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

[Peter Zijlstra]

On Fri, Jun 28, 2019 at 10:20:56AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 28, 2019 at 06:04:08PM +0200, Peter Zijlstra wrote:
> > On Fri, Jun 28, 2019 at 08:54:04AM -0700, Paul E. McKenney wrote:
> > > Thank you!  Plus it looks like scheduler_ipi() takes an early exit if
> > > ->wake_list is empty, regardless of TIF_NEED_RESCHED, right?
> > 
> > Yes, TIF_NEED_RESCHED is checked in the interrupt return path.
> 
> OK, got it.  So the following sequence would be a valid way to get the
> scheduler's attention on the current CPU shortly after interrupts
> are re-enabled, even if the current CPU is already holding some
> rq or pi locks, correct?
> 
> 	set_tsk_need_resched(current);
> 	set_preempt_need_resched();
> 	smp_send_reschedule(smp_processor_id());

I'm not sure if smp_send_reschedule() can be used as self-IPI, some
hardware doesn't particularly like that IIRC. That is, hardware might
only have interfaces to IPI _other_ CPUs, but not self.

The normal scheduler code takes care to not call smp_send_reschedule()
to self.