Re: Query regarding synchronize_sched_expedited and resched_cpu

[Mike Galbraith <efault@gmx.de>]

On Tue, 2017-09-19 at 13:37 +0800, Boqun Feng wrote:
> On Mon, Sep 18, 2017 at 09:04:56PM -0700, Paul E. McKenney wrote:
> > On Tue, Sep 19, 2017 at 11:48:22AM +0900, Byungchul Park wrote:
> > > On Mon, Sep 18, 2017 at 07:33:29PM -0700, Paul E. McKenney wrote:
> > > > > > Hello Paul and Steven,
> > > > > > 
> 
> So I think this is another false positive, and the reason is we use
> st->done for multiple purposes.
> 
> > > > > > This is saying:
> > > > > > 
> > > > > > Thread A
> > > > > > --------
> > > > > > takedown_cpu()
> > > > > >    irq_lock_sparse()
> > > > > >    wait_for_completion(&st->done) // Wait for completion of B
> 
> Thread A wait for the idle task on the outgoing to set the st->state to 
> CPUHP_AP_IDLE_DEAD(i.e. the corresponding complete() is the one in
> cpuhp_complete_idle_dead()), and it happens when we try to _offline_ a
> cpu.
> 
> > > > > >    irq_unlock_sparse()
> > > > > > 
> > > > > > Thread B
> > > > > > --------
> > > > > > cpuhp_invoke_callback()
> > > > > >    irq_lock_sparse() // Wait for A to irq_unlock_sparse()
> 
> irq_affinity_online_cpu() is called here, so it happens when we try to
> _online_ a cpu.
> 
> > > > > >    (on the way going to complete(&st->done))
> 
> and we are going to complete(&st->done) in a hotplug thread context to
> indicate the hotplug thread has finished its job(i.e. this complete() is
> the one in cpuhp_thread_fun()).
> 
> 
> So even though the &st->done are the same instance, the deadlock could
> not happen, I think, as we could not up/down a same cpu at the same
> time?
> 
> If I'm not missing something subtle. To fix this we can either
> 
> 1)	have dedicated completion instances for different wait purposes
> 	in cpuhp_cpu_state.
> 
> or
> 
> 2)	extend crossrelease to have the "subclass" concept, so that
> 	callsite of complete() and wait_for_completion() for the same
> 	completion instance but with different purposes could be
> 	differed by lockdep.
> 
> Thoughts?

https://lkml.org/lkml/2017/9/5/184

Peter's patches worked for me, but per tglx, additional (non-
grasshopper level) hotplug-fu is required.

	-Mike