drm/i915: Hold struct_mutex for per-file stats in debugfs/i915_gem_object

As we walk the obj->vma_list in per_file_stats(), we need to hold
struct_mutex to prevent alteration of that list.

Fixes: 1d2ac403ae3b ("drm: Protect dev->filelist with its own mutex")
Fixes: c84455b4bacc ("drm/i915: Move debug only per-request pid tracking from request to ctx")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101460
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170617115744.4452-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
(cherry picked from commit 0caf81b5c53d9bd332a95dbcb44db8de0b397a7c)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -292,6 +292,8 @@ static int per_file_stats(int id, void *ptr, void *data)
 	struct file_stats *stats = data;
 	struct i915_vma *vma;
 
+	lockdep_assert_held(&obj->base.dev->struct_mutex);
+
 	stats->count++;
 	stats->total += obj->base.size;
 	if (!obj->bind_count)
@@ -476,6 +478,8 @@ static int i915_gem_object_info(struct seq_file *m, void *data)
 		struct drm_i915_gem_request *request;
 		struct task_struct *task;
 
+		mutex_lock(&dev->struct_mutex);
+
 		memset(&stats, 0, sizeof(stats));
 		stats.file_priv = file->driver_priv;
 		spin_lock(&file->table_lock);
@@ -487,7 +491,6 @@ static int i915_gem_object_info(struct seq_file *m, void *data)
 		 * still alive (e.g. get_pid(current) => fork() => exit()).
 		 * Therefore, we need to protect this ->comm access using RCU.
 		 */
-		mutex_lock(&dev->struct_mutex);
 		request = list_first_entry_or_null(&file_priv->mm.request_list,
 						   struct drm_i915_gem_request,
 						   client_link);
@@ -497,6 +500,7 @@ static int i915_gem_object_info(struct seq_file *m, void *data)
 				PIDTYPE_PID);
 		print_file_stats(m, task ? task->comm : "<unknown>", stats);
 		rcu_read_unlock();
+
 		mutex_unlock(&dev->struct_mutex);
 	}
 	mutex_unlock(&dev->filelist_mutex);