Re: [RFC][PATCH] memcg: fix a race when setting memcg.swappiness

Li Zefan
2009-01-14T06:48:13Z

KAMEZAWA Hiroyuki wrote:
> On Wed, 14 Jan 2009 11:24:18 +0800
> Li Zefan <lizf@cn.fujitsu.com> wrote:
>
>> (suppose: memcg->use_hierarchy == 0 and memcg->swappiness == 60)
>>
>> echo 10 > /memcg/0/swappiness |
>> mem_cgroup_swappiness_write() |
>> ... | echo 1 > /memcg/0/use_hierarchy
>> | mkdir /mnt/0/1
>> | sub_memcg->swappiness = 60;
>> memcg->swappiness = 10; |
>>
>> In the above scenario, we end up having 2 different swappiness
>> values in a single hierarchy.
>>
>> Note we can't use hierarchy_lock here, because it doesn't protect
>> the create() method.
>>
>> Though IMO use cgroup_lock() in simple write functions is OK,
>> Paul would like to avoid it. And he sugguested use a counter to
>> count the number of children instead of check cgrp->children list:
>>
>> =================
>> create() does:
>>
>> lock memcg_parent
>> memcg->swappiness = memcg->parent->swappiness;
>> memcg_parent->child_count++;
>> unlock memcg_parent
>>
>> and write() does:
>>
>> lock memcg
>> if (!memcg->child_count) {
>> memcg->swappiness = swappiness;
>> } else {
>> report error;
>> }
>> unlock memcg
>>
>> destroy() does:
>> lock memcg_parent
>> memcg_parent->child_count--;
>> unlock memcg_parent
>>
>> =================
>>
>> And there is a suble differnce with checking cgrp->children,
>> that a cgroup is removed from parent's list in cgroup_rmdir(),
>> while memcg->child_count is decremented in cgroup_diput().
>>
>>
>> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
>
> Seems reasonable, but, hmm...
>

Do you mean you agree to avoid using cgroup_lock()?

> Why hierarchy_mutex can't be used for create() ?
>

We can make hierarchy_mutex work for this race by:

@@ -2403,16 +2403,18 @@ static long cgroup_create(struct cgroup *parent, struct
if (notify_on_release(parent))
set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);

+ cgroup_lock_hierarchy(root);
+
for_each_subsys(root, ss) {
struct cgroup_subsys_state *css = ss->create(ss, cgrp);
if (IS_ERR(css)) {
+ cgroup_unlock_hierarchy(root);
err = PTR_ERR(css);
goto err_destroy;
}
init_cgroup_css(css, ss, cgrp);
}

- cgroup_lock_hierarchy(root);
list_add(&cgrp->sibling, &cgrp->parent->children);
cgroup_unlock_hierarchy(root);
root->number_of_cgroups++;

But this may not be what we want, because hierarchy_mutex is meant to be
lightweight, so it's not held while subsys callbacks are invoked, except
bind().