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 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.)

							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.

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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