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From: Rusty Russell on 25 Jun 2010 09:10
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(a)rustcorp.com.au>
Cc: Arnd Bergmann <arnd(a)arndb.de>
Cc: anton(a)samba.org
Cc: KOSAKI Motohiro <kosaki.motohiro(a)jp.fujitsu.com>
Cc: Mike Travis <travis(a)sgi.com>
---
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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