memcg: res_counter_read_u64(): fix potential races on 32-bit machines

res_counter_read_u64 reads u64 value without lock. It's dangerous in a
32bit environment. Add locking.

Signed-off-by: KAMEZAWA Hiroyuki <[e-mail]>
Cc: Greg Thelen <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: KOSAKI Motohiro <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/res_counter.c

···
 			pos, buf, s - buf);
 }
 
+#if BITS_PER_LONG == 32
+u64 res_counter_read_u64(struct res_counter *counter, int member)
+{
+	unsigned long flags;
+	u64 ret;
+
+	spin_lock_irqsave(&counter->lock, flags);
+	ret = *res_counter_member(counter, member);
+	spin_unlock_irqrestore(&counter->lock, flags);
+
+	return ret;
+}
+#else
 u64 res_counter_read_u64(struct res_counter *counter, int member)
 {
 	return *res_counter_member(counter, member);
 }
+#endif
 
 int res_counter_memparse_write_strategy(const char *buf,
 					unsigned long long *res)