freezer: use lock_task_sighand() in fake_signal_wake_up()

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>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Paul Menage <paul@paulmenage.org>

1 files changed, 4 insertions, 3 deletions

diff --git a/kernel/freezer.c b/kernel/freezer.c
index c3496c6a91b2..389549f0a94e 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -93,9 +93,10 @@ static void fake_signal_wake_up(struct task_struct *p)
 {
         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);
+        }
 }
 
 /**