Add comment on manual registration of "ust" channel

diff --git a/libust/tracectl.c b/libust/tracectl.c
index 873b0f66b432e150853363fb1456f5c0b3e7fc55..f9312b4340238e79e3e2dd6abf51d82bd962cca1 100644 (file)
--- a/libust/tracectl.c
+++ b/libust/tracectl.c
@@ -949,7 +949,24 @@ static void __attribute__((constructor)) init()
                /* Ensure markers are initialized */
                init_markers();
 
-               /* In case. */
+               /* FIXME: When starting early tracing (here), depending on the
+                * order of constructors, it is very well possible some marker
+                * sections are not yet registered. Because of this, some
+                * channels may not be registered. Yet, we are about to ask the
+                * daemon to collect the channels. Channels which are not yet
+                * registered will not be collected.
+                *
+                * Currently, in LTTng, there is no way to add a channel after
+                * trace start. The reason for this is that it induces complex
+                * concurrency issues on the trace structures, which can only
+                * be resolved using RCU. This has not been done yet. As a
+                * workaround, we are forcing the registration of the "ust"
+                * channel here. This is the only channel (apart from metadata)
+                * that can be reliably used in early tracing.
+                *
+                * Non-early tracing does not have this problem and can use
+                * arbitrary channel names.
+                */
                ltt_channels_register("ust");
 
                result = ltt_trace_setup(trace_name);