RE: [PATCH] Drivers: hv: vmbus: prevent new subchannel creation on device shutdown

[KY Srinivasan <kys@microsoft.com>]

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com]
> Sent: Monday, July 13, 2015 5:19 AM
> To: devel@linuxdriverproject.org
> Cc: KY Srinivasan; Haiyang Zhang; Dexuan Cui; linux-kernel@vger.kernel.org
> Subject: [PATCH] Drivers: hv: vmbus: prevent new subchannel creation on
> device shutdown
> 
> When a new subchannel offer from host comes during device shutdown
> (e.g.
> when a netvsc/storvsc module is unloadedshortly after it was loaded) a
> crash can happen as vmbus_process_offer() is not anyhow serialized with
> vmbus_remove(). As an example we can try calling subchannel create
> callback when the module is already unloaded.
> The following crash was observed while keeping loading/unloading
> hv_netvsc
> module on 64-CPU guest:
> 
>   hv_netvsc vmbus_14: net device safe to remove
>   BUG: unable to handle kernel paging request at 000000000000a118
>   IP: [<ffffffffa01c1b21>] netvsc_sc_open+0x31/0xb0 [hv_netvsc]
>   PGD 1f3946a067 PUD 1f38a5f067 PMD 0
>   Oops: 0000 [#1] SMP
>   ...
>   Call Trace:
>   [<ffffffffa0055cd7>] vmbus_onoffer+0x477/0x530 [hv_vmbus]
>   [<ffffffff81092e1f>] ? move_linked_works+0x5f/0x80
>   [<ffffffffa0055fd3>] vmbus_onmessage+0x33/0xa0 [hv_vmbus]
>   [<ffffffffa0052f81>] vmbus_onmessage_work+0x21/0x30 [hv_vmbus]
>   [<ffffffff81095cde>] process_one_work+0x18e/0x4e0
>   ...
> 
> The issue cannot be solved by just resetting sc_creation_callback on
> driver removal as while we search for the parent channel with channel_lock
> held we release it after the channel was found and it can disapper beneath
> our feet while we're still in vmbus_process_offer();
> 
> Introduce new sc_create_lock mutex and take it in vmbus_remove() to
> ensure
> no new subchannels are created after we started the removal procedure.
> Check its state with mutex_trylock in vmbus_process_offer().
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>

Thanks Vitaly; strangely enough, I too am looking at this exact problem. I was planning to
address this problem a little differently: I was planning to hold the context that performed
the initial (primary) channel open until all of the "open activity" was completed and this would include
opening of the sub-channels for multi-channel devices. 

Regards,

K. Y

> ---
>  drivers/hv/channel.c      |  3 +++
>  drivers/hv/channel_mgmt.c | 20 ++++++++++++++++++--
>  drivers/hv/vmbus_drv.c    |  6 ++++++
>  include/linux/hyperv.h    |  4 ++++
>  4 files changed, 31 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/channel.c b/drivers/hv/channel.c
> index 603ce97..faa91fe 100644
> --- a/drivers/hv/channel.c
> +++ b/drivers/hv/channel.c
> @@ -555,6 +555,9 @@ static int vmbus_close_internal(struct vmbus_channel
> *channel)
>  	if (channel->rescind)
>  		hv_process_channel_removal(channel,
>  					   channel->offermsg.child_relid);
> +	else if (mutex_is_locked(&channel->sc_create_lock))
> +		mutex_unlock(&channel->sc_create_lock);
> +
>  	return ret;
>  }
> 
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> index 4506a66..96f8b96 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -150,6 +150,8 @@ static struct vmbus_channel *alloc_channel(void)
>  	INIT_LIST_HEAD(&channel->sc_list);
>  	INIT_LIST_HEAD(&channel->percpu_list);
> 
> +	mutex_init(&channel->sc_create_lock);
> +
>  	return channel;
>  }
> 
> @@ -158,6 +160,7 @@ static struct vmbus_channel *alloc_channel(void)
>   */
>  static void free_channel(struct vmbus_channel *channel)
>  {
> +	mutex_destroy(&channel->sc_create_lock);
>  	kfree(channel);
>  }
> 
> @@ -247,8 +250,18 @@ static void vmbus_process_offer(struct
> vmbus_channel *newchannel)
>  			newchannel->offermsg.offer.if_type) &&
>  			!uuid_le_cmp(channel->offermsg.offer.if_instance,
>  				newchannel->offermsg.offer.if_instance)) {
> -			fnew = false;
> -			break;
> +			if (mutex_trylock(&channel->sc_create_lock)) {
> +				fnew = false;
> +				break;
> +			}
> +			/*
> +			 * If we fail to acquire mutex here it means we're
> +			 * closing parent channel at this moment. We can't
> +			 * create new subchannel in this case.
> +			 */
> +
> 	spin_unlock_irqrestore(&vmbus_connection.channel_lock,
> +					       flags);
> +			goto err_free_chan;
>  		}
>  	}
> 
> @@ -297,6 +310,7 @@ static void vmbus_process_offer(struct
> vmbus_channel *newchannel)
>  	if (!fnew) {
>  		if (channel->sc_creation_callback != NULL)
>  			channel->sc_creation_callback(newchannel);
> +		mutex_unlock(&channel->sc_create_lock);
>  		return;
>  	}
> 
> @@ -340,6 +354,8 @@ err_deq_chan:
>  	}
> 
>  err_free_chan:
> +	if (!fnew)
> +		mutex_unlock(&channel->sc_create_lock);
>  	free_channel(newchannel);
>  }
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index cf20400..7ad7fcc 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -539,6 +539,12 @@ static int vmbus_remove(struct device
> *child_device)
>  	struct hv_device *dev = device_to_hv_device(child_device);
>  	u32 relid = dev->channel->offermsg.child_relid;
> 
> +	/*
> +	 * Device is being removed, prevent creating new subchannels.
> Mutex
> +	 * will be released in vmbus_close_internal().
> +	 */
> +	mutex_lock(&dev->channel->sc_create_lock);
> +
>  	if (child_device->driver) {
>  		drv = drv_to_hv_drv(child_device->driver);
>  		if (drv->remove)
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index 30d3a1f..f1af05c 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -748,6 +748,10 @@ struct vmbus_channel {
>  	 */
>  	struct vmbus_channel *primary_channel;
>  	/*
> +	 * Protects sub-channel create path.
> +	 */
> +	struct mutex sc_create_lock;
> +	/*
>  	 * Support per-channel state for use by vmbus drivers.
>  	 */
>  	void *per_channel_state;
> --
> 2.4.3