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SubjectRe: [PATCH] cgroups: fix incorrect using rcu_dereference() in cgroup_subsys_state()
Paul Menage wrote:
> On Fri, Nov 21, 2008 at 12:49 AM, Lai Jiangshan <laijs@cn.fujitsu.com> wrote:
>> It's task->cgroups protected by RCU. and struct css_set.subsys[subsys_id]
>> is readonly(after init). so we don't need rcu_dereference() for
>> struct css_set.subsys[subsys_id].
>>
>> the ways using cgroup_subsys_state() safely:
>>
>> #1:
>> rcu_read_lock() / task_lock();
>> c = cgroup_subsys_state(tsk, id);
>> use c;
>> rcu_read_unlock() / task_unlock();
>
> You need to qualify that with the fact that if you're just using RCU,
> the subsys state may no longer be the state for the task that you're
> interested in, since we don't guarantee that the task won't move
> directly after you read the state pointer.
>
>>
>> #2: use cgroup_lock() for _current_ task.
>> cgroup_lock();
>> c = cgroup_subsys_state(current, id);
>> use c;
>> cgroup_unlock();
>
> No, if you use cgroup_lock() you can do this for any task.
> cgroup_lock() is the cgroups equivalent of the BKL, and definitely
> prevents all task movement between groups.

cgroup_exit() will defeat you.

>> static inline struct cgroup_subsys_state *task_subsys_state(
>> struct task_struct *task, int subsys_id)
>> {
>> - return rcu_dereference(task->cgroups->subsys[subsys_id]);
>> + /*
>> + * ->subsys[subsys_id] are read-only data, so we do not need
>> + * rcu_dereference() for it.
>> + */
>> + return rcu_dereference(task->cgroups)->subsys[subsys_id];
>> }
>
> Change looks OK but I think we can lose the additional comment.
>
> Paul
>
>
>

I just remembered I had deferred Li Zefan's patch.
(I'm also RCU developer, I had been writing CGROUP VS RCU then,
I thought these patches should be sent together, So I deferred his patch)

From: Li Zefan <lizf@cn.fujitsu.com>
Date: Mon, 25 Aug 2008 11:05:28 +0800
Subject: [PATCH] cgroup: fix wrong rcu_dereference()

It is tsk->cgroups which is protected by RCU.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
---
include/linux/cgroup.h | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index c98dd7c..d911dc7 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -355,7 +355,7 @@ static inline struct cgroup_subsys_state *cgroup_subsys_state(
static inline struct cgroup_subsys_state *task_subsys_state(
struct task_struct *task, int subsys_id)
{
- return rcu_dereference(task->cgroups->subsys[subsys_id]);
+ return rcu_dereference(task->cgroups)->subsys[subsys_id];
}

static inline struct cgroup* task_cgroup(struct task_struct *task,


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