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

Paul E. McKenney
2019-06-28T17:31:26Z

On Fri, Jun 28, 2019 at 12:45:59PM -0400, Joel Fernandes wrote:
> On Fri, Jun 28, 2019 at 12:40:08PM -0400, Joel Fernandes wrote:
> > On Thu, Jun 27, 2019 at 11:41:07AM -0700, Paul E. McKenney wrote:
> > [snip]
> > > > > > 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?
> > > >
> > > > I am not sure if this will fix all cases though?
> > > >
> > > > I think the crux of the problem is doing a recursive wake up. The threaded
> > > > IRQ probably just happens to be causing it here, it seems to me this problem
> > > > can also occur on a non-threaded irq system (say current_reader() in your
> > > > example executed in a scheduler path in process-context and not from an
> > > > interrupt). Is that not possible?
> > >
> > > In the non-threaded case, invoking raise_softirq*() from hardirq context
> > > just sets a bit in a per-CPU variable. Now, to Sebastian's point, we
> > > are only sort of in hardirq context in this case due to being called
> > > from irq_exit(), but the failure we are seeing might well be a ways
> > > downstream of the actual root-cause bug.
> >
> > Hi Paul,
> > I was talking about calling of rcu_read_unlock_special from a normal process
> > context from the scheduler.
> >
> > In the below traces, it shows that only the PREEMPT_MASK offset is set at the
> > time of the issue. Both HARD AND SOFT IRQ masks are not enabled, which means
> > the lock up is from a normal process context.
> >
> > I think I finally understood why the issue shows up only with threadirqs in
> > my setup. If I build x86_64_defconfig, the CONFIG_IRQ_FORCED_THREADING=y
> > option is set. And booting this with threadirqs, it always tries to
> > wakeup_ksoftirqd in invoke_softirq.
> >
> > I believe what happens is, at an in-opportune time when the .blocked field is
> > set for the preempted task, an interrupt is received. This timing is quite in
> > auspicious because t->rcu_read_unlock_special just happens to have its
> > .blocked field set even though it is not in a reader-section.

Thank you for tracing through this!

> I believe the .blocked field remains set even though we are not any more in a
> reader section because of deferred processing of the blocked lists that you
> mentioned yesterday.

That can indeed happen. However, in current -rcu, that would mean
that .deferred_qs is also set, which (if in_irq()) would prevent
the raise_softirq_irqsoff() from being invoked. Which was why I was
asking the questions about whether in_irq() returns true within threaded
interrupts yesterday. If it does, I need to find if there is some way
of determining whether rcu_read_unlock_special() is being called from
a threaded interrupt in order to suppress the call to raise_softirq()
in that case.

But which version of the kernel are you using here? Current -rcu?
v5.2-rc1? Something else?

Thanx, Paul