ja_lookup_lower_equal(struct cds_ja *ja, uint64_t key)
        }
 
        level++;
+
+       /*
+        * From this point, we should be able to find a "lower than"
+        * match. The only reason why we could fail to find such a match
+        * would be due to a concurrent removal of the branch that
+        * contains the match. If this happens, we have no choice but to
+        * retry the entire lookup. Indeed, just because we reached a
+        * dead-end due to concurrent removal of the branch does not
+        * mean that other match don't exist. However, this requires
+        * going up into the tree, hence the retry.
+        */
+
        /* Find rightmost child of rightmost child (recursively). */
        for (; level < tree_depth; level++) {
                node_flag = ja_node_get_rightmost(node_flag);
@@ -1807,13 +1824,16 @@ struct cds_hlist_head cds_ja_lookup_lower_equal(struct cds_ja *ja, uint64_t key)
                        break;
        }
 
-       if (level == tree_depth) {
-               /* Last level lookup succeded. We got a "lower than" match. */
-               head.next = (struct cds_hlist_node *) node_flag;
-               return head;
+       if (level != tree_depth) {
+               /*
+                * We did not get a match. Caused by concurrent removal.
+                * We need to retry the entire lookup.
+                */
+               goto retry;
        }
 
-       /* No match */
+       /* Last level lookup succeded. We got a "lower than" match. */
+       head.next = (struct cds_hlist_node *) node_flag;
        return head;
 }