linux-kernel - [PATCH] freezer_cg: remove task_lock from freezer

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Message-ID: <490FF047.2030008@cn.fujitsu.com>
Date:	Tue, 04 Nov 2008 14:48:39 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Matt Helsley <matthltc@...ibm.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	linux-pm@...ts.linux-foundation.org,
	Cedric Le Goater <clg@...ibm.com>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	Paul Menage <menage@...gle.com>
Subject: [PATCH] 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@...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
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