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

[Andrew Morton]

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?

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