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

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Message-ID: <20191107154413.GZ3622521@devbig004.ftw2.facebook.com>
Date:   Thu, 7 Nov 2019 07:44:13 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        stable@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: hugetlb: switch to css_tryget() in
 hugetlb_cgroup_charge_cgroup()

On Wed, Nov 06, 2019 at 02:51:31PM -0800, Roman Gushchin 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>
> Cc: stable@...r.kernel.org
> Cc: Tejun Heo <tj@...nel.org>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun