sync_file: Protect access to driver and timeline name

Protect the access to driver and timeline name which otherwise could be
freed as dma-fence exported is signalling fences.

This prepares the code for incoming dma-fence API changes which will start
asserting these accesses are done from a RCU locked section.

Signed-off-by: Tvrtko Ursulin <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Tvrtko Ursulin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/dma-buf/sync_file.c

···
 		strscpy(buf, sync_file->user_name, len);
 	} else {
 		struct dma_fence *fence = sync_file->fence;
+		const char __rcu *timeline;
+		const char __rcu *driver;
 
+		rcu_read_lock();
+		driver = dma_fence_driver_name(fence);
+		timeline = dma_fence_timeline_name(fence);
 		snprintf(buf, len, "%s-%s%llu-%lld",
-			 dma_fence_driver_name(fence),
-			 dma_fence_timeline_name(fence),
+			 rcu_dereference(driver),
+			 rcu_dereference(timeline),
 			 fence->context,
 			 fence->seqno);
+		rcu_read_unlock();
 	}
 
 	return buf;
···
 static int sync_fill_fence_info(struct dma_fence *fence,
 				 struct sync_fence_info *info)
 {
-	strscpy(info->obj_name, dma_fence_timeline_name(fence),
+	const char __rcu *timeline;
+	const char __rcu *driver;
+
+	rcu_read_lock();
+
+	driver = dma_fence_driver_name(fence);
+	timeline = dma_fence_timeline_name(fence);
+
+	strscpy(info->obj_name, rcu_dereference(timeline),
 		sizeof(info->obj_name));
-	strscpy(info->driver_name, dma_fence_driver_name(fence),
+	strscpy(info->driver_name, rcu_dereference(driver),
 		sizeof(info->driver_name));
 
 	info->status = dma_fence_get_status(fence);
···
 		dma_fence_is_signaled(fence) ?
 			ktime_to_ns(dma_fence_timestamp(fence)) :
 			ktime_set(0, 0);
+
+	rcu_read_unlock();
 
 	return info->status;
 }