Document relay_add_stream RCU locking

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>

diff --git a/src/bin/lttng-relayd/main.c b/src/bin/lttng-relayd/main.c
index 537ef877882ad5ac9e006c94f9979e3a928fb925..6b0ff97fecfd10b9202cc82d5002c5715a700bd4 100644 (file)
--- a/src/bin/lttng-relayd/main.c
+++ b/src/bin/lttng-relayd/main.c
@@ -1248,7 +1248,7 @@ int relay_add_stream(struct lttcomm_relayd_hdr *recv_hdr,
        struct relay_session *session = conn->session;
        struct relay_stream *stream = NULL;
        struct lttcomm_relayd_status_stream reply;
-       struct ctf_trace *trace;
+       struct ctf_trace *trace = NULL;
 
        if (!session || conn->version_check_done == 0) {
                ERR("Trying to add a stream before version check");
@@ -1276,7 +1276,6 @@ int relay_add_stream(struct lttcomm_relayd_hdr *recv_hdr,
                goto err_free_stream;
        }
 
-       rcu_read_lock();
        stream->stream_handle = ++last_relay_stream_id;
        stream->prev_seq = -1ULL;
        stream->session_id = session->id;
@@ -1309,6 +1308,8 @@ int relay_add_stream(struct lttcomm_relayd_hdr *recv_hdr,
                DBG("Tracefile %s/%s created", stream->path_name, stream->channel_name);
        }
 
+       /* Protect access to "trace" */
+       rcu_read_lock();
        trace = ctf_trace_find_by_path(session->ctf_traces_ht, stream->path_name);
        if (!trace) {
                trace = ctf_trace_create(stream->path_name);
@@ -1336,6 +1337,9 @@ int relay_add_stream(struct lttcomm_relayd_hdr *recv_hdr,
        /*
         * Both in the ctf_trace object and the global stream ht since the data
         * side of the relayd does not have the concept of session.
+        *
+        * rcu_read_lock() is kept to protect the stream which is now part of
+        * the relay_streams_ht.
         */
        lttng_ht_add_unique_u64(relay_streams_ht, &stream->node);
        cds_list_add_tail(&stream->trace_list, &trace->stream_list);
@@ -1363,7 +1367,13 @@ end:
                ERR("Relay sending stream id");
                ret = send_ret;
        }
+       /*
+        * rcu_read_lock() was held to protect either "trace" OR the "stream" at
+        * this point.
+        */
        rcu_read_unlock();
+       trace = NULL;
+       stream = NULL;
 
 end_no_session:
        return ret;