RE: [PATCH 3/4] Drivers: hv: vmbus: protect vmbus_get_outgoing_channel() against channel removal

[Dexuan Cui <decui@microsoft.com>]

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com]
> Sent: Wednesday, February 4, 2015 1:01 AM
> To: KY Srinivasan; devel@linuxdriverproject.org
> Cc: Haiyang Zhang; linux-kernel@vger.kernel.org; Dexuan Cui; Jason Wang
> Subject: [PATCH 3/4] Drivers: hv: vmbus: protect vmbus_get_outgoing_channel()
> against channel removal
> 
> list_for_each_safe() we have in vmbus_get_outgoing_channel() works, however,
> we
> are not protected against the channel being removed (e.g. after receiving
> rescind
> offer). Users of this function (storvsc_do_io() is the only one at this moment)
> can get a link to an already freed channel. Make vmbus_get_outgoing_channel()
> search holding primary->lock as child channels are not being freed unless they're
> removed from parent's list.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  drivers/hv/channel_mgmt.c  | 10 +++++++---
>  drivers/scsi/storvsc_drv.c |  2 ++
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> index fdccd16..af6243c 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -881,18 +881,20 @@ cleanup:
>   */
>  struct vmbus_channel *vmbus_get_outgoing_channel(struct vmbus_channel
> *primary)
>  {
> -	struct list_head *cur, *tmp;
> +	struct list_head *cur;
>  	int cur_cpu;
>  	struct vmbus_channel *cur_channel;
>  	struct vmbus_channel *outgoing_channel = primary;
>  	int cpu_distance, new_cpu_distance;
> +	unsigned long flags;
> 
>  	if (list_empty(&primary->sc_list))
> -		return outgoing_channel;
> +		return vmbus_get_channel(outgoing_channel);
> 
>  	cur_cpu = hv_context.vp_index[get_cpu()];
>  	put_cpu();
> -	list_for_each_safe(cur, tmp, &primary->sc_list) {
> +	spin_lock_irqsave(&primary->lock, flags);
hmm, we should avoid the locking here because it's a performance killer...
How about adding vmbus_get/put_channel() in vmbus_open/close()?

Thanks,
-- Dexuan

> +	list_for_each(cur, &primary->sc_list) {
>  		cur_channel = list_entry(cur, struct vmbus_channel, sc_list);
>  		if (cur_channel->state != CHANNEL_OPENED_STATE)
>  			continue;
> @@ -913,6 +915,8 @@ struct vmbus_channel
> *vmbus_get_outgoing_channel(struct vmbus_channel *primary)
> 
>  		outgoing_channel = cur_channel;
>  	}
> +	outgoing_channel = vmbus_get_channel(outgoing_channel);
> +	spin_unlock_irqrestore(&primary->lock, flags);
> 
>  	return outgoing_channel;
>  }
> diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> index 4cff0dd..3b9b851 100644
> --- a/drivers/scsi/storvsc_drv.c
> +++ b/drivers/scsi/storvsc_drv.c
> @@ -1370,6 +1370,8 @@ static int storvsc_do_io(struct hv_device *device,
> 
> VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED);
>  	}
> 
> +	vmbus_put_channel(outgoing_channel);
> +
>  	if (ret != 0)
>  		return ret;
> 
> --
> 1.9.3