[PATCH] freezer_cg: remove task_lock from freezer_fork()

[Li Zefan <lizf@cn.fujitsu.com>]

In theory the task can be moved to another cgroup and the freezer will
be freed right after task_lock is dropped, so the lock results in zero
protection.

But in the case of freezer_fork() no lock is needed, since the task
is not in tasklist yet so it won't be moved to another cgroup, so
task->cgroups won't be changed or invalidated.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
---
 kernel/cgroup_freezer.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 7fa476f..6605907 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -184,9 +184,13 @@ static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
 {
 	struct freezer *freezer;
 
-	task_lock(task);
+	/*
+	 * No lock is needed, since the task isn't on tasklist yet,
+	 * so it can't be moved to another cgroup, which means the
+	 * freezer won't be removed and will be valid during this
+	 * function call.
+	 */
 	freezer = task_freezer(task);
-	task_unlock(task);
 
 	spin_lock_irq(&freezer->lock);
 	BUG_ON(freezer->state == CGROUP_FROZEN);
-- 
1.5.4.rc3