linux-kernel - Re: rcu: BUG on exit

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Message-ID: <CA+1xoqcXMVG0J4r8XhtzcxxDqLuyR30x5a3o8BsrBqDakDdHgg@mail.gmail.com>
Date:	Fri, 4 May 2012 06:08:34 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	"linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>, yinghan@...gle.com,
	kosaki.motohiro@...fujitsu.com,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: rcu: BUG on exit_group

On Thu, May 3, 2012 at 7:01 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Thu, May 03, 2012 at 05:55:14PM +0200, Sasha Levin wrote:
>> On Thu, May 3, 2012 at 5:41 PM, Paul E. McKenney
>> <paulmck@...ux.vnet.ibm.com> wrote:
>> > On Thu, May 03, 2012 at 10:57:19AM +0200, Sasha Levin wrote:
>> >> Hi Paul,
>> >>
>> >> I've hit a BUG similar to the schedule_tail() one when. It happened
>> >> when I've started fuzzing exit_group() syscalls, and all of the traces
>> >> are starting with exit_group() (there's a flood of them).
>> >>
>> >> I've verified that it indeed BUGs due to the rcu preempt count.
>> >
>> > Hello, Sasha,
>> >
>> > Which version of -next are you using?  I did some surgery on this
>> > yesterday based on some bugs Hugh Dickins tracked down, so if you
>> > are using something older, please move to the current -next.
>>
>> I'm using -next from today (3.4.0-rc5-next-20120503-sasha-00002-g09f55ae-dirty).
>
> Hmmm...  Looking at this more closely, it looks like there really is
> an attempt to acquire a mutex within an RCU read-side critical section,
> which is illegal.  Could you please bisect this?

Right, the issue is as you described, taking a mutex inside rcu_read_lock().

The offending commit is (I've cc'ed all parties from it):

commit adf79cc03092ee4aec70da10e91b05fb8116ac7b
Author: Ying Han <yinghan@...gle.com>
Date:   Thu May 3 15:44:01 2012 +1000

    memcg: add mlock statistic in memory.stat

With the issue there being is that in munlock_vma_page(), it now does
a mem_cgroup_begin_update_page_stat() which takes the rcu_read_lock(),
so when the older code that was there previously will try taking a
mutex you'll get a BUG.

Thanks.
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