Update comment a bit.

6cec0211 · Frans Kaashoek · da91a3a4 · 6cec0211
--- a/spinlock.c
+++ b/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();