Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs

Paul E. McKenney
2019-06-27T17:38:39Z

On Thu, Jun 27, 2019 at 12:47:24PM -0400, Joel Fernandes wrote:
> On Thu, Jun 27, 2019 at 11:55 AM Paul E. McKenney <paulmck@linux.ibm.com> wrote:
> >
> > On Thu, Jun 27, 2019 at 11:30:31AM -0400, Joel Fernandes wrote:
> > > On Thu, Jun 27, 2019 at 10:34:55AM -0400, Steven Rostedt wrote:
> > > > On Thu, 27 Jun 2019 10:24:36 -0400
> > > > Joel Fernandes <joel@joelfernandes.org> wrote:
> > > >
> > > > > > What am I missing here?
> > > > >
> > > > > This issue I think is
> > > > >
> > > > > (in normal process context)
> > > > > spin_lock_irqsave(rq_lock); // which disables both preemption and interrupt
> > > > > // but this was done in normal process context,
> > > > > // not from IRQ handler
> > > > > rcu_read_lock();
> > > > > <---------- IPI comes in and sets exp_hint
> > > >
> > > > How would an IPI come in here with interrupts disabled?
> > > >
> > > > -- Steve
> > >
> > > This is true, could it be rcu_read_unlock_special() got called for some
> > > *other* reason other than the IPI then?
> > >
> > > Per Sebastian's stack trace of the recursive lock scenario, it is happening
> > > during cpu_acct_charge() which is called with the rq_lock held.
> > >
> > > The only other reasons I know off to call rcu_read_unlock_special() are if
> > > 1. the tick indicated that the CPU has to report a QS
> > > 2. an IPI in the middle of the reader section for expedited GPs
> > > 3. preemption in the middle of a preemptible RCU reader section
> >
> > 4. Some previous reader section was IPIed or preempted, but either
> > interrupts, softirqs, or preemption was disabled across the
> > rcu_read_unlock() of that previous reader section.
>
> Hi Paul, I did not fully understand 4. The previous RCU reader section
> could not have been IPI'ed or been preempted if interrupts were
> disabled across. Also, if softirq/preempt is disabled across the
> previous reader section, the previous reader could not be preempted in
> these case.

Like this, courtesy of the consolidation of RCU flavors:

previous_reader()
{
rcu_read_lock();
do_something(); /* Preemption happened here. */
local_irq_disable(); /* Cannot be the scheduler! */
do_something_else();
rcu_read_unlock(); /* Must defer QS, task still queued. */
do_some_other_thing();
local_irq_enable();
}

current_reader() /* QS from previous_reader() is still deferred. */
{
local_irq_disable(); /* Might be the scheduler. */
do_whatever();
rcu_read_lock();
do_whatever_else();
rcu_read_unlock(); /* Must still defer reporting QS. */
do_whatever_comes_to_mind();
local_irq_enable();
}

Both instances of rcu_read_unlock() need to cause some later thing
to report the quiescent state, and in some cases it will do a wakeup.
Now, previous_reader()'s IRQ disabling cannot be due to scheduler rq/pi
locks due to the rule about holding them across the entire RCU reader
if they are held across the rcu_read_unlock(). But current_reader()'s
IRQ disabling might well be due to the scheduler rq/pi locks, so
current_reader() must be careful about doing wakeups.

> That leaves us with the only scenario where the previous reader was
> IPI'ed while softirq/preempt was disabled across it. Is that what you
> meant?

No, but that can also happen.

> But in this scenario, the previous reader should have set
> exp_hint to false in the previous reader's rcu_read_unlock_special()
> invocation itself. So I would think t->rcu_read_unlock_special should
> be 0 during the new reader's invocation thus I did not understand how
> rcu_read_unlock_special can be called because of a previous reader.

Yes, exp_hint would unconditionally be set to false in the first
reader's rcu_read_unlock(). But .blocked won't be.

> I'll borrow some of that confused color paint if you don't mind ;-)
> And we should document this somewhere for future sanity preservation
> :-D

Or adjust the code and requirements to make it more sane, if feasible.

My current (probably wildly unreliable) guess that the conditions in
rcu_read_unlock_special() need adjusting. I was assuming that in_irq()
implies a hardirq context, in other words that in_irq() would return
false from a threaded interrupt handler. If in_irq() instead returns
true from within a threaded interrupt handler, then this code in
rcu_read_unlock_special() needs fixing:

if ((exp || in_irq()) && irqs_were_disabled && use_softirq &&
(in_irq() || !t->rcu_read_unlock_special.b.deferred_qs)) {
// Using softirq, safe to awaken, and we get
// no help from enabling irqs, unlike bh/preempt.
raise_softirq_irqoff(RCU_SOFTIRQ);

The fix would be replacing the calls to in_irq() with something that
returns true only if called from within a hardirq context.

Thoughts?

Ugh. Same question about IRQ work. Will the current use of it by
rcu_read_unlock_special() cause breakage in the presence of threaded
interrupt handlers?

Thanx, Paul

> thanks,
> - Joel
>
>
>
> >
> > I -think- that this is what Sebastian is seeing.
> >
> > Thanx, Paul
> >
> > > 1. and 2. are not possible because interrupts are disabled, that's why the
> > > wakeup_softirq even happened.
> > > 3. is not possible because we are holding rq_lock in the RCU reader section.
> > >
> > > So I am at a bit of a loss how this can happen :-(
> > >
> > > Spurious call to rcu_read_unlock_special() may be when it should not have
> > > been called?
> > >
> > > thanks,
> > >
> > > - Joel