linux-kernel - Re: [PATCH 3.10] cgroup: break infinite loop in __css

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Message-ID: <20141202165553.GB10918@htj.dyndns.org>
Date:	Tue, 2 Dec 2014 11:55:53 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Roman Gushchin <klamm@...dex-team.ru>
Cc:	linux-kernel@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
	stable@...r.kernel.org#3.10
Subject: Re: [PATCH 3.10] cgroup: break infinite loop in __css_tryget()

Hello, Roman.

On Fri, Nov 28, 2014 at 07:47:54PM +0300, Roman Gushchin wrote:
> If cgroup_destroy_locked() sets the css refcount to a negative value,
> we get an infinite loop in __css_tryget().
> 
> In this case css_refcnt() returns modified (non-negative value), so
> both (t == v) and (t < 0) conditions are always false.

I don't follow.  The count is biased and modified by unbiasing iff the
value is negative.  Here, @v is the unbiased value and @t is the
verbatim value.  If @v is different from @t due to unbiasing, @t must
be negative satisfying the second condition and returning NULL, no?

Thanks.

-- 
tejun
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