Re: [PATCH] hugetlb/cgroup: Simplify pre_destroy callback

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Wed, 18 Jul 2012 11:04:09 +0530
> "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
>
>> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
>> 
>> 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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