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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Dexuan Cui <decui@microsoft.com> writes:

>> -----Original Message-----
>> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com]
>> Sent: Wednesday, January 28, 2015 20:09 PM
>> To: Dexuan Cui
>> Cc: KY Srinivasan; devel@linuxdriverproject.org; Haiyang Zhang; linux-
>> kernel@vger.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@microsoft.com> writes:
>> 
>> >> -----Original Message-----
>> >> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com]
>> >> Sent: Tuesday, January 20, 2015 23:45 PM
>> >> To: KY Srinivasan; devel@linuxdriverproject.org
>> >> Cc: Haiyang Zhang; linux-kernel@vger.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. :-)

Ah, I see, CHANNELMSG_RELID_RELEASED is expected from us in any
case. I'll doing that in a separate patch is noone objects.

Thanks for the review,

>
>>  If we still need to do something we need to
>> add support for already freed channel to the rescind offer processing path.
>> 
>
> Thanks,
> -- Dexuan

-- 
  Vitaly