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

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Message-ID: <F792CF86EFE20D4AB8064279AFBA51C61EA9510B@SIXPRD3002MB028.064d.mgd.msft.net>
Date:	Wed, 28 Jan 2015 12:51:42 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>
CC:	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>,
	Jason Wang <jasowang@...hat.com>,
	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: 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. :-)

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

Thanks,
-- Dexuan