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

[KY Srinivasan]

> -----Original Message-----
> From: Dexuan Cui
> Sent: Thursday, January 29, 2015 8:21 PM
> To: Jason Wang
> Cc: Vitaly Kuznetsov; KY Srinivasan; devel@xxxxxxxxxxxxxxxxxxxxxx; Haiyang
> Zhang; linux-kernel@xxxxxxxxxxxxxxx; Radim KrÄmÃÅ; Dan Carpenter
> Subject: RE: [PATCH v3 3/3] Drivers: hv: vmbus: serialize Offer and Rescind
> offer
> 
> > -----Original Message-----
> > From: Jason Wang [mailto:jasowang@xxxxxxxxxx]
> > Sent: Thursday, January 29, 2015 18:09 PM
> > To: Dexuan Cui
> > Cc: Vitaly Kuznetsov; KY Srinivasan; devel@xxxxxxxxxxxxxxxxxxxxxx;
> > Haiyang Zhang; linux-kernel@xxxxxxxxxxxxxxx; Radim KrÄmÃÅ; 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@xxxxxxxxxxxxx>
> wrote:
> > >>  -----Original Message-----
> > >>  From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
> > >>  Sent: Wednesday, January 28, 2015 20:09 PM
> > >>  To: Dexuan Cui
> > >>  Cc: KY Srinivasan; devel@xxxxxxxxxxxxxxxxxxxxxx; Haiyang Zhang;
> > >> linux-
> > >>  kernel@xxxxxxxxxxxxxxx; Jason Wang; Radim KrÄmÃÅ; Dan Carpenter
> > >>  Subject: Re: [PATCH v3 3/3] Drivers: hv: vmbus: serialize Offer
> > >> and Rescind  offer
> > >>
> > >>  Dexuan Cui <decui@xxxxxxxxxxxxx> writes:
> > >>
> > >>  >> -----Original Message-----
> > >>  >> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]  >> Sent:
> > >> Tuesday, January 20, 2015 23:45 PM  >> To: KY Srinivasan;
> > >> devel@xxxxxxxxxxxxxxxxxxxxxx  >> Cc: Haiyang Zhang;
> > >> linux-kernel@xxxxxxxxxxxxxxx; Dexuan Cui; Jason Wang;  >> Radim
> > >> KrÄmÃÅ; 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.

That is correct. This change was I think introduced recently (requiring the guest to release the RelID).

K. Y
> 
> > >
> > >
> > >>   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