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

[Jason Wang]

On Wed, Jan 28, 2015 at 8:51 PM, Dexuan Cui <de...@microsoft.com> wrote:
 -----Original Message-----
 From: Vitaly Kuznetsov [mailto:vkuzn...@redhat.com]
 Sent: Wednesday, January 28, 2015 20:09 PM
 To: Dexuan Cui
 Cc: KY Srinivasan; de...@linuxdriverproject.org; Haiyang Zhang; 
linux-
 ker...@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 <de...@microsoft.com> writes:
 
 >> -----Original Message-----
 >> From: Vitaly Kuznetsov [mailto:vkuzn...@redhat.com]
 >> Sent: Tuesday, January 20, 2015 23:45 PM
 >> To: KY Srinivasan; de...@linuxdriverproject.org
 >> Cc: Haiyang Zhang; linux-ker...@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. 
:-)

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?

Thanks


  If we still need to do something we need to
 add support for already freed channel to the rescind offer 
processing path.
 

Thanks,
-- Dexuan

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