Update comment a bit.

spinlock.c

@@ -61,7 +61,7 @@ release(struct spinlock *lk)
 
   // Release the lock, equivalent to lk->locked = 0.
   // This code can't use a C assignment, since it might
-  // not be atomic.
+  // not be atomic. A real OS would use C atomics here.
   asm volatile("movl $0, %0" : "+m" (lk->locked) : );
 
   popcli();