[lttng-dev] [lttng-tools PATCH] Fix UST channel error handling

[David Goulet]

When an application registers, if an ust app channel failed to get
created on the tracer, it was *not* removed from the ust app session
hash table and thus creating a segfault on start trace.

Adds a safe_delete_ust_app_channel function which deletes the channel
object from the session hash table and free() the memory safely using an
RCU call.

Commit fixes #89

Signed-off-by: David Goulet <dgoulet at efficios.com>
---
 src/bin/lttng-sessiond/ust-app.c |   43 ++++++++++++++++++++++++++++++++++++-
 1 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/src/bin/lttng-sessiond/ust-app.c b/src/bin/lttng-sessiond/ust-app.c
index 0303582..3d4132d 100644
--- a/src/bin/lttng-sessiond/ust-app.c
+++ b/src/bin/lttng-sessiond/ust-app.c
@@ -207,6 +207,20 @@ void delete_ust_app(struct ust_app *app)
 }
 
 /*
+ * Safe RCU protected for freeing ust app channel memory.
+ */
+static void delete_ust_app_channel_rcu(struct rcu_head *head)
+{
+	struct lttng_ht_node_str *node =
+		caa_container_of(head, struct lttng_ht_node_str, head);
+	struct ust_app_channel *ua_chan =
+		caa_container_of(node, struct ust_app_channel, node);
+
+	DBG3("Call RCU deleting app channel %s", ua_chan->name);
+	delete_ust_app_channel(-1, ua_chan);
+}
+
+/*
  * URCU intermediate call to delete an UST app.
  */
 static
@@ -222,6 +236,26 @@ void delete_ust_app_rcu(struct rcu_head *head)
 }
 
 /*
+ * Safely remove a ust app channel from the ust app session channel hash table
+ * and free the memory inside a RCU call.
+ */
+static void safe_delete_ust_app_channel(struct ust_app_session *ua_sess,
+		struct ust_app_channel *ua_chan)
+{
+	struct lttng_ht_node_str *node;
+	struct lttng_ht_iter iter;
+
+	lttng_ht_lookup(ua_sess->channels, (void *) ua_chan->name, &iter);
+	node = lttng_ht_iter_get_node_str(&iter);
+	if (node == NULL) {
+		return;
+	}
+
+	lttng_ht_del(ua_sess->channels, &iter);
+	call_rcu(&node->head, delete_ust_app_channel_rcu);
+}
+
+/*
  * Alloc new UST app session.
  */
 static
@@ -2257,7 +2291,13 @@ void ust_app_global_update(struct ltt_ust_session *usess, int sock)
 			node.node) {
 		ret = create_ust_channel(app, ua_sess, ua_chan);
 		if (ret < 0) {
-			/* FIXME: Should we quit here or continue... */
+			/*
+			 * This means that something went wrong with what we've passed to
+			 * the tracer so we must destroy this object from the ust app
+			 * session since it's not valid anymore. This fix is related to
+			 * issue #89
+			 */
+			safe_delete_ust_app_channel(ua_sess, ua_chan);
 			continue;
 		}
 
@@ -2270,7 +2310,6 @@ void ust_app_global_update(struct ltt_ust_session *usess, int sock)
 			}
 		}
 
-
 		/* For each events */
 		cds_lfht_for_each_entry(ua_chan->events->ht, &uiter.iter, ua_event,
 				node.node) {
-- 
1.7.9