Fix RCU-related hangs: incorrect lttng_ht_destroy use

[lttng-tools.git] / src / bin / lttng-sessiond / ust-registry.c

diff --git a/src/bin/lttng-sessiond/ust-registry.c b/src/bin/lttng-sessiond/ust-registry.c
index b538a8fa9f37d6f5108c609a3021a27173cbe9ac..7250dd12d751b0edcce93ffd4b886f8bdd7c7395 100644 (file)
--- a/src/bin/lttng-sessiond/ust-registry.c
+++ b/src/bin/lttng-sessiond/ust-registry.c
@@ -308,12 +308,26 @@ void ust_registry_destroy_event(struct ust_registry_channel *chan,
        return;
 }
 
+/*
+ * We need to execute ht_destroy outside of RCU read-side critical
+ * section, so we postpone its execution using call_rcu. It is simpler
+ * than to change the semantic of the many callers of
+ * destroy_channel().
+ */
+static
+void destroy_channel_rcu(struct rcu_head *head)
+{
+       struct ust_registry_channel *chan =
+               caa_container_of(head, struct ust_registry_channel, rcu_head);
+
+       lttng_ht_destroy(chan->ht);
+       free(chan);
+}
+
 /*
  * Destroy every element of the registry and free the memory. This does NOT
  * free the registry pointer since it might not have been allocated before so
  * it's the caller responsability.
- *
- * This *MUST NOT* be called within a RCU read side lock section.
  */
 static void destroy_channel(struct ust_registry_channel *chan)
 {
@@ -329,9 +343,7 @@ static void destroy_channel(struct ust_registry_channel *chan)
                ust_registry_destroy_event(chan, event);
        }
        rcu_read_unlock();
-
-       lttng_ht_destroy(chan->ht);
-       free(chan);
+       call_rcu(&chan->rcu_head, destroy_channel_rcu);
 }
 
 /*
@@ -435,12 +447,6 @@ void ust_registry_channel_del_free(struct ust_registry_session *session,
        ret = lttng_ht_del(session->channels, &iter);
        assert(!ret);
        rcu_read_unlock();
-
-       /*
-        * Destroying the hash table should be done without RCU
-        * read-side lock held. Since we own "chan" now, it is OK to use
-        * it outside of RCU read-side critical section.
-        */
        destroy_channel(chan);
 
 end:
@@ -535,8 +541,8 @@ void ust_registry_session_destroy(struct ust_registry_session *reg)
                assert(!ret);
                destroy_channel(chan);
        }
-       lttng_ht_destroy(reg->channels);
        rcu_read_unlock();
 
+       lttng_ht_destroy(reg->channels);
        free(reg->metadata);
 }