on 2012/7/19 10:55, Aneesh Kumar K.V wrote:
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:
>
>> On Wed, 18 Jul 2012 11:04:09 +0530
>> "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote:
>>
>>> From: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
>>>
>>> Since we cannot fail in hugetlb_cgroup_move_parent, we don't really
>>> need to check whether cgroup have any change left after that. Also skip
>>> those hstates for which we don't have any charge in this cgroup.
>>>
>>> ...
>>>
>>> + for_each_hstate(h) {
>>> + /*
>>> + * if we don't have any charge, skip this hstate
>>> + */
>>> + idx = hstate_index(h);
>>> + if (res_counter_read_u64(&h_cg->hugepage[idx], RES_USAGE) == 0)
>>> + continue;
>>> + spin_lock(&hugetlb_lock);
>>> + list_for_each_entry(page, &h->hugepage_activelist, lru)
>>> + hugetlb_cgroup_move_parent(idx, cgroup, page);
>>> + spin_unlock(&hugetlb_lock);
>>> + VM_BUG_ON(res_counter_read_u64(&h_cg->hugepage[idx], RES_USAGE));
>>> + }
>>> out:
>>> return ret;
>>> }
>>
>> This looks fishy.
>>
>> We test RES_USAGE before taking hugetlb_lock. What prevents some other
>> thread from increasing RES_USAGE after that test?
>>
>> After walking the list we test RES_USAGE after dropping hugetlb_lock.
>> What prevents another thread from incrementing RES_USAGE before that
>> test, triggering the BUG?
>
> IIUC core cgroup will prevent a new task getting added to the cgroup
> when we are in pre_destroy. Since we already check that the cgroup doesn't
> have any task, the RES_USAGE cannot increase in pre_destroy.
>
You're wrong here. We release cgroup_lock before calling pre_destroy and retrieve
the lock after that, so a task can be attached to the cgroup in this interval.
See 3fa59dfbc3b223f02c26593be69ce6fc9a940405 ("cgroup: fix potential deadlock in pre_destroy")
But I think the memcg->pre_destroy has been reworked and now we can safely hold
cgroup_lock when calling the callback, and this can make the code a bit simpler.
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