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

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Message-Id: <1242948762.3373.143.camel@Joe-Laptop.home>
Date:	Thu, 21 May 2009 16:32:42 -0700
From:	Joe Perches <joe@...ches.com>
To:	Randy Dunlap <randy.dunlap@...cle.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>,
	Rusty Russell <rusty@...tcorp.com.au>,
	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 Thu, 2009-05-21 at 16:27 -0700, Randy Dunlap wrote:
> Joe Perches wrote:
> > Add a DO_ONCE statement expression analogous to printk_once
> > that executes any arbitrary statement exactly once.
> Since x is of arbitrary size & time duration, I think that x; should follow
> 		__done = true;
> instead of being before it, as was done in printk_once().  Otherwise there
> could be a sizable window there x could be done more than once.

Makes sense to me.

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.

Signed-off-by: Joe Perches <joe@...ches.com>
---
 include/linux/kernel.h |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 883cd44..a5520c9 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -637,7 +637,18 @@ static inline void ftrace_dump(void) { }
 #define swap(a, b) \
 	do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
 
+/*
+ * Do something once (analogous to WARN_ONCE() et al):
+ */
+#define DO_ONCE(x...) ({			\
+	static bool __done = false;		\
+						\
+	if (!__done) {				\
+		__done = true;			\
+		x;				\
+	}					\
+})
+
 /**
  * container_of - cast a member of a structure out to the containing structure
  * @ptr:	the pointer to the member.
-- 
1.6.3.1.10.g659a0.dirty



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