[PATCH for tip] tracing: Don't use tracing_record_cmdline() in workqueue tracer fix

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

commit c3ffc7a40b7e94b094efe1c8ab4e24370a782b65 "Don't use tracing_record_cmdline() 
in workqueue tracer" have a race window.

find_task_by_vpid() require task_list_lock(). but the patch doesn't.

fixing here.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 kernel/trace/trace_workqueue.c |   20 ++++++++++++++------
 1 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace_workqueue.c b/kernel/trace/trace_workqueue.c
index fb5ccac..9ab035b 100644
--- a/kernel/trace/trace_workqueue.c
+++ b/kernel/trace/trace_workqueue.c
@@ -193,12 +193,20 @@ static int workqueue_stat_show(struct seq_file *s, void *p)
 	struct cpu_workqueue_stats *cws = p;
 	unsigned long flags;
 	int cpu = cws->cpu;
-	struct task_struct *tsk = find_task_by_vpid(cws->pid);
-
-	seq_printf(s, "%3d %6d     %6u       %s\n", cws->cpu,
-		   atomic_read(&cws->inserted),
-		   cws->executed,
-		   tsk ? tsk->comm : "<...>");
+	struct pid *pid;
+	struct task_struct *tsk;
+
+	pid = find_get_pid(cws->pid);
+	if (pid) {
+		tsk = get_pid_task(pid, PIDTYPE_PID);
+		if (tsk) {
+			seq_printf(s, "%3d %6d     %6u       %s\n", cws->cpu,
+				   atomic_read(&cws->inserted), cws->executed,
+				   tsk->comm);
+			put_task_struct(tsk);
+		}
+		put_pid(pid);
+	}
 
 	spin_lock_irqsave(&workqueue_cpu_stat(cpu)->lock, flags);
 	if (&cws->list == workqueue_cpu_stat(cpu)->list.next)
-- 
1.6.0.6