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.
-aneesh
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