rculfhash: comment shrink operation

[urcu.git] / rculfhash.c

diff --git a/rculfhash.c b/rculfhash.c
index f95ef5dc083aa4e22618512708beb37cee22e1e3..8dd5a7327ad81be4da1b3dddace433b4fd8aadc4 100644 (file)
--- a/rculfhash.c
+++ b/rculfhash.c
@@ -752,10 +752,8 @@ struct cds_lfht_node *_cds_lfht_add(struct cds_lfht *ht,
                iter = rcu_dereference(iter_prev->p.next);
                assert(iter_prev->p.reverse_hash <= node->p.reverse_hash);
                for (;;) {
-                       /* TODO: check if removed */
                        if (unlikely(!clear_flag(iter)))
                                goto insert;
-                       /* TODO: check if removed */
                        if (likely(clear_flag(iter)->p.reverse_hash > node->p.reverse_hash))
                                goto insert;
                        next = rcu_dereference(clear_flag(iter)->p.next);
@@ -883,6 +881,11 @@ void init_table_hash(struct cds_lfht *ht, unsigned long i,
        }
 }
 
+/*
+ * Holding RCU read lock to protect _cds_lfht_add against memory
+ * reclaim that could be performed by other call_rcu worker threads (ABA
+ * problem).
+ */
 static
 void init_table_link(struct cds_lfht *ht, unsigned long i, unsigned long len)
 {
@@ -905,11 +908,6 @@ void init_table_link(struct cds_lfht *ht, unsigned long i, unsigned long len)
        ht->cds_lfht_rcu_thread_offline();
 }
 
-/*
- * Holding RCU read lock to protect _cds_lfht_add against memory
- * reclaim that could be performed by other call_rcu worker threads (ABA
- * problem).
- */
 static
 void init_table(struct cds_lfht *ht,
                unsigned long first_order, unsigned long len_order)
@@ -948,6 +946,26 @@ void init_table(struct cds_lfht *ht,
        }
 }
 
+/*
+ * Holding RCU read lock to protect _cds_lfht_remove against memory
+ * reclaim that could be performed by other call_rcu worker threads (ABA
+ * problem).
+ * For a single level, we logically remove and garbage collect each node.
+ *
+ * As a design choice, we perform logical removal and garbage collection on a
+ * node-per-node basis to simplify this algorithm. We also assume keeping good
+ * cache locality of the operation would overweight possible performance gain
+ * that could be achieved by batching garbage collection for multiple levels.
+ * However, this would have to be justified by benchmarks.
+ *
+ * Concurrent removal and add operations are helping us perform garbage
+ * collection of logically removed nodes. We guarantee that all logically
+ * removed nodes have been garbage-collected (unlinked) before call_rcu is
+ * invoked to free a hole level of dummy nodes (after a grace period).
+ *
+ * Logical removal and garbage collection can therefore be done in batch or on a
+ * node-per-node basis, as long as the guarantee above holds.
+ */
 static
 void remove_table(struct cds_lfht *ht, unsigned long i, unsigned long len)
 {
@@ -972,11 +990,6 @@ void remove_table(struct cds_lfht *ht, unsigned long i, unsigned long len)
        ht->cds_lfht_rcu_thread_offline();
 }
 
-/*
- * Holding RCU read lock to protect _cds_lfht_remove against memory
- * reclaim that could be performed by other call_rcu worker threads (ABA
- * problem).
- */
 static
 void fini_table(struct cds_lfht *ht,
                unsigned long first_order, unsigned long len_order)