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

From: Paul E. McKenney
Date: Sat Jan 24 2015 - 16:18:45 EST

On Sat, Jan 24, 2015 at 10:18:05AM +0800, Lai Jiangshan wrote:
> 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.

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?

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@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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