linux-kernel - Re: generic swap()

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Date:	Fri, 16 Jan 2009 10:45:36 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Wu Fengguang <wfg@...ux.intel.com>,
	L-K <linux-kernel@...r.kernel.org>
Subject: Re: generic swap()

On Fri, 2009-01-16 at 01:38 -0800, Andrew Morton wrote:
> On Fri, 16 Jan 2009 10:26:34 +0100 Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Hi,
> > 
> > Where did this patch come from? -- I can't seem to find it on lkml at all..
> 
> That's a damn good question.  I received:

Hehe, one of the mysteries of life then :-)

> > The reason I ask it that I wonder why swap() has a return value?
> 
> Accident, I guess.
> 
> > +#define swap(a, b) ({ typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; })
> 
> You think it should use the do{}while(0) thing?

That was what I was thinking indeed. Non of the current users appear to
make use of the (somewhat arbitrary) return value, so lets change it
before someone gets creative ;-)

Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 343df9e..7fa3718 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -480,7 +480,8 @@ static inline char *pack_hex_byte(char *buf, u8 byte)
 /*
  * swap - swap value of @a and @b
  */
-#define swap(a, b) ({ typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; })
+#define swap(a, b) \
+	do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
 
 /**
  * container_of - cast a member of a structure out to the containing structure


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