cpumask: remove struct cpumask definition when CONFIG_CPUMASK_OFFSTACK=y

We're about to change CONFIG_CPUMASK_OFFSTACK so it only allocate nr_cpu_ids bits for all cpumasks. We need to make sure that when CONFIG_CPUMASK_OFFSTACK is set:

1) Noone uses the old bitmap ops, which use NR_CPUS bits (use cpumask_*)
2) Noone uses assignment of struct cpumask (use cpumask_copy)
3) Noone passes a struct cpumask (pass a pointer)
4) Noone declares them on the stack (use cpumask_var_t)

So we finally remove the definition of struct cpumask when CONFIG_CPUMASK_OFFSTACK=y. This means that these usages will hit a compile error the moment that config option is turned on.

Note that it also means you can't declare a static cpumask. You should avoid this anyway (use cpumask_var_t), but there's a deliberately-ugly workaround for special cases, using DECLARE_BITMAP() and to_cpumask().

---
 include/linux/cpumask.h |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

@@ -10,7 +10,15 @@
 #include <linux/threads.h>
 #include <linux/bitmap.h>
 
+#ifdef CONFIG_CPUMASK_OFFSTACK
+/*
+ * This is never declared, so you can't accidentally create one (see
+ * cpumask_var_t) or copy them by assignment (see cpumask_copy).
+ */
+struct cpumask;
+#else
 typedef struct cpumask { DECLARE_BITMAP(bits, NR_CPUS); } cpumask_t;
+#endif /* CONFIG_CPUMASK_OFFSTACK */
 
 /**
  * cpumask_bits - get the bits in a cpumask

@@ -19,7 +28,14 @@ typedef struct cpumask { DECLARE_BITMAP(
  * You should only assume nr_cpu_ids bits of this mask are valid.  This is
  * a macro so it's const-correct.
  */
+#ifdef CONFIG_CPUMASK_OFFSTACK
+static inline unsigned long *cpumask_bits(const struct cpumask *mask)
+{
+	return (unsigned long *)mask;
+}
+#else
 #define cpumask_bits(maskp) ((maskp)->bits)
+#endif
 
 #if NR_CPUS == 1
 #define nr_cpu_ids		1