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

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Message-ID: <54C5A184.20105@cn.fujitsu.com>
Date:	Mon, 26 Jan 2015 10:08:04 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	<paulmck@...ux.vnet.ibm.com>
CC:	Sasha Levin <sasha.levin@...cle.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 01/25/2015 05:18 AM, Paul E. McKenney wrote:

> 
> Good point!  In my scenario, CPU 0 would not yet have switched away from
> Task A.  Hmmm...  Yet Sasha really does see this failure.  Will give it
> some more thought.
> 
> Any ideas?

I don't known which commit was merged from the rcu-git-tree in Sasha's test
I try to review it.

We can fallback to git-bitsect if the reviews fails.

Thanks,
Lai

> 
> 							Thanx, Paul
> 
>> Thanks,
>> Lai
>>
>>>     
>>> 6.	Once in rcu_read_unlock_special(), the fact that
>>>     	current->rcu_read_unlock_special.b.need_qs is true becomes
>>>     	apparent, so rcu_read_unlock_special() invokes rcu_preempt_qs().
>>>     	Recursively, given that we interrupted out of that same
>>>     	function in the preceding step.
>>>     
>>> 7.	Because rcu_preempt_data.passed_quiesce is now true,
>>>     	rcu_preempt_qs() does nothing, and simply returns.
>>>     
>>> 8.	Upon return to rcu_read_unlock_special(), it is noted that
>>>     	current->rcu_read_unlock_special is still nonzero (because
>>>     	the interrupted rcu_preempt_qs() had not yet gotten around
>>>     	to clearing current->rcu_read_unlock_special.b.need_qs).
>>>     
>>> 9.	Execution proceeds to the WARN_ON_ONCE(), which notes that
>>>     	we are in an interrupt handler and thus duly splats.
>>>
>>> The solution, as noted above, is to make rcu_read_unlock_special()
>>> clear out current->rcu_read_unlock_special.b.need_qs after calling
>>> rcu_preempt_qs().  The interrupted rcu_preempt_qs() will clear it again,
>>> but this is harmless.  The worst that happens is that we clobber another
>>> attempt to set this field, but this is not a problem because we just
>>> got done reporting a quiescent state.
>>>
>>> Reported-by: Sasha Levin <sasha.levin@...cle.com>
>>> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>>>
>>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>>> index 8669de884445..ec99dc16aa38 100644
>>> --- a/kernel/rcu/tree_plugin.h
>>> +++ b/kernel/rcu/tree_plugin.h
>>> @@ -322,6 +322,7 @@ void rcu_read_unlock_special(struct task_struct *t)
>>>  	special = t->rcu_read_unlock_special;
>>>  	if (special.b.need_qs) {
>>>  		rcu_preempt_qs();
>>> +		t->rcu_read_unlock_special.need_qs = false;
>>>  		if (!t->rcu_read_unlock_special.s) {
>>>  			local_irq_restore(flags);
>>>  			return;
>>>
>>> .
>>>
>>
> 
> .
> 

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