linux-kernel - Re: [PATCH 1/2] Drivers: hv: vmbus: Introduce table of VMBus protocol versions

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Message-ID: <20191009095423.GA9510@andrea.guest.corp.microsoft.com>
Date:   Wed, 9 Oct 2019 11:54:23 +0200
From:   Andrea Parri <parri.andrea@...il.com>
To:     Dexuan Cui <decui@...rosoft.com>
Cc:     vkuznets <vkuznets@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>
Subject: Re: [PATCH 1/2] Drivers: hv: vmbus: Introduce table of VMBus
 protocol versions

On Tue, Oct 08, 2019 at 10:41:42PM +0000, Dexuan Cui wrote:
> > From: Vitaly Kuznetsov <vkuznets@...hat.com>
> > Sent: Tuesday, October 8, 2019 6:00 AM
> >  ...
> > > Looking at the uses of VERSION_INVAL, I find one remaining occurrence
> > > of this macro in vmbus_bus_resume(), which does:
> > >
> > > 	if (vmbus_proto_version == VERSION_INVAL ||
> > > 	    vmbus_proto_version == 0) {
> > > 		...
> > > 	}
> > >
> > > TBH I'm looking at vmbus_bus_resume() and vmbus_bus_suspend() for the
> > > first time and I'm not sure about how to change such check yet... any
> > > suggestions?
> > 
> > Hm, I don't think vmbus_proto_version can ever become == VERSION_INVAL
> > if we rewrite the code the way you suggest, right? So you'll reduce this
> > to 'if (!vmbus_proto_version)' meaning we haven't negotiated any version
> > (yet).
> 
> Yeah, Vitaly is correct. The check may be a little paranoid as I believe 
> "vmbus_proto_version" must be a negotiated value in vmbus_bus_resume()
> and vmbus_bus_suspend().  I added the check just in case.
> 
> > > Mmh, I see that vmbus_bus_resume() and vmbus_bus_suspend() can access
> > > vmbus_connection.conn_state: can such accesses race with a concurrent
> > > vmbus_connect()?
> > 
> > Let's summon Dexuan who's the author! :-)
> 
> There should not be an issue:
> 
> vmbus_connect() is called in the early subsys_initcall(hv_acpi_init).
> 
> vmbus_bus_suspend() is called late in the PM code after the kernel boots up, e.g.
> in the hibernation function hibernation_snapshot() -> dpm_suspend(). 
> 
> vmbus_bus_resume() is also called later in late_initcall_sync(software_resume).
> 
> In the hibernatin process, vmbus_bus_suspend()/resume() can also be called a
> few times, and vmbus_bus_resume() calls vmbus_negotiate_version(). As I
> checked, there is no issue, either.

Thank you both for these remarks.

So, I'll proceed with the removal of VERSION_INVAL in v2 of this series.

Thanks,
  Andrea