Re: [PATCH] smp: Do not warn if smp_call_function_single() is doing a self call.

[Peter Zijlstra]

On Mon, Apr 15, 2019 at 11:39:57PM +0000, Dexuan Cui wrote:
> > From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Sent: Monday, April 15, 2019 5:21 AM
> > To: Dexuan Cui <decui@xxxxxxxxxxxxx>
> > 
> > On Fri, Apr 12, 2019 at 11:53:57PM +0000, Dexuan Cui wrote:
> > > If smp_call_function_single() is calling the function for itself, it's safe
> > > to run with irqs_disabled() == true.
> > >
> > > I hit the warning because I'm in the below path in the .suspend callback of
> > > a "syscore_ops" to support hibernation for a VM running on Hyper-V:
> > >
> > >   hv_synic_cleanup() ->
> > >     clockevents_unbind_device() ->
> > >       clockevents_unbind() ->
> > >         smp_call_function_single().
> > >
> > > When the .suspend callback runs, only CPU0 is online and irqs_disabled() is
> > > true.
> > 
> > Pray tell, how well do you think mutex_lock() works with interrupts
> > disabled?
> 
> Good point. I realized generally speaking this patch makes no sense, so let me
> try the solution proposed by Vitaly, i.e. fix clockevents_unbind() instead.

That's still not the problem. You're calling clockevents_unbind_device()
with IRQs disabled, that's not correct. It doesn't matter what
clockevents_unbind() does thereafter.

You simply cannot do any of this with IRQs disabled, end of story.