[PATCH v2] cgroup: Remove task_lock() from cgroup_post_fork()

[PATCH v2] cgroup: Remove task_lock() from cgroup_post_fork()

[Frederic Weisbecker]

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>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: 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 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1042b3c..0ccfc25 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4593,10 +4593,19 @@ 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);
 	}
 }
-- 
1.7.5.4

Re: [PATCH v2] cgroup: Remove task_lock() from cgroup_post_fork()

[Tejun Heo]

On Fri, Dec 23, 2011 at 04:25:23AM +0100, Frederic Weisbecker wrote:
> 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>
> Cc: Li Zefan <lizf@cn.fujitsu.com>
> Cc: 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>

Li, does this look good to you?

Thanks.

-- 
tejun

Re: [PATCH v2] cgroup: Remove task_lock() from cgroup_post_fork()

[Li Zefan]

Tejun Heo wrote:
> On Fri, Dec 23, 2011 at 04:25:23AM +0100, Frederic Weisbecker wrote:
>> 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>
>> Cc: Li Zefan <lizf@cn.fujitsu.com>
>> Cc: 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>
> 
> Li, does this look good to you?
> 

Acked-by: Li Zefan <lizf@cn.fujitsu.com>

Re: [PATCH v2] cgroup: Remove task_lock() from cgroup_post_fork()

[Tejun Heo]

On Fri, Dec 23, 2011 at 04:25:23AM +0100, Frederic Weisbecker wrote:
> 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>
> Cc: Li Zefan <lizf@cn.fujitsu.com>
> Cc: 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>

Applied to cgroup/for-3.3.  Thank you.

-- 
tejun