freezer_cg: fix improper BUG_ON() causing oops

The BUG_ON() should be protected by freezer->lock, otherwise it can be
triggered easily when a task has been unfreezed but the corresponding
cgroup hasn't been changed to FROZEN state.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Cedric Le Goater <clg@fr.ibm.com>
Acked-by: Matt Helsley <matthltc@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index e95056954498..7f54d1c42953 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -190,8 +190,9 @@ static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
         freezer = task_freezer(task);
         task_unlock(task);
 
-        BUG_ON(freezer->state == CGROUP_FROZEN);
         spin_lock_irq(&freezer->lock);
+        BUG_ON(freezer->state == CGROUP_FROZEN);
+
         /* Locking avoids race with FREEZING -> THAWED transitions. */
         if (freezer->state == CGROUP_FREEZING)
                 freeze_task(task, true);