Re: rcu, sched: WARNING: CPU: 30 PID: 23771 at kernel/rcu/tree_plugin.h:337 rcu_read_unlock_special+0x369/0x550()

[Lai Jiangshan]

On 01/23/2015 05:36 PM, Paul E. McKenney wrote:
> On Thu, Jan 22, 2015 at 10:55:42PM -0800, Paul E. McKenney wrote:
>> On Thu, Jan 22, 2015 at 11:05:45PM -0500, Sasha Levin wrote:
>>> 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.
>>
>> OK, that was what I thought was the situation.  I have not yet fully
>> worked out how RCU gets into that state, but in the meantime, here
>> is a patch that should prevent the splats.  (It requires a subtle
>> interaction of quiescent-state detection and the scheduling-clock
>> interrupt.)
> 
> And I did finally figure out how this can happen.  Please see below
> for an updated patch with this information recorded in the commit log.
> Sasha, I am impressed -- your testing not only located a true RCU bug,
> but an RCU bug that can happen on a uniprocessor!  ;-)
> 
> As far as I know, the bug is harmless apart from the splat, but still...
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> rcu: Clear need_qs flag to prevent splat
> 
> If the scheduling-clock interrupt sets the current tasks need_qs flag,
> but if the current CPU passes through a quiescent state in the meantime,
> then rcu_preempt_qs() will fail to clear the need_qs flag, which can fool
> RCU into thinking that additional rcu_read_unlock_special() processing
> is needed.  This commit therefore clears the need_qs flag before checking
> for additional processing.
> 
> For this problem to occur, we need rcu_preempt_data.passed_quiesce equal
> to true and current->rcu_read_unlock_special.b.need_qs also equal to true.
> This condition can occur as follows:


Hi, Paul
I still can't draw the sequence map.

> 
> 1.	CPU 0 is aware of the current preemptible RCU grace period,
>     	but has not yet passed through a quiescent state.  Among other
>     	things, this means that rcu_preempt_data.passed_quiesce is false.
>     
> 2.	Task A running on CPU 0 enters a preemptible RCU read-side
>     	critical section.
>     
> 3.	CPU 0 takes a scheduling-clock interrupt, which notices the
>     	RCU read-side critical section and the need for a quiescent state,
>     	and thus sets current->rcu_read_unlock_special.b.need_qs to true.


So, Task A is still in the preemptible RCU read-side critical section here.

>     
> 4.	Task A is preempted, enters the scheduler, eventually invoking
>     	rcu_preempt_note_context_switch() which in turn invokes
>     	rcu_preempt_qs().
>     
>     	Because rcu_preempt_data.passed_quiesce is false,
>     	control enters the body of the "if" statement, which sets
>     	rcu_preempt_data.passed_quiesce to true.
>     
> 5.	At this point, CPU 0 takes an interrupt.  The interrupt
>     	handler contains an RCU read-side critical section, and
>     	the rcu_read_unlock() notes that current->rcu_read_unlock_special
>     	is nonzero, and thus invokes rcu_read_unlock_special().

If the previous critical section is not ended, this new critical section
is nested, and this new critical section will not call rcu_read_unlock_special().

If the previous critical section is ended, the conditions were changed between
step#3,#4,#5, and the following #6... can't happen.

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@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> 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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