perf tools: Make perf_session__register_idle_thread drop the refcount

Note that since the thread was already inserted to the session
list, it will be released when the session is released.
Also, in perf_session__register_idle_thread() failure path,
the thread should be put before returning.

Refcnt debugger shows that the perf_session__register_idle_thread
gets the returned thread, but the caller (__cmd_top) does not
put the returned idle thread.

  ----
  ==== [0] ====
  Unreclaimed thread@0x24e6240
  Refcount +1 => 0 at
    ./perf(thread__new+0xe5) [0x4c8a75]
    ./perf(machine__findnew_thread+0x9a) [0x4bbdba]
    ./perf(perf_session__register_idle_thread+0x28) [0x4c63c8]
    ./perf(cmd_top+0xd7d) [0x43cf6d]
    ./perf() [0x47ba35]
    ./perf(main+0x617) [0x4225b7]
    /lib64/libc.so.6(__libc_start_main+0xf5) [0x7f06027c5af5]
    ./perf() [0x42272d]
  Refcount +1 => 1 at
    ./perf(thread__get+0x2c) [0x4c8bcc]
    ./perf(machine__findnew_thread+0xee) [0x4bbe0e]
    ./perf(perf_session__register_idle_thread+0x28) [0x4c63c8]
    ./perf(cmd_top+0xd7d) [0x43cf6d]
    ./perf() [0x47ba35]
    ./perf(main+0x617) [0x4225b7]
    /lib64/libc.so.6(__libc_start_main+0xf5) [0x7f06027c5af5]
    ./perf() [0x42272d]
  Refcount +1 => 2 at
    ./perf(thread__get+0x2c) [0x4c8bcc]
    ./perf(machine__findnew_thread+0x112) [0x4bbe32]
    ./perf(perf_session__register_idle_thread+0x28) [0x4c63c8]
    ./perf(cmd_top+0xd7d) [0x43cf6d]
    ./perf() [0x47ba35]
    ./perf(main+0x617) [0x4225b7]
    /lib64/libc.so.6(__libc_start_main+0xf5) [0x7f06027c5af5]
    ./perf() [0x42272d]
  ----
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20151209021122.10245.69707.stgit@localhost.localdomain
[ Drop the refcount in perf_session__register_idle_thread() ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-top.c

@@ -964,7 +964,7 @@ static int __cmd_top(struct perf_top *top)
 	if (ret)
 		goto out_delete;
 
-	if (perf_session__register_idle_thread(top->session) == NULL)
+	if (perf_session__register_idle_thread(top->session) < 0)
 		goto out_delete;
 
 	machine__synthesize_threads(&top->session->machines.host, &opts->target,

tools/perf/util/session.c

@@ -1311,17 +1311,20 @@ struct thread *perf_session__findnew(struct perf_session *session, pid_t pid)
 	return machine__findnew_thread(&session->machines.host, -1, pid);
 }
 
-struct thread *perf_session__register_idle_thread(struct perf_session *session)
+int perf_session__register_idle_thread(struct perf_session *session)
 {
 	struct thread *thread;
+	int err = 0;
 
 	thread = machine__findnew_thread(&session->machines.host, 0, 0);
 	if (thread == NULL || thread__set_comm(thread, "swapper", 0)) {
 		pr_err("problem inserting idle task.\n");
-		thread = NULL;
+		err = -1;
 	}
 
-	return thread;
+	/* machine__findnew_thread() got the thread, so put it */
+	thread__put(thread);
+	return err;
 }
 
 static void perf_session__warn_about_errors(const struct perf_session *session)
@@ -1676,7 +1679,7 @@ int perf_session__process_events(struct perf_session *session)
 	u64 size = perf_data_file__size(session->file);
 	int err;
 
-	if (perf_session__register_idle_thread(session) == NULL)
+	if (perf_session__register_idle_thread(session) < 0)
 		return -ENOMEM;
 
 	if (!perf_data_file__is_pipe(session->file))

tools/perf/util/session.h

@@ -89,7 +89,7 @@ struct machine *perf_session__findnew_machine(struct perf_session *session, pid_
 }
 
 struct thread *perf_session__findnew(struct perf_session *session, pid_t pid);
-struct thread *perf_session__register_idle_thread(struct perf_session *session);
+int perf_session__register_idle_thread(struct perf_session *session);
 
 size_t perf_session__fprintf(struct perf_session *session, FILE *fp);