From 7e3aa30ac8c904a706518b725c451bb486daaae9 Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <fweisbec@gmail.com>
Date: Fri, 23 Dec 2011 04:25:23 +0100
Subject: [PATCH] cgroup: Remove task_lock() from cgroup_post_fork()
cgroup_post_fork() is protected between threadgroup_change_begin()
and threadgroup_change_end() against concurrent changes of the
child's css_set in cgroup_task_migrate(). Also the child can't
exit and call cgroup_exit() at this stage, this means it's css_set
can't be changed with init_css_set concurrently.
For these reasons, we don't need to hold task_lock() on the child
because it's css_set can only remain stable in this place.
Let's remove the lock there.
v2: Update comment to explain that we are safe against
cgroup_exit()
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Containers <containers@lists.linux-foundation.org>
Cc: Cgroups <cgroups@vger.kernel.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>
Cc: Mandeep Singh Baines <msb@chromium.org>
---
kernel/cgroup.c | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index c6bd67b3fcf6e..548d8d4e86d03 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4595,10 +4595,19 @@ void cgroup_post_fork(struct task_struct *child)
{
if (use_task_css_set_links) {
write_lock(&css_set_lock);
- task_lock(child);
- if (list_empty(&child->cg_list))
+ if (list_empty(&child->cg_list)) {
+ /*
+ * It's safe to use child->cgroups without task_lock()
+ * here because we are protected through
+ * threadgroup_change_begin() against concurrent
+ * css_set change in cgroup_task_migrate(). Also
+ * the task can't exit at that point until
+ * wake_up_new_task() is called, so we are protected
+ * against cgroup_exit() setting child->cgroup to
+ * init_css_set.
+ */
list_add(&child->cg_list, &child->cgroups->tasks);
- task_unlock(child);
+ }
write_unlock(&css_set_lock);
}
}
--
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