linux-kernel - Re: rcu, sched: WARNING: CPU: 30 PID: 23771 at kernel/rcu/tree_plugin.h:337 rcu_read_unlock

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Date:	Thu, 22 Jan 2015 23:05:45 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Lai Jiangshan <laijs@...fujitsu.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"davej@...emonkey.org.uk >> Dave Jones" <davej@...emonkey.org.uk>
Subject: Re: rcu, sched: WARNING: CPU: 30 PID: 23771 at kernel/rcu/tree_plugin.h:337
 rcu_read_unlock_special+0x369/0x550()

On 01/22/2015 11:02 PM, Sasha Levin wrote:
> On 01/22/2015 10:51 PM, Paul E. McKenney wrote:
>> On Thu, Jan 22, 2015 at 10:29:01PM -0500, Sasha Levin wrote:
>>>> On 01/21/2015 07:43 PM, Paul E. McKenney wrote:
>>>>>> On Wed, Jan 21, 2015 at 10:44:57AM -0500, Sasha Levin wrote:
>>>>>>>> On 01/20/2015 09:57 PM, Paul E. McKenney wrote:
>>>>>>>>>>>>>> So RCU believes that an RCU read-side critical section that ended within
>>>>>>>>>>>>>>>>>> an interrupt handler (in this case, an hrtimer) somehow got preempted.
>>>>>>>>>>>>>>>>>> Which is not supposed to happen.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Do you have CONFIG_PROVE_RCU enabled?  If not, could you please enable it
>>>>>>>>>>>>>>>>>> and retry?
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> I did have CONFIG_PROVE_RCU, and didn't see anything else besides what I pasted here.
>>>>>>>>>> OK, fair enough.  I do have a stack of RCU CPU stall-warning changes on
>>>>>>>>>> their way in, please see v3.19-rc1..630181c4a915 in -rcu, which is at:
>>>>>>>>>>
>>>>>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
>>>>>>>>>>
>>>>>>>>>> These handle the problems that Dave Jones, yourself, and a few others
>>>>>>>>>> located this past December.  Could you please give them a spin?
>>>>>>>>
>>>>>>>> They seem to be a part of -next already, so this testing already includes them.
>>>>>>>>
>>>>>>>> I seem to be getting them about once a day, anything I can add to debug it?
>>>>>>
>>>>>> Could you please try reproducing with the following patch?
>>>>
>>>> Yes, and I've got mixed results. It reproduced, and all I got was:
>>>>
>>>> [  717.645572] ===============================
>>>> [  717.645572] [ INFO: suspicious RCU usage. ]
>>>> [  717.645572] 3.19.0-rc5-next-20150121-sasha-00064-g3c37e35-dirty #1809 Tainted: G        W
>>>> [  717.645572] -------------------------------
>>>> [  717.645572] kernel/rcu/tree_plugin.h:337 rcu_read_unlock() from irq or softirq with blocking in critical section!!!
>>>> [  717.645572] !
>>>> [  717.645572]
>>>> [  717.645572] other info that might help us debug this:
>>>> [  717.645572]
>>>> [  717.645572]
>>>> [  717.645572] rcu_scheduler_active = 1, debug_locks = 1
>>>> [  717.645572] 3 locks held by trinity-c29/16497:
>>>> [  717.645572]  #0:  (&sb->s_type->i_mutex_key){+.+.+.}, at: [<ffffffff81bec373>] lookup_slow+0xd3/0x420
>>>> [  717.645572]  #1:
>>>> [hang]
>>>>
>>>> So the rest of the locks/stack trace didn't get printed, nor the pr_alert() which
>>>> should follow that.
>>>>
>>>> I've removed the lockdep call and will re-run it.
>> Thank you!  You are keeping the pr_alert(), correct?
> 
> Yup, just the lockdep call goes away.

Okay, this reproduced faster than I anticipated:

[  786.160131] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[  786.239513] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[  786.240503] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[  786.242575] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[  786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[  786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[  786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[  786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)
[  786.243565] ->rcu_read_unlock_special: 0x100 (b: 0, nq: 1)

It seems like the WARN_ON_ONCE was hiding the fact it actually got hit couple
of times in a very short interval. Maybe that would also explain lockdep crapping
itself.


Thanks,
Sasha

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