Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()

From: Michal Hocko <mhocko@kernel.org>
To: Roman Gushchin <guro@fb.com>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Kernel Team <Kernel-team@fb.com>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()
Date: Fri, 8 Nov 2019 09:53:37 +0100	[thread overview]
Message-ID: <20191108085337.GA15658@dhcp22.suse.cz> (raw)
In-Reply-To: <20191107224107.GA8219@castle.DHCP.thefacebook.com>

On Thu 07-11-19 22:41:13, Roman Gushchin wrote:
> On Thu, Nov 07, 2019 at 06:02:00PM +0100, Michal Hocko wrote:
> > On Thu 07-11-19 16:42:41, Roman Gushchin wrote:
[...]
> > > It's an exiting task with the PF_EXITING flag set and it's in their late stages
> > > of life.
> > 
> > This is a signal delivery path AFAIU (get_signal) and the coredumping
> > happens before do_exit. My understanding is that that unlinking
> > happens from cgroup_exit. So either I am misreading the backtrace or
> > there is some other way to leave cgroups or there is something more
> > going on.
> 
> Yeah, you're right. I have no better explanation for this and the similar,
> mentioned in the commit bsd accounting issue,

Tejun mentioned bsd accounting issue as well, but I do not see any
explicit reference to it in neither of the two patches.

> than some very rare race condition
> that allows cgroups to be offlined with a task inside.
> 
> I'll think more about it.

Thanks a lot. As I've said, I am not opposing this change once we have a
proper changelog but I find the explanation really weak. If there is a
race then it should be fixed as well.

Thanks!
-- 
Michal Hocko
SUSE Labs