gpu: ion: Fix race between ion_import and ion_free

If preemted during ion_free after the refcount is updated but
before the handle can be removed from the rb_tree, import
might find that handle in the tree and try to reuse it
when execution returns to free, the handle will be cleaned
up leaving the caller of import with a corrupt handle.
This patch modifies the locking to protect agains this race.
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/ion/ion.c

@@ -253,8 +253,6 @@ static void ion_handle_destroy(struct kref *kref)
 	struct ion_client *client = handle->client;
 	struct ion_buffer *buffer = handle->buffer;
 
-	mutex_lock(&client->lock);
-
 	mutex_lock(&buffer->lock);
 	while (handle->kmap_cnt)
 		ion_handle_kmap_put(handle);
@@ -262,7 +260,6 @@ static void ion_handle_destroy(struct kref *kref)
 
 	if (!RB_EMPTY_NODE(&handle->node))
 		rb_erase(&handle->node, &client->handles);
-	mutex_unlock(&client->lock);
 
 	ion_buffer_put(buffer);
 	kfree(handle);
@@ -406,13 +403,13 @@ void ion_free(struct ion_client *client, struct ion_handle *handle)
 
 	mutex_lock(&client->lock);
 	valid_handle = ion_handle_validate(client, handle);
-	mutex_unlock(&client->lock);
 
 	if (!valid_handle) {
 		WARN(1, "%s: invalid handle passed to free.\n", __func__);
 		return;
 	}
 	ion_handle_put(handle);
+	mutex_unlock(&client->lock);
 }
 EXPORT_SYMBOL(ion_free);