perf tools: Make fork event processing more resilient

When processing a fork event, the tools lookup the parent thread by its
tid.  In a couple of cases, it is possible for that thread to have the
wrong pid.

That can happen if the data is being processed out of order, or if the
(fork) event that would have removed the erroneous thread was lost.

Assume the latter case, print a dump message, remove the erroneous
thread, create a new one with the correct pid, and keep going.

Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1439994561-27436-3-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/machine.c

@@ -1387,6 +1387,24 @@ int machine__process_fork_event(struct machine *machine, union perf_event *event
 							event->fork.ptid);
 	int err = 0;
 
+	if (dump_trace)
+		perf_event__fprintf_task(event, stdout);
+
+	/*
+	 * There may be an existing thread that is not actually the parent,
+	 * either because we are processing events out of order, or because the
+	 * (fork) event that would have removed the thread was lost. Assume the
+	 * latter case and continue on as best we can.
+	 */
+	if (parent->pid_ != (pid_t)event->fork.ppid) {
+		dump_printf("removing erroneous parent thread %d/%d\n",
+			    parent->pid_, parent->tid);
+		machine__remove_thread(machine, parent);
+		thread__put(parent);
+		parent = machine__findnew_thread(machine, event->fork.ppid,
+						 event->fork.ptid);
+	}
+
 	/* if a thread currently exists for the thread id remove it */
 	if (thread != NULL) {
 		machine__remove_thread(machine, thread);
@@ -1395,8 +1413,6 @@ int machine__process_fork_event(struct machine *machine, union perf_event *event
 
 	thread = machine__findnew_thread(machine, event->fork.pid,
 					 event->fork.tid);
-	if (dump_trace)
-		perf_event__fprintf_task(event, stdout);
 
 	if (thread == NULL || parent == NULL ||
 	    thread__fork(thread, parent, sample->time) < 0) {