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Date: Thu, 7 Mar 2013 14:21:29 +0800
From: Li Zefan <lizefan@huawei.com>
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To: Michael Wang <wangyun@linux.vnet.ibm.com>
CC: Sasha Levin <sasha.levin@oracle.com>, Tejun Heo <tj@kernel.org>,
        Dave Jones <davej@redhat.com>, Cgroups <cgroups@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: cgroup: INFO: suspicious RCU usage. in cgroup_name
References: <5137687E.50701@oracle.com> <5138309A.2060804@linux.vnet.ibm.com>
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On 2013/3/7 14:15, Michael Wang wrote:
> On 03/07/2013 12:02 AM, Sasha Levin wrote:
>> Hi all,
>>
>> While fuzzing with trinity inside a KVM tools guest running latest -next kernel
>> I've stumbled on the following:
>>
>> [  450.180599] ===============================
>> [  450.181392] [ INFO: suspicious RCU usage. ]
>> [  450.182101] 3.9.0-rc1-next-20130305-sasha-00048-g35e9ec5-dirty #1032 Tainted: G        W
>> [  450.183482] -------------------------------
>> [  450.184343] include/linux/cgroup.h:429 suspicious rcu_dereference_check() usage!
>> [  450.185575]
>> [  450.185575] other info that might help us debug this:
>> [  450.185575]
>> [  450.186961]
>> [  450.186961] rcu_scheduler_active = 1, debug_locks = 1
>> [  450.188001] 4 locks held by kworker/u:0/6:
>> [  450.188646]  #0:  (khelper){.+.+.+}, at: [<ffffffff8112ba98>] process_one_work+0x238/0x570
>> [  450.190186]  #1:  ((&sub_info->work)){+.+.+.}, at: [<ffffffff8112ba98>] process_one_work+0x238/0x570
>> [  450.191824]  #2:  (&(&p->alloc_lock)->rlock){+.+.-.}, at: [<ffffffff812036d3>] dump_header+0x43/0xe0
> 
> Hi, Sasha
> 
> I suppose this is the warn context:
> 
> cpuset_print_task_mems_allowed() -> task_cs() -> task_subsys_state()
> 
> and this is the definition of task_subsys_state():
> 
> #define task_subsys_state_check(task, subsys_id, __c)                   \
>         rcu_dereference_check(task->cgroups->subsys[subsys_id],         \
>                               lockdep_is_held(&task->alloc_lock) ||     \
>                               cgroup_lock_is_held() || (__c))
> 
> the condition "lockdep_is_held(&task->alloc_lock)" should match (#2
> lock), the warn doesn't make sense to me...
> 

nope..note this is 3.9-rc1-next, not 3.9-rc1.

The warning is from this:

/* Caller should hold rcu_read_lock() */
static inline const char *cgroup_name(const struct cgroup *cgrp)
{
        return rcu_dereference(cgrp->name)->name;
}

I've cooked up a patch. Will send out today.

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