RE: [PATCH 1/4] Drivers: hv: vmbus: implement get/put usage workflow for vmbus channels

[Dexuan Cui]

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
> Sent: Wednesday, February 4, 2015 1:01 AM
> To: KY Srinivasan; devel@xxxxxxxxxxxxxxxxxxxxxx
> Cc: Haiyang Zhang; linux-kernel@xxxxxxxxxxxxxxx; Dexuan Cui; Jason Wang
> Subject: [PATCH 1/4] Drivers: hv: vmbus: implement get/put usage workflow for
> vmbus channels
> 
> free_channel() function frees the channel unconditionally so we need to make
> sure nobody has any link to it. This is not trivial and there are several
> examples of races we have:
> 
> 1) In vmbus_onoffer_rescind() we check for channel existence with
>    relid2channel() and then use it. This can go wrong if we're in the middle
>    of channel removal (free_channel() was already called).
> 
> 2) In process_chn_event() we check for channel existence with
>    pcpu_relid2channel() and then use it. This can also go wrong.
> 
> 3) vmbus_free_channels() just frees all channels, in case we're in the middle
>    of vmbus_process_rescind_offer() crash is possible.
> 
> The issue can be solved by holding vmbus_connection.channel_lock everywhere,
> however, it looks like a way to deadlocks and performance degradation. Get/put
> workflow fits here the best.
> 
> Implement vmbus_get_channel()/vmbus_put_channel() pair instead of
> free_channel().
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  drivers/hv/channel_mgmt.c | 45
> ++++++++++++++++++++++++++++++++++++++-------
>  drivers/hv/connection.c   |  7 +++++--
>  drivers/hv/hyperv_vmbus.h |  4 ++++
>  include/linux/hyperv.h    | 13 +++++++++++++
>  4 files changed, 60 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> index 36bacc7..eb9ce94 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -147,6 +147,8 @@ static struct vmbus_channel *alloc_channel(void)
>  		return NULL;
> 
>  	channel->id = atomic_inc_return(&chan_num);
> +	atomic_set(&channel->count, 1);
> +
>  	spin_lock_init(&channel->inbound_lock);
>  	spin_lock_init(&channel->lock);
> 
> @@ -178,19 +180,47 @@ static void release_channel(struct work_struct *work)
>  }
> 
>  /*
> - * free_channel - Release the resources used by the vmbus channel object
> + * vmbus_put_channel - Decrease the channel usage counter and release the
> + * resources when this counter reaches zero.
>   */
> -static void free_channel(struct vmbus_channel *channel)
> +void vmbus_put_channel(struct vmbus_channel *channel)
>  {
> +	unsigned long flags;
> 
>  	/*
>  	 * We have to release the channel's workqueue/thread in the vmbus's
>  	 * workqueue/thread context
>  	 * ie we can't destroy ourselves.
>  	 */
> -	INIT_WORK(&channel->work, release_channel);
> -	queue_work(vmbus_connection.work_queue, &channel->work);
> +	spin_lock_irqsave(&channel->lock, flags);
> +	if (atomic_dec_and_test(&channel->count)) {
> +		channel->dying = true;
> +		INIT_WORK(&channel->work, release_channel);
> +		spin_unlock_irqrestore(&channel->lock, flags);
> +		queue_work(vmbus_connection.work_queue, &channel->work);
> +	} else
> +		spin_unlock_irqrestore(&channel->lock, flags);
> +}
> +EXPORT_SYMBOL_GPL(vmbus_put_channel);
> +
> +/* vmbus_get_channel - Get additional reference to the channel */
> +struct vmbus_channel *vmbus_get_channel(struct vmbus_channel *channel)
> +{
> +	unsigned long flags;
> +	struct vmbus_channel *ret = NULL;
> +
> +	if (!channel)
> +		return NULL;
> +
> +	spin_lock_irqsave(&channel->lock, flags);
> +	if (!channel->dying) {
> +		atomic_inc(&channel->count);
> +		ret = channel;
> +	}
> +	spin_unlock_irqrestore(&channel->lock, flags);
> +	return ret;
>  }
> +EXPORT_SYMBOL_GPL(vmbus_get_channel);
> 
>  static void percpu_channel_enq(void *arg)
>  {
> @@ -253,7 +283,7 @@ static void vmbus_process_rescind_offer(struct
> work_struct *work)
>  		list_del(&channel->sc_list);
>  		spin_unlock_irqrestore(&primary_channel->lock, flags);
>  	}
> -	free_channel(channel);
> +	vmbus_put_channel(channel);
>  }
> 
>  void vmbus_free_channels(void)
> @@ -262,7 +292,7 @@ void vmbus_free_channels(void)
> 
>  	list_for_each_entry(channel, &vmbus_connection.chn_list, listentry) {
>  		vmbus_device_unregister(channel->device_obj);
> -		free_channel(channel);
> +		vmbus_put_channel(channel);
>  	}
>  }
> 
> @@ -391,7 +421,7 @@ done_init_rescind:
>  	spin_unlock_irqrestore(&newchannel->lock, flags);
>  	return;
>  err_free_chan:
> -	free_channel(newchannel);
> +	vmbus_put_channel(newchannel);
>  }
> 
>  enum {
> @@ -549,6 +579,7 @@ static void vmbus_onoffer_rescind(struct
> vmbus_channel_message_header *hdr)
>  		queue_work(channel->controlwq, &channel->work);
> 
>  	spin_unlock_irqrestore(&channel->lock, flags);
> +	vmbus_put_channel(channel);
>  }
> 
>  /*
> diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
> index c4acd1c..d1ce134 100644
> --- a/drivers/hv/connection.c
> +++ b/drivers/hv/connection.c
> @@ -247,7 +247,8 @@ void vmbus_disconnect(void)
>   * Map the given relid to the corresponding channel based on the
>   * per-cpu list of channels that have been affinitized to this CPU.
>   * This will be used in the channel callback path as we can do this
> - * mapping in a lock-free fashion.
> + * mapping in a lock-free fashion. Takes additional reference to the
> + * channel, all users are supposed to do vmbus_put_channel().
>   */
>  static struct vmbus_channel *pcpu_relid2channel(u32 relid)
>  {
> @@ -263,7 +264,7 @@ static struct vmbus_channel *pcpu_relid2channel(u32
> relid)
>  		}
>  	}
> 
> -	return found_channel;
> +	return vmbus_get_channel(found_channel);
>  }
> 
>  /*
> @@ -297,6 +298,7 @@ struct vmbus_channel *relid2channel(u32 relid)
>  			}
>  		}
>  	}
> +	found_channel = vmbus_get_channel(found_channel);
>  	spin_unlock_irqrestore(&vmbus_connection.channel_lock, flags);
> 
>  	return found_channel;
> @@ -360,6 +362,7 @@ static void process_chn_event(u32 relid)
>  		pr_err("no channel callback for relid - %u\n", relid);
>  	}
> 
> +	vmbus_put_channel(channel);
>  }
> 
>  /*
> diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
> index b055e53..40d70f0 100644
> --- a/drivers/hv/hyperv_vmbus.h
> +++ b/drivers/hv/hyperv_vmbus.h
> @@ -687,6 +687,10 @@ void vmbus_device_unregister(struct hv_device
> *device_obj);
>  /* VmbusChildDeviceDestroy( */
>  /* struct hv_device *); */
> 
> +/*
> + * Get the channel by its relid. Takes additional reference to the channel so
> + * all users are supposed to do vmbus_put_channel() when they're done.
> + */
>  struct vmbus_channel *relid2channel(u32 relid);
> 
>  void vmbus_free_channels(void);
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index e73cfeb..c576d2d 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -649,6 +649,9 @@ struct vmbus_channel {
>  	/* Unique channel id */
>  	int id;
> 
> +	/* Active reference count */
> +	atomic_t count;
> +
>  	struct list_head listentry;
> 
>  	struct hv_device *device_obj;
> @@ -666,6 +669,7 @@ struct vmbus_channel {
>  	u8 monitor_bit;
> 
>  	bool rescind; /* got rescind msg */
> +	bool dying; /* channel is dying */
> 
>  	u32 ringbuffer_gpadlhandle;
> 
> @@ -907,6 +911,15 @@ extern int vmbus_recvpacket_raw(struct
> vmbus_channel *channel,
> 
>  extern void vmbus_ontimer(unsigned long data);
> 
> +/*
> + * Decrease reference count for the channel. Frees the channel when its usage
> + * count reaches zero.
> + */
> +extern void vmbus_put_channel(struct vmbus_channel *channel);
> +
> +/* Get additional reference to the channel */
> +extern struct vmbus_channel *vmbus_get_channel(struct vmbus_channel
> *channel);
> +
>  /* Base driver object */
>  struct hv_driver {
>  	const char *name;
> --

Hi Vitaly,
Thanks for the patchset! 
I once tried to do the same work, but just didn't find enough time. :-)

Please see my below questions:

Since we always get channel->lock when accessing channel->count, I don't
think the counter has to be of atomic_t?

I suggest we add vmbus_get/put_channel() in vmbus_open/close().
In this way, IMO patch 3/4 and 4/4 would be unnecessary?

In vmbus_exit(), I suspect we should swap the order of the 2 lines:
        vmbus_free_channels();
        bus_unregister(&hv_bus);
I suppose in bus_unregister(), the .remove callbacks of all the drivers are
invoked, and vmbus_close() is  invoked for every channel.


BTW, I just noticed: alloc_channel() -> alloc_workqueue(,,max_active==1,):
due to max_active==1 here, a channel's process_offer() and
process_rescind_offer() is already serialized.
I'm not sure if this fact can be used to simplify the logic?
Is the previous commit "Drivers: hv: vmbus: serialize Offer and Rescind offer"
necessary?

BTW2, I think there is an unsafe race in the 2 paths (this is an existing issue,
not related to this patch of yours):

vmbus_on_event() -> process_chn_event() -> pcpu_relid2channel -> 
	list_for_each_entry(channel, pcpu_head, percpu_list) 

and

vmbus_process_rescind_offer() -> percpu_channel_deq() ->
	list_del(&channel->percpu_list)

because the former is running in tasklet and hence can preempt the latter (running
in process context).  percpu_channel_enq() should have the same issue(?)

We should add local_bh_disable()/enable() accordingly?
Can you please help to fix this?

vmbus_exit() -> vmbus_free_channels() -> vmbus_put_channel() may have
a race with vmbus_process_rescind_offer()?
After vmbus_process_rescind_offer() ->  vmbus_device_unregister(), the count
can already be the initial 1, later vmbus_free_channels() -> vmbus_put_channel() can
kfree the channel before vmbus_process_rescind_offer() completely finishes?

Thanks,
-- Dexuan

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