fsnotify: remove destroy_list from fsnotify_mark

destroy_list is used to track marks which still need waiting for srcu
period end before they can be freed. However by the time mark is added to
destroy_list it isn't in group's list of marks anymore and thus we can
reuse fsnotify_mark->g_list for queueing into destroy_list. This saves
two pointers for each fsnotify_mark.

Signed-off-by: Jan Kara <[e-mail]>
Cc: Eric Paris <[e-mail]>
Cc: Heinrich Schuchardt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/notify/mark.c

···
 	mutex_unlock(&group->mark_mutex);
 
 	spin_lock(&destroy_lock);
-	list_add(&mark->destroy_list, &destroy_list);
+	list_add(&mark->g_list, &destroy_list);
 	spin_unlock(&destroy_lock);
 	wake_up(&destroy_waitq);
 	/*
···
 	spin_unlock(&mark->lock);
 
 	spin_lock(&destroy_lock);
-	list_add(&mark->destroy_list, &destroy_list);
+	list_add(&mark->g_list, &destroy_list);
 	spin_unlock(&destroy_lock);
 	wake_up(&destroy_waitq);
 
···
 
 		synchronize_srcu(&fsnotify_mark_srcu);
 
-		list_for_each_entry_safe(mark, next, &private_destroy_list, destroy_list) {
-			list_del_init(&mark->destroy_list);
+		list_for_each_entry_safe(mark, next, &private_destroy_list, g_list) {
+			list_del_init(&mark->g_list);
 			fsnotify_put_mark(mark);
 		}
 

include/linux/fsnotify_backend.h

···
 	 * in kernel that found and may be using this mark. */
 	atomic_t refcnt;		/* active things looking at this mark */
 	struct fsnotify_group *group;	/* group this mark is for */
-	struct list_head g_list;	/* list of marks by group->i_fsnotify_marks */
+	struct list_head g_list;	/* list of marks by group->i_fsnotify_marks
+					 * Also reused for queueing mark into
+					 * destroy_list when it's waiting for
+					 * the end of SRCU period before it can
+					 * be freed */
 	spinlock_t lock;		/* protect group and inode */
 	struct hlist_node obj_list;	/* list of marks for inode / vfsmount */
 	struct list_head free_list;	/* tmp list used when freeing this mark */
···
 #define FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY	0x08
 #define FSNOTIFY_MARK_FLAG_ALIVE		0x10
 	unsigned int flags;		/* vfsmount or inode mark? */
-	struct list_head destroy_list;
 	void (*free_mark)(struct fsnotify_mark *mark); /* called on final put+free */
 };