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

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Date:	Tue, 02 Dec 2014 21:40:45 +0300
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	Tejun Heo <tj@...nel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH 3.10] cgroup: break infinite loop in __css_tryget()

Hi, Tejun!

02.12.2014, 19:56, "Tejun Heo" <tj@...nel.org>:
> 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?

Yep. I missed that we compare t with 0 (not v).
Thanks!
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