linux-kernel - Re: [PATCH] smp: Do not warn if smp_call_function

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Date:   Tue, 16 Apr 2019 13:20:19 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Dexuan Cui <decui@...rosoft.com>
Cc:     "tglx\@linutronix.de" <tglx@...utronix.de>,
        "riel\@surriel.com" <riel@...riel.com>,
        "jpoimboe\@redhat.com" <jpoimboe@...hat.com>,
        "luto\@kernel.org" <luto@...nel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <Alexander.Levin@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        "marcelo.cerri\@canonical.com" <marcelo.cerri@...onical.com>,
        "apw\@canonical.com" <apw@...onical.com>,
        "olaf\@aepfle.de" <olaf@...fle.de>,
        "jasowang\@redhat.com" <jasowang@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] smp: Do not warn if smp_call_function_single() is doing a self call.

Peter Zijlstra <peterz@...radead.org> writes:

> On Mon, Apr 15, 2019 at 11:39:57PM +0000, Dexuan Cui wrote:
>> > From: Peter Zijlstra <peterz@...radead.org>
>> > Sent: Monday, April 15, 2019 5:21 AM
>> > To: Dexuan Cui <decui@...rosoft.com>
>> > 
>> > 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