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

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Dexuan Cui <decui@microsoft.com>
Cc: "tglx\@linutronix.de" <tglx@linutronix.de>,
	"riel\@surriel.com" <riel@surriel.com>,
	"jpoimboe\@redhat.com" <jpoimboe@redhat.com>,
	"luto\@kernel.org" <luto@kernel.org>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Sasha Levin <Alexander.Levin@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	KY Srinivasan <kys@microsoft.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Michael Kelley <mikelley@microsoft.com>,
	"marcelo.cerri\@canonical.com" <marcelo.cerri@canonical.com>,
	"apw\@canonical.com" <apw@canonical.com>,
	"olaf\@aepfle.de" <olaf@aepfle.de>,
	"jasowang\@redhat.com" <jasowang@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] smp: Do not warn if smp_call_function_single() is doing a self call.
Date: Tue, 16 Apr 2019 13:20:19 +0200	[thread overview]
Message-ID: <87o9562o3w.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20190416093123.GR11158@hirez.programming.kicks-ass.net>

Peter Zijlstra <peterz@infradead.org> writes:

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