linux-kernel - Re: [PATCH 2/2] mm: hugetlb: switch to css_tryget() in hugetlb_cgroup_charge

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Date:   Wed, 6 Nov 2019 18:31:56 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Roman Gushchin <guro@...com>
Cc:     Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>, stable@...r.kernel.org,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 2/2] mm: hugetlb: switch to css_tryget() in hugetlb_cgroup_charge_cgroup()

On Wed, Nov 6, 2019 at 2:53 PM Roman Gushchin <guro@...com> wrote:
>
> An exiting task might belong to an offline cgroup. In this case
> an attempt to grab a cgroup reference from the task can end up
> with an infinite loop in hugetlb_cgroup_charge_cgroup(), because
> neither the cgroup will become online, neither the task will
> be migrated to a live cgroup.
>
> Fix this by switching over to css_tryget(). As css_tryget_online()
> can't guarantee that the cgroup won't go offline, in most cases
> the check doesn't make sense. In this particular case users of
> hugetlb_cgroup_charge_cgroup() are not affected by this change.
>
> A similar problem is described by commit 18fa84a2db0e ("cgroup: Use
> css_tryget() instead of css_tryget_online() in task_get_css()").
>
> Signed-off-by: Roman Gushchin <guro@...com>

Reviewed-by: Shakeel Butt <shakeelb@...gle.com>

> Cc: stable@...r.kernel.org
> Cc: Tejun Heo <tj@...nel.org>
> ---
>  mm/hugetlb_cgroup.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
> index f1930fa0b445..2ac38bdc18a1 100644
> --- a/mm/hugetlb_cgroup.c
> +++ b/mm/hugetlb_cgroup.c
> @@ -196,7 +196,7 @@ int hugetlb_cgroup_charge_cgroup(int idx, unsigned long nr_pages,
>  again:
>         rcu_read_lock();
>         h_cg = hugetlb_cgroup_from_task(current);
> -       if (!css_tryget_online(&h_cg->css)) {
> +       if (!css_tryget(&h_cg->css)) {
>                 rcu_read_unlock();
>                 goto again;
>         }
> --
> 2.17.1
>