[PATCH 4/5] cpumask: remove struct cpumask definition when CONFIG_CPUMASK_OFFSTACK=y

[Rusty Russell]

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().

Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: anton@xxxxxxxxx
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: Mike Travis <travis@xxxxxxx>
---
 include/linux/cpumask.h |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -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

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