filter: bytecode already in the list should go before

Background
==========
This `seqnum` (sequence number) feature is currently unused. It was
designed so that the session daemon could tell the tracer the order in
which the bytecode should be run.

Issue
=====
The current implementation of the session daemon doesn't use this
feature so there is only ever a single bytecode to execute per callsite.

During work on an upcoming feature uses this `seqnum` became useful and
it was realized that the current bytecode linking code would reverse the
order in which the bytecode were executed when all bytecodes have the
same `seqnum` value.

This is due to the fact that the `cds_list_for_each_entry_reverse` loops
until it finds a `seqnum` smaller than the new one.

So if all bytecodes have the same `seqnum`, the new bytecode will be
added at the beginning of the list.

This is not technically a problem since it's the session daemon's job to
set the sequence number if it wants a particular ordering. Even
considering that, we found it counterintuitive that new bytecodes are
added at the beginning of the list in those cases.

Solution
========
This commit makes it so that on equality, the insertion is done after
the existing bytecodes.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I784887e3e6085f9344a2bb429d4f0d30586ebc57

diff --git a/lttng-filter.c b/lttng-filter.c
index 45319d6f0ef7a0ccd60270d21e50d77a77cc1d71..6ff6be8f2a6888b4ecd7a61bce8ef25ff9830738 100644 (file)
--- a/lttng-filter.c
+++ b/lttng-filter.c
@@ -510,11 +510,12 @@ void lttng_enabler_event_link_bytecode(struct lttng_event *event,
 
                /*
                 * Insert at specified priority (seqnum) in increasing
-                * order.
+                * order. If there already is a bytecode of the same priority,
+                * insert the new bytecode right after it.
                 */
                list_for_each_entry_reverse(runtime,
                                &event->bytecode_runtime_head, node) {
-                       if (runtime->bc->bc.seqnum < bc->bc.seqnum) {
+                       if (runtime->bc->bc.seqnum <= bc->bc.seqnum) {
                                /* insert here */
                                insert_loc = &runtime->node;
                                goto add_within;