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

[Vitaly Kuznetsov]

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;
-- 
1.9.3

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