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

[KY Srinivasan]

> -----Original Message-----
> From: Jason Wang [mailto:jasowang@xxxxxxxxxx]
> Sent: Thursday, January 29, 2015 2:07 AM
> 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 7:57 PM, Dexuan Cui <decui@xxxxxxxxxxxxx> wrote:
> >>  -----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
> >>
> >>  Commit 4b2f9abea52a ("staging: hv: convert channel_mgmt.c to not
> >> call  osd_schedule_callback")' was written under an assumption that
> >> we never  receive  Rescind offer while we're still processing the
> >> initial Offer request. However,  the issue we fixed in 04a258c162a8
> >> could be caused by this assumption not  always being true.
> >>
> >>  In particular, we need to protect against the following:
> >>  1) Receiving a Rescind offer after we do queue_work() for processing
> >> an  Offer
> >>     request and before we actually enter vmbus_process_offer().
> >> work.func
> >>  points
> >>     to vmbus_process_offer() at this moment and in
> >> vmbus_onoffer_rescind()
> >>  we do
> >>     another queue_work() without a check so we'll enter
> >>  vmbus_process_offer()
> >>     twice.
> >>  2) Receiving a Rescind offer after we enter vmbus_process_offer()
> >> and
> >>     especially after we set >state = CHANNEL_OPEN_STATE. Many things
> >> can go
> >>     wrong in that case, e.g. we can call free_channel() while we're
> >> still using
> >>     it.
> >>
> >>  Implement the required protection by changing work->func at the very
> >> end  of
> >>  vmbus_process_offer() and checking work->func in
> >> vmbus_onoffer_rescind().
> >>  In
> >>  case we receive rescind offer during or before
> >> vmbus_process_offer() is
> >>  done
> >>  we set rescind flag to true and we check it at the end of
> >>  vmbus_process_offer()
> >>  so such offer will not get lost.
> >>
> >>  Suggested-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> >>  Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> >>  ---
> >>   drivers/hv/channel_mgmt.c | 30 ++++++++++++++++++++++--------
> >>   1 file changed, 22 insertions(+), 8 deletions(-)
> >>
> >>  diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> >> index c6fdd74..877a944 100644
> >>  --- a/drivers/hv/channel_mgmt.c
> >>  +++ b/drivers/hv/channel_mgmt.c
> >>  @@ -279,9 +279,6 @@ static void vmbus_process_offer(struct
> >> work_struct
> >>  *work)
> >>   	int ret;
> >>   	unsigned long flags;
> >>
> >>  -	/* The next possible work is rescind handling */
> >>  -	INIT_WORK(&newchannel->work, vmbus_process_rescind_offer);
> >>  -
> >>   	/* Make sure this is a new offer */
> >>   	spin_lock_irqsave(&vmbus_connection.channel_lock, flags);
> >>
> >>  @@ -341,7 +338,7 @@ static void vmbus_process_offer(struct
> >> work_struct
> >>  *work)
> >>   			if (channel->sc_creation_callback != NULL)
> >>   				channel->sc_creation_callback(newchannel);
> >>
> >>  -			goto out;
> >>  +			goto done_init_rescind;
> >>   		}
> >>
> >>   		goto err_free_chan;
> >>  @@ -382,7 +379,14 @@ static void vmbus_process_offer(struct
> >> work_struct
> >>  *work)
> >>   		kfree(newchannel->device_obj);
> >>   		goto err_free_chan;
> >>   	}
> >>  -out:
> >>  +done_init_rescind:
> >>  +	spin_lock_irqsave(&newchannel->lock, flags);
> >>  +	/* The next possible work is rescind handling */
> >>  +	INIT_WORK(&newchannel->work, vmbus_process_rescind_offer);
> >>  +	/* Check if rescind offer was already received */
> >>  +	if (newchannel->rescind)
> >>  +		queue_work(newchannel->controlwq, &newchannel-
> >work);
> >>  +	spin_unlock_irqrestore(&newchannel->lock, flags);
> >>   	return;
> >>   err_free_chan:
> >>   	free_channel(newchannel);
> >>  @@ -520,6 +524,7 @@ static void vmbus_onoffer_rescind(struct
> >> vmbus_channel_message_header *hdr)
> >>   {
> >>   	struct vmbus_channel_rescind_offer *rescind;
> >>   	struct vmbus_channel *channel;
> >>  +	unsigned long flags;
> >>
> >>   	rescind = (struct vmbus_channel_rescind_offer *)hdr;
> >>   	channel = relid2channel(rescind->child_relid);
> >>  @@ -528,11 +533,20 @@ static void vmbus_onoffer_rescind(struct
> >> vmbus_channel_message_header *hdr)
> >>   		/* Just return here, no channel found */
> >>   		return;
> >>
> >>  +	spin_lock_irqsave(&channel->lock, flags);
> >>   	channel->rescind = true;
> >>  +	/*
> >>  +	 * channel->work.func != vmbus_process_rescind_offer means we
> >>  are still
> >>  +	 * processing offer request and the rescind offer processing
> >> should
> >>  be
> >>  +	 * postponed. It will be done at the very end of
> >>  vmbus_process_offer()
> >>  +	 * as rescind flag is being checked there.
> >>  +	 */
> >>  +	if (channel->work.func == vmbus_process_rescind_offer)
> >>  +		/* work is initialized for vmbus_process_rescind_offer() from
> >>  +		 * vmbus_process_offer() where the channel got created */
> >>  +		queue_work(channel->controlwq, &channel->work);
> >>
> >>  -	/* work is initialized for vmbus_process_rescind_offer() from
> >>  -	 * vmbus_process_offer() where the channel got created */
> >>  -	queue_work(channel->controlwq, &channel->work);
> >>  +	spin_unlock_irqrestore(&channel->lock, flags);
> >>   }
> >>
> >>   /*
> >>  --
> >
> > 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.
> >
> > 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.
> 
> If CHANNELMSG_RELID_RELEASED is mandatory, need INIT_WORK during
> onoffer_rescind() unconditionally and and we need post this message
> without the help of relid2channel() since:
> 
> - relid2channel() only valid when the channel was added to list, so either in
> the case of question 2 or before list_add_tail() in
> vmbus_process_offer()
> - the channel rescind offer message has a relid
> 
Once the channel is rescinded from the host, we need to send the RELID_RELEASED message to the host
for the host to potentially resend the offer.

K. Y 
> >
> >
> > Thanks,
> > -- Dexuan