memcg: avoid use cmpxchg in swap cgroup maintainance

swap_cgroup uses 2bytes data and uses cmpxchg in a new operation. 2byte
cmpxchg/xchg is not available on some archs. This patch replaces
cmpxchg/xchg with operations under lock.

Signed-off-by: KAMEZAWA Hiroyuki <[e-mail]>
Reported-by: Sachin Sant <[e-mail]> wrote:
Acked-by: Balbir Singh <[e-mail]>
Acked-by: Daisuke Nishimura <[e-mail]>
Cc: Li Zefan <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/page_cgroup.c

···
 struct swap_cgroup_ctrl {
 	struct page **map;
 	unsigned long length;
+	spinlock_t	lock;
 };
 
 struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
···
 	struct swap_cgroup_ctrl *ctrl;
 	struct page *mappage;
 	struct swap_cgroup *sc;
+	unsigned long flags;
+	unsigned short retval;
 
 	ctrl = &swap_cgroup_ctrl[type];
 
 	mappage = ctrl->map[idx];
 	sc = page_address(mappage);
 	sc += pos;
-	if (cmpxchg(&sc->id, old, new) == old)
-		return old;
+	spin_lock_irqsave(&ctrl->lock, flags);
+	retval = sc->id;
+	if (retval == old)
+		sc->id = new;
 	else
-		return 0;
+		retval = 0;
+	spin_unlock_irqrestore(&ctrl->lock, flags);
+	return retval;
 }
 
 /**
···
 	struct page *mappage;
 	struct swap_cgroup *sc;
 	unsigned short old;
+	unsigned long flags;
 
 	ctrl = &swap_cgroup_ctrl[type];
 
 	mappage = ctrl->map[idx];
 	sc = page_address(mappage);
 	sc += pos;
-	old = xchg(&sc->id, id);
+	spin_lock_irqsave(&ctrl->lock, flags);
+	old = sc->id;
+	sc->id = id;
+	spin_unlock_irqrestore(&ctrl->lock, flags);
 
 	return old;
 }
···
 	mutex_lock(&swap_cgroup_mutex);
 	ctrl->length = length;
 	ctrl->map = array;
+	spin_lock_init(&ctrl->lock);
 	if (swap_cgroup_prepare(type)) {
 		/* memory shortage */
 		ctrl->map = NULL;