perf synthetic-events: Ignore dead threads during event synthesis

When it synthesize various task events, it scans the list of task
first and then accesses later.  There's a window threads can die
between the two and proc entries may not be available.

Instead of bailing out, we can ignore that thread and move on.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20220701205458.985106-2-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/synthetic-events.c

@@ -767,11 +767,12 @@ static int __event__synthesize_thread(union perf_event *comm_event,
 		if (*end)
 			continue;
 
-		rc = -1;
+		/* some threads may exit just after scan, ignore it */
 		if (perf_event__prepare_comm(comm_event, pid, _pid, machine,
 					     &tgid, &ppid, &kernel_thread) != 0)
-			break;
+			continue;
 
+		rc = -1;
 		if (perf_event__synthesize_fork(tool, fork_event, _pid, tgid,
 						ppid, process, machine) < 0)
 			break;