linux-kernel - Re: [PATCH 1/3] kernel.h: Add DO_ONCE statement expression macro

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Date:	Mon, 25 May 2009 16:15:07 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
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@...r.kernel.org, Len Brown <len.brown@...el.com>,
	Mike Travis <travis@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
Subject: Re: [PATCH 1/3] kernel.h: Add DO_ONCE statement expression macro

On Fri, 22 May 2009 08:30:22 am Joe Perches wrote:
> Add a DO_ONCE statement expression analogous to printk_once
> that executes any arbitrary statement exactly once.
>
> This will take the place of printk_once so that
> DO_ONCE(pr_<foo>) or any other statement performed
> a single time may be easily written.

If you're going to use a statement expression, was the intent to make
it usable for a test?  If so, you want something vaguely like:

#define DO_ONCE(x...) ({			\
	static bool __done = false;		\
	bool did = !__done;			\
	if (!__done) {				\
		x;				\
		__done = true;			\
	}					\
	did;					\
})

Otherwise, why not just use a normal do {} while (0) ?

Anyway, statements in macros leave me uncomfortable. I prefer explicit
ONCE() macro which can be tested.

Cheers,
Rusty.
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