linux-kernel - Re: [PATCH] hugetlb/cgroup: Simplify pre

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Message-Id: <20120718142628.76bf78b3.akpm@linux-foundation.org>
Date:	Wed, 18 Jul 2012 14:26:28 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	linux-mm@...ck.org, kamezawa.hiroyu@...fujitsu.com, mhocko@...e.cz,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hugetlb/cgroup: Simplify pre_destroy callback

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

> From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.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?

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