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Date: Mon, 7 Jun 2010 17:19:29 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Miles Lane <miles.lane@gmail.com>
Cc: Vivek Goyal <vgoyal@redhat.com>, Eric Paris <eparis@redhat.com>,
       Lai Jiangshan <laijs@cn.fujitsu.com>, Ingo Molnar <mingo@elte.hu>,
       Peter Zijlstra <peterz@infradead.org>,
       LKML <linux-kernel@vger.kernel.org>, nauman@google.com,
       eric.dumazet@gmail.com, netdev@vger.kernel.org,
       Jens Axboe <jens.axboe@oracle.com>,
       Gui Jianfeng <guijianfeng@cn.fujitsu.com>,
       Li Zefan <lizf@cn.fujitsu.com>,
       Johannes Berg <johannes@sipsolutions.net>
Subject: Re: 2.6.35-rc2-git1 - include/linux/cgroup.h:534 invoked
 rcu_dereference_check() without protection!
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References: <AANLkTin2pPqOUx--9fIX3BH3e-cU6oCRufijcx_4ozx5@mail.gmail.com>
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On Mon, Jun 07, 2010 at 02:14:25PM -0400, Miles Lane wrote:
> Hi All,
> 
> I just reproduced a warning I reported quite a while ago.? Is a patch
> for this in the pipeline?

I proposed a patch, thinking that it was a false positive.  Peter Zijlstra
pointed out that there was a real race, and proposed an alternative patch,
which may be found at http://lkml.org/lkml/2010/4/22/603.

Could you please test Peter's patch and let us know if it cures the problem?

							Thanx, Paul

> [??? 0.167267] [ INFO: suspicious rcu_dereference_check() usage. ]
> [??? 0.167396] ---------------------------------------------------
> [??? 0.167526] include/linux/cgroup.h:534 invoked
> rcu_dereference_check() without protection!
> [??? 0.167728]
> [??? 0.167729] other info that might help us debug this:
> [??? 0.167731]
> [??? 0.168092]
> [??? 0.168093] rcu_scheduler_active = 1, debug_locks = 1
> [??? 0.168337] no locks held by watchdog/0/5.
> [??? 0.168462]
> [??? 0.168463] stack backtrace:
> [??? 0.168704] Pid: 5, comm: watchdog/0 Not tainted 2.6.35-rc2-git1 #8
> [??? 0.168834] Call Trace:
> [??? 0.168965]? [<ffffffff81064e9c>] lockdep_rcu_dereference+0x9d/0xa5
> [??? 0.169100]? [<ffffffff8102c1ce>] task_subsys_state+0x59/0x70
> [??? 0.169232]? [<ffffffff8103189b>] __sched_setscheduler+0x19d/0x2f8
> [??? 0.169365]? [<ffffffff8102a5ef>] ? need_resched+0x1e/0x28
> [??? 0.169497]? [<ffffffff813c7d01>] ? schedule+0x586/0x619
> [??? 0.169628]? [<ffffffff81081c33>] ? watchdog+0x0/0x8c
> [??? 0.169758]? [<ffffffff81031a11>] sched_setscheduler+0xe/0x10
> [??? 0.169889]? [<ffffffff81081c5d>] watchdog+0x2a/0x8c
> [??? 0.170010]? [<ffffffff81081c33>] ? watchdog+0x0/0x8c
> [??? 0.170141]? [<ffffffff81054a82>] kthread+0x89/0x91
> [??? 0.170274]? [<ffffffff81003054>] kernel_thread_helper+0x4/0x10
> [??? 0.170405]? [<ffffffff813ca480>] ? restore_args+0x0/0x30
> [??? 0.170536]? [<ffffffff810549f9>] ? kthread+0x0/0x91
> [??? 0.170667]? [<ffffffff81003050>] ? kernel_thread_helper+0x0/0x10
> [    0.176751] lockdep: fixing up alternatives.
--
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