linux-kernel - Re: [PATCH V2 0/3] Introduce and use DO

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Message-ID: <20090522052652.GX8633@ZenIV.linux.org.uk>
Date:	Fri, 22 May 2009 06:26:52 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Joe Perches <joe@...ches.com>
Cc:	linux-kernel@...r.kernel.org, cpufreq@...r.kernel.org,
	Dave Jones <davej@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	x86@...nel.org, Len Brown <len.brown@...el.com>,
	Mike Travis <travis@....com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH V2 0/3] Introduce and use DO_ONCE statement expression
	macro

On Thu, May 21, 2009 at 09:27:29PM -0700, Joe Perches wrote:
> The printk_once macro in kernel.h is limited to printk
> 
> This generalizes the functionality of printk_once and
> allows statements like DO_ONCE(pr_info("foo\n"))
> and DO_ONCE(initialize(foo));

Sigh...  Please, don't do introduce new control structures without extremely
good reasons.

printk_once() has syntax of function call; it's far more tolerable for
casual reader (e.g. somebody looking through the code while trying to
localize a bug) since it doesn't interrupt the flow - it parses as
"some function call, apparently doing some debugging output, let's see
if there are any obvious side effects in the arguments and move on".
Your DO_ONCE(....) parses as "what the fuck is that?" followed by
grepping for definition, and the cost is much higher.

Don't do that.  Staying close to normal syntax is a very good thing;
preprocessor can be used in a lot of ways that are harmful and introduction
of (effectively) new kinds of statements is a prime example.  It *does*
have its place, but it should be done very sparingly.
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