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Date:	Thu, 26 Mar 2009 09:42:39 +0530
From:	Nikanth Karthikesan <>
To:	Rusty Russell <>
Subject: Re: Strange code in include/linux/cpumask.h

On Thursday 26 March 2009 04:14:46 Rusty Russell wrote:
> On Wednesday 25 March 2009 17:14:30 Nikanth Karthikesan wrote:
> > On Wednesday 25 March 2009 10:41:11 Rusty Russell wrote:
> > > Yes, except that this insists that bitmap be an unsigned long * or
> > > you'll get a warning.  Otherwise the macro could be used on anything. 
> > > And it needs to be a macro to use it as a static initializer.
> >
> > Ah, got it. Thanks a lot for the explanation. May be a comment could be
> > added to the source.
> Well, it's not that unusual a trick in the kernel, but an explanation might
> help.

I _assumed_ that kernel developers are good programmers who do not need type
enforcements/warnings. And tricky code for non-performance optimizations wont
be allowed. Yes, this is not very tricky once you are aware that such tricks
are done in the kernel just for type-safety. Anyway lets document it.


Add a comment explaining the purpose of the tricky code in the macro
to_cpumask(). The __check_is_bitmap() is used to enforce type-safety.

Signed-off-by: Nikanth Karthikesan <>


diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 9f31538..32b802b 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -279,6 +279,7 @@ static inline void __cpus_shift_left(cpumask_t *dstp,
  * expose the definition of 'struct cpumask'.
  * This does the conversion, and can be used as a constant initializer.
+ * __check_is_bitmap() enforces bitmap to be an 'unsigned long *'.
 #define to_cpumask(bitmap)						\
 	((struct cpumask *)(1 ? (bitmap)				\

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