linux-kernel - RE: [PATCH v3 3/3] Drivers: hv: vmbus: serialize Offer and Rescind offer

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Date:	Fri, 30 Jan 2015 04:21:24 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	Jason Wang <jasowang@...hat.com>
CC:	Vitaly Kuznetsov <vkuznets@...hat.com>,
	KY Srinivasan <kys@...rosoft.com>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Dan Carpenter <dan.carpenter@...cle.com>
Subject: RE: [PATCH v3 3/3] Drivers: hv: vmbus: serialize Offer and Rescind
 offer

> -----Original Message-----
> From: Jason Wang [mailto:jasowang@...hat.com]
> Sent: Thursday, January 29, 2015 18:09 PM
> To: Dexuan Cui
> Cc: Vitaly Kuznetsov; KY Srinivasan; devel@...uxdriverproject.org; Haiyang Zhang;
> linux-kernel@...r.kernel.org; Radim Krčmář; Dan Carpenter
> Subject: RE: [PATCH v3 3/3] Drivers: hv: vmbus: serialize Offer and Rescind offer
> 
> 
> 
> On Wed, Jan 28, 2015 at 8:51 PM, Dexuan Cui <decui@...rosoft.com> wrote:
> >>  -----Original Message-----
> >>  From: Vitaly Kuznetsov [mailto:vkuznets@...hat.com]
> >>  Sent: Wednesday, January 28, 2015 20:09 PM
> >>  To: Dexuan Cui
> >>  Cc: KY Srinivasan; devel@...uxdriverproject.org; Haiyang Zhang;
> >> linux-
> >>  kernel@...r.kernel.org; Jason Wang; Radim Krčmář; Dan Carpenter
> >>  Subject: Re: [PATCH v3 3/3] Drivers: hv: vmbus: serialize Offer and
> >> Rescind
> >>  offer
> >>
> >>  Dexuan Cui <decui@...rosoft.com> writes:
> >>
> >>  >> -----Original Message-----
> >>  >> From: Vitaly Kuznetsov [mailto:vkuznets@...hat.com]
> >>  >> Sent: Tuesday, January 20, 2015 23:45 PM
> >>  >> To: KY Srinivasan; devel@...uxdriverproject.org
> >>  >> Cc: Haiyang Zhang; linux-kernel@...r.kernel.org; Dexuan Cui;
> >> Jason Wang;
> >>  >> Radim Krčmář; Dan Carpenter
> >>  >> Subject: [PATCH v3 3/3] Drivers: hv: vmbus: serialize Offer and
> >> Rescind
> >>  offer
> >>  ...
> >>  >
> >>  > Hi Vitaly and all,
> >>  > I have 2 questions:
> >>  > In vmbus_process_offer(),  in the cases of "goto err_free_chan",
> >>  > should we consider the possibility a rescind message could be
> >> pending for
> >>  > the new channel?
> >>  > In the cases,  because we don't run
> >>  > "INIT_WORK(&newchannel->work, vmbus_process_rescind_offer); ",
> >>  > vmbus_onoffer_rescind() will do nothing and as a result,
> >>  > vmbus_process_rescind_offer() won't be invoked.
> >>
> >>  Yes, but processing the rescind offer results in freeing the channel
> >>  (and this processing supposes the channel wasn't freed before) so
> >>  there is no difference... or is it?
> >>
> >>  >
> >>  > Question 2: in vmbus_process_offer(), in the case
> >>  > vmbus_device_register() fails, we'll run
> >>  > "list_del(&newchannel->listentry);" -- just after this line,
> >>  >  what will happen at this time if relid2channel() returns NULL
> >>  > in vmbus_onoffer_rescind()?
> >>  >
> >>  > I think we'll lose the rescind message.
> >>  >
> >>
> >>  Yes, but same logic applies - we already freed the channes so no
> >> rescind
> >>  proccessing required.
> > free_channel() and vmbus_process_rescind_offer() are different,
> > because
> >  the latter does more work, e.g., sending the host a message
> > CHANNELMSG_RELID_RELEASED.
> >
> > In the cases of "goto err_free_chan" + "a pending rescind message",
> > the host may expect the message CHANNELMSG_RELID_RELEASED and
> > could reoffer the channel once the message is received.
> >
> > It would be better if the VM doesn't lose the rescind message here.
> > :-)
> 
> It's interesting that rescind needs a ack from guest. But looks like
> the offer does not need it? Is there a spec for this for us for
> reference?

My understanding is:
The host may reuse the same relid after the VM acks the rescind message.

I don't have a VMBus spec either.

> >
> >
> >>   If we still need to do something we need to
> >>  add support for already freed channel to the rescind offer
> >> processing path.
> >>
This sounds reasonable to me.
Error handling is always full of  various corner cases...

Thanks,
-- Dexuan