From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Wed, 22 Feb 2012 00:06:52 +0000 (-0500)
Subject: RCU lock-free hash table: implement cds_lfht_is_node_deleted()
X-Git-Tag: v0.7.0~43^2
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=refs%2Fheads%2Furcu%2Fht-shrink;p=userspace-rcu.git

RCU lock-free hash table: implement cds_lfht_is_node_deleted()

Some thoughts on how to use the RCU lock-free hash table brought me to
figure out that using the lock-free hash table along with a per-node
mutex is a quite interesting way to deal with lookup vs teardown
coherency.

A per-node lock can be used to protect concurrent modifications of an
entry from one another, as well as concurrent read vs modification. In
addition, if we ensure that each reader/updater of the node checks if
the node has been removed right after taking the mutex, and if we
perform the node removal from the hash table with the per-node mutex
held, we can ensure that readers/updaters will never access unlinked
data.

struct mynode {
	pthread_mutex_t mutex;
	struct cds_lfht node;
}

CPU A (lookup destroy and free)           CPU B (lookup and read/modify)

                                          rcu_read_lock()
                                          mynode = caa_container_of(
                                                cds_lfht_lookup(...), ...);
                                          mutex_lock(&mynode->mutex);
                                          if (cds_lfht_is_node_deleted(
							&mynode->node))
                                               goto unlock;

                                          read/modify structure....

                                        unlock:
                                          mutex_unlock(&mynode->mutex);
                                          rcu_read_unlock()

rcu_read_lock()
mynode = caa_container_of(
        cds_lfht_lookup(...), ...);

mutex_lock(&mynode->mutex);

cds_lfht_del(ht, &mynode->node);

- perform extra teardown operations
  with side-effects, for which call_rcu
  delay is not appropriate

mutex_unlock(&mynode->mutex);
rcu_read_unlock()
call_rcu(free, mynode);

To perform this efficiently, we need an API function to return whether
the node previously looked-up has been deleted since then.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>
CC: Stephen Hemminger <shemminger@vyatta.com>
---

diff --git a/rculfhash.c b/rculfhash.c
index b114e0c..9436605 100644
--- a/rculfhash.c
+++ b/rculfhash.c
@@ -1553,6 +1553,11 @@ int cds_lfht_del(struct cds_lfht *ht, struct cds_lfht_node *node)
 	return ret;
 }
 
+int cds_lfht_is_node_deleted(struct cds_lfht_node *node)
+{
+	return is_removed(rcu_dereference(node->next));
+}
+
 static
 int cds_lfht_delete_bucket(struct cds_lfht *ht)
 {
diff --git a/urcu/rculfhash.h b/urcu/rculfhash.h
index 96dc5a0..8baf31d 100644
--- a/urcu/rculfhash.h
+++ b/urcu/rculfhash.h
@@ -380,6 +380,20 @@ int cds_lfht_replace(struct cds_lfht *ht,
  */
 int cds_lfht_del(struct cds_lfht *ht, struct cds_lfht_node *node);
 
+/*
+ * cds_lfht_is_node_deleted - query if a node is removed from hash table.
+ *
+ * Return non-zero if the node is deleted from the hash table, 0
+ * otherwise.
+ * Node can be looked up with cds_lfht_lookup and cds_lfht_next,
+ * followed by use of cds_lfht_iter_get_node.
+ * RCU read-side lock must be held between lookup and call to this
+ * function.
+ * Call with rcu_read_lock held.
+ * Threads calling this API need to be registered RCU read-side threads.
+ */
+int cds_lfht_is_node_deleted(struct cds_lfht_node *node);
+
 /*
  * cds_lfht_resize - Force a hash table resize
  * @ht: the hash table.