Re: [PATCH 2/2 v2] cgroup: Drop task_lock(parent) on cgroup_fork()

Li Zefan
2011-12-21T03:14:07Z

>>> @@ -4556,7 +4556,7 @@ static const struct file_operations proc_cgroupstats_operations = {
>>> *
>>> * A pointer to the shared css_set was automatically copied in
>>> * fork.c by dup_task_struct(). However, we ignore that copy, since
>>> - * it was not made under the protection of RCU or cgroup_mutex, so
>>> + * it was not made under the protection of threadgroup_change_begin(), so
>>
>> I think the original comment still stands, but now threadgroup_change_begin()
>> can also protect the cgroup pointer from becoming invalid.
>
> Right but I'm not sure it's worth quoting RCU and cgroup_mutex. The reason
> why we use threadgroup_change_begin() is not only to ensure the pointer
> validity but also to synchronize the whole cgroup proc logic. This way
> when we attach a whole proc with cgroup_attach_proc(), we are sure that
> no thread forked too soon or too late such that it wouldn't be migrated with
> the rest.
>
> RCU or cgroup_mutex on dup_task_struct() (+ a get_css_set()) would have
> protected the pointer validity but not the whole above described machinery.
> So I don't think it's even worth quoting those solutions. But if you prefer
> I can keep the old comment.
>

No, I don't have strong opinion here.

So I'll ack this patch.

> OTOH what I think is missing in the comment is that explanation on the synchronization
> against entire proc migration. I can edit that.
>

I would appreciate this. :)

>>
>>> * might no longer be a valid cgroup pointer. cgroup_attach_task() might
>>> * have already changed current->cgroups, allowing the previously
>>> * referenced cgroup group to be removed and freed.
>>> @@ -4566,10 +4566,14 @@ static const struct file_operations proc_cgroupstats_operations = {
>>> */
>>> void cgroup_fork(struct task_struct *child)
>>> {
>>> - task_lock(current);
>>> + /*
>>> + * We don't need to task_lock() current because current->cgroups
>>> + * can't be changed concurrently here. The parent obviously hasn't
>>> + * exited and called cgroup_exit(), and we are synchronized against
>>> + * cgroup migration through threadgroup_change_begin().
>>> + */
>>> child->cgroups = current->cgroups;
>>> get_css_set(child->cgroups);
>>> - task_unlock(current);
>>> INIT_LIST_HEAD(&child->cg_list);
>>> }
>>>
>