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

[Dexuan Cui]

> -----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. 

Thanks,
-- Dexuan