[PATCH pm-freezer 4/4] freezer: use lock_task_sighand() in fake_signal_wake_up()

[Tejun Heo <tj@kernel.org>]

cgroup_freezer calls freeze_task() without holding tasklist_lock and,
if the task is exiting, its ->sighand may be gone by the time
fake_signal_wake_up() is called.  Use lock_task_sighand() instead of
accessing ->sighand directly.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Paul Menage <paul@paulmenage.org>
---
 kernel/freezer.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

Index: work/kernel/freezer.c
===================================================================
--- work.orig/kernel/freezer.c
+++ work/kernel/freezer.c
@@ -95,9 +95,10 @@ static void fake_signal_wake_up(struct t
 {
 	unsigned long flags;
 
-	spin_lock_irqsave(&p->sighand->siglock, flags);
-	signal_wake_up(p, 0);
-	spin_unlock_irqrestore(&p->sighand->siglock, flags);
+	if (lock_task_sighand(p, &flags)) {
+		signal_wake_up(p, 0);
+		unlock_task_sighand(p, &flags);
+	}
 }
 
 /**