From: Li Zefan <lizf@cn.fujitsu.com>
Date: Wed, 12 Nov 2008 21:26:49 +0000 (-0800)
Subject: freezer_cg: remove task_lock from freezer_fork()
X-Git-Tag: v2.6.28-rc5~50
X-Git-Url: http://pilppa.com/gitweb/?a=commitdiff_plain;h=687446760bd008df96655cb8c5900f8e48a7118c;p=linux-2.6-omap-h63xx.git

freezer_cg: remove task_lock from freezer_fork()

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>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: "Serge E. Hallyn" <serue@us.ibm.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 7fa476f01d0..66059071040 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);