do { } while (0) question

Hi Andrew,

your commit e2c2770096b686b4d2456173f53cb50e01aa635c does this:

---
Always use do {} while (0).  Failing to do so can cause subtle compile
failures or bugs.

-#define hotcpu_notifier(fn, pri)
-#define register_hotcpu_notifier(nb)
-#define unregister_hotcpu_notifier(nb)
+#define hotcpu_notifier(fn, pri)	do { } while (0)
+#define register_hotcpu_notifier(nb)	do { } while (0)
+#define unregister_hotcpu_notifier(nb)	do { } while (0)
---

I'm really wondering what these subtle compile failures or bugs are.
Could you please explain?
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---

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