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

[Vitaly Kuznetsov]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> 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.
>

True. And before we start digging deeper into this, let's step back: why
do we need to do clockevents_unbind_device() on hybernation? Can we just
disable the device and re-enable it back on resume?

Actually, all usages of clockevents_unbind_device() in kernel are
limited to Hyper-V and with Michael's patches moving this out of VMBus
driver I think it can go away completely.

-- 
Vitaly