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From: Jason Wang <jasowang@redhat.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>,
	devel@linuxdriverproject.org,
	Haiyang Zhang <haiyangz@microsoft.com>,
	linux-kernel@vger.kernel.org, Dexuan Cui <decui@microsoft.com>
Subject: Re: [PATCH 1/4] Drivers: hv: vmbus: implement get/put usage workflow for vmbus channels
Date: Wed, 04 Feb 2015 09:22:25 +0008	[thread overview]
Message-ID: <1423041265.10558.6@smtp.corp.redhat.com> (raw)
In-Reply-To: <1422982839-3948-2-git-send-email-vkuznets@redhat.com>



On Wed, Feb 4, 2015 at 1:00 AM, Vitaly Kuznetsov <vkuznets@redhat.com> 
wrote:
> 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@redhat.com>
> ---
>  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);

Looks like we can use atomic_inc_return_safe() here to avoid extra 
dying. And then there's also no need for the spinlock.

if (atomic_inc_return_safe(&channel->count) > 0)
	return channel;
else
	return NULL;
> 
> +	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;
> -- 
> 1.9.3
> 


  parent reply	other threads:[~2015-02-04  9:14 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-03 17:00 [PATCH 0/4] Drivers: hv: Further protection for the rescind path Vitaly Kuznetsov
2015-02-03 17:00 ` [PATCH 1/4] Drivers: hv: vmbus: implement get/put usage workflow for vmbus channels Vitaly Kuznetsov
2015-02-04  8:18   ` Dexuan Cui
2015-02-04  9:32     ` Vitaly Kuznetsov
2015-02-04  9:54       ` Dexuan Cui
2015-02-04  9:14   ` Jason Wang [this message]
2015-02-04  9:33     ` Vitaly Kuznetsov
2015-02-03 17:00 ` [PATCH 2/4] Drivers: hv: vmbus: do not lose rescind offer on failure in vmbus_process_offer() Vitaly Kuznetsov
2015-02-04  7:42   ` Dexuan Cui
2015-02-03 17:00 ` [PATCH 3/4] Drivers: hv: vmbus: protect vmbus_get_outgoing_channel() against channel removal Vitaly Kuznetsov
2015-02-04  7:27   ` Dexuan Cui
2015-02-03 17:00 ` [PATCH 4/4] hyperv: netvsc: improve protection against rescind offer Vitaly Kuznetsov
2015-02-04  7:29   ` Dexuan Cui
2015-02-04 18:26 ` [PATCH 0/4] Drivers: hv: Further protection for the rescind path KY Srinivasan
2015-02-05 10:10   ` Vitaly Kuznetsov
2015-02-05 12:44     ` Dexuan Cui
2015-02-05 22:47       ` KY Srinivasan
2015-02-06 14:53         ` Dexuan Cui
2015-02-07 16:27           ` KY Srinivasan
2015-02-05 15:52     ` KY Srinivasan

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