perf tools: Avoid deadlock when map_groups are broken

Attempting to clone map groups onto themselves will deadlock.

It only happens because of other bugs, but the code should protect
itself anyway.

Reported-by: Linus Torvalds <[e-mail]>
Signed-off-by: Adrian Hunter <[e-mail]>
Tested-by: Jiri Olsa <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
[ Use pr_debug() instead of dump_fprintf() ]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/thread.c

···
 	if (thread->pid_ == parent->pid_)
 		return 0;
 
+	if (thread->mg == parent->mg) {
+		pr_debug("broken map groups on thread %d/%d parent %d/%d\n",
+			 thread->pid_, thread->tid, parent->pid_, parent->tid);
+		return 0;
+	}
+
 	/* But this one is new process, copy maps. */
 	for (i = 0; i < MAP__NR_TYPES; ++i)
 		if (map_groups__clone(thread->mg, parent->mg, i) < 0)