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

[Paul E. McKenney]

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:

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.
    
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().
    
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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