Enhance documentation to fork separation and join

kernel/behave.c

@@ -391,6 +391,18 @@ int pbm_task_end(PBM* pbm) {
     return 0;
 }
 
+/**
+ * Why is the code concerning the forking separated into the two functions
+ * pbm_fork_parent_new_task() and pbm_fork() instead of simply putting it at the end of _do_fork?
+ *
+ * The separation is necessary since in the _do_fork a context switch from the parent to the
+ * child process takes place which is problematic since we want to end (and restart) perf-measuring
+ * the parent as well as the child process and the measurements (in pbm_task_end()) can only happen
+ * from the process itself. But in the beginning of _do_fork the child process does not exist yet.
+ * Therefore we have to split the code into the two functions to be able to measure the parent
+ * before the context switch as well as initialize the child-measuring after switching to the child.
+ */
+
 /* Stop previous task and start new task for the parent process and also reset the perf counter
  * Returns a pointer to the fork-task-node which the forked process can use as a time information.
  *
@@ -572,6 +584,7 @@ int pbm_exit(pid_t pid, pid_t parent_pid) {
 int pbm_join(PBM* child_pbm) {
     pbm_NODE* fork_node;
     pbm_NODE* join_node;
+    pid_t join_label;
 
     if(!child_pbm)
         return 0;
@@ -579,7 +592,11 @@ int pbm_join(PBM* child_pbm) {
     printk(KERN_WARNING "Joining thread %u with fork_node: %lli\n", child_pbm->last->thread_id, task_2_index(child_pbm->fork_date));
     fork_node = child_pbm->fork_date;
 
-    join_node = _pbm_create_node(JOIN, child_pbm->last->thread_id/*fork_node->thread_id*/); // FIXME????
+    // the child process is used to label the join operation to know which process the join belongs
+    // to since using the parent as the label would be ambiguous since more than one child could
+    // have been spawned by the same parent
+    join_label = child_pbm->last->thread_id;
+    join_node = _pbm_create_node(JOIN, child_pbm->last->thread_id);
     if(!join_node) {
         printk(KERN_WARNING "ERROR: Could not create node!\n");
         return 0;