linux-kernel - Re: [PATCH] cgroups: fix probable race with put_css_set[_taskexit] and find_css

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Date:	Wed, 10 Sep 2008 14:25:57 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Greg KH <greg@...ah.com>
CC:	Paul Menage <menage@...gle.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cgroups: fix probable race with put_css_set[_taskexit]
 and find_css_set

Greg KH wrote:
> On Tue, Sep 09, 2008 at 10:31:24PM -0700, Paul Menage wrote:
>> On Tue, Sep 9, 2008 at 10:01 PM, Greg KH <greg@...ah.com> wrote:
>>> What are you trying to solve here with this change?  I agree, it does
>>> seem a bit "chaotic" :)
>> There's a place in cgroups that uses kref_put() to release an object;
>> the release function *then* takes a write-lock and removes the object
>> from a lookup table; it could race with another thread that searches
>> the lookup table (while holding a read-lock) and does kref_get() on
>> the same object.
> 
> Ick, yeah that's not good.
> 
> What about the way everyone else solves this, grab the lock before you
> call kref_put()?
> 

do_exit()
  cgroup_exit()
    put_css_set_taskexit()
      kref_put()

If we grab the lock before kref_put(), we add overhead to do_exit(), which
is what we are trying to avoid here.

>> The current fix is for the release function to recheck inside the lock
>> that the object's refcount is still zero, and only actually
>> unlink/free it if so. And actually I've just realised that this isn't
>> actually even safe, since the thread that just acquired the object
>> could kref_put() it almost immediately, which would leave two threads
>> both trying to unlink/free the object.
> 
> Yeah, don't do that :)
> 
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