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SubjectRe: Query regarding synchronize_sched_expedited and resched_cpu
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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

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