dmaengine: fix broken device refcounting

When a DMA device is unregistered, its reference count is decremented twice
for each channel: Once dma_class_dev_release() and once in
dma_chan_cleanup().  This may result in the DMA device driver's remove()
function completing before all channels have been cleaned up, causing lots
of use-after-free fun.

Fix it by incrementing the device's reference count twice for each
channel during registration.

[dan.j.williams@intel.com: kill unnecessary client refcounting]
Signed-off-by: NHaavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NShannon Nelson <shannon.nelson@intel.com>
Cc: <stable@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/dma/dmaengine.c

@@ -182,10 +182,9 @@ static void dma_client_chan_alloc(struct dma_client *client)
 				/* we are done once this client rejects
 				 * an available resource
 				 */
-				if (ack == DMA_ACK) {
+				if (ack == DMA_ACK)
 					dma_chan_get(chan);
-					kref_get(&device->refcount);
-				} else if (ack == DMA_NAK)
+				else if (ack == DMA_NAK)
 					return;
 			}
 		}
@@ -272,11 +271,8 @@ static void dma_clients_notify_removed(struct dma_chan *chan)
 		/* client was holding resources for this channel so
 		 * free it
 		 */
-		if (ack == DMA_ACK) {
+		if (ack == DMA_ACK)
 			dma_chan_put(chan);
-			kref_put(&chan->device->refcount,
-				dma_async_device_cleanup);
-		}
 	}
 
 	mutex_unlock(&dma_list_mutex);
@@ -316,11 +312,8 @@ void dma_async_client_unregister(struct dma_client *client)
 			ack = client->event_callback(client, chan,
 				DMA_RESOURCE_REMOVED);
 
-			if (ack == DMA_ACK) {
+			if (ack == DMA_ACK)
 				dma_chan_put(chan);
-				kref_put(&chan->device->refcount,
-					dma_async_device_cleanup);
-			}
 		}
 
 	list_del(&client->global_node);
@@ -397,6 +390,8 @@ int dma_async_device_register(struct dma_device *device)
 			goto err_out;
 		}
 
+		/* One for the channel, one of the class device */
+		kref_get(&device->refcount);
 		kref_get(&device->refcount);
 		kref_init(&chan->refcount);
 		chan->slow_ref = 0;