Message-ID: <1381872280.22110.41.camel@joe-AO722>
Subject: Re: [PATCH 2/3] core: Convert printk_once to use DO_ONCE
From: Joe Perches <joe@perches.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Liu Chuansheng <chuansheng.liu@intel.com>,
        Ingo Molnar <mingo@kernel.org>
Date: Tue, 15 Oct 2013 14:24:40 -0700
In-Reply-To: <20131015211251.GE3269@localhost.localdomain>
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	 <1381870805.22110.37.camel@joe-AO722>
	 <20131015211251.GE3269@localhost.localdomain>
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On Tue, 2013-10-15 at 23:12 +0200, Frederic Weisbecker wrote:
> On Tue, Oct 15, 2013 at 02:00:05PM -0700, Joe Perches wrote:
> > On Tue, 2013-10-15 at 22:50 +0200, Frederic Weisbecker wrote:
> > []
> > > diff --git a/include/linux/printk.h b/include/linux/printk.h
> > []
> > > @@ -252,14 +253,7 @@ extern asmlinkage void dump_stack(void) __cold;
> > >  
> > >  #ifdef CONFIG_PRINTK
> > >  #define printk_once(fmt, ...)			\
> > > -({						\
> > > -	static bool __print_once;		\
> > > -						\
> > > -	if (!__print_once) {			\
> > > -		__print_once = true;		\
> > > -		printk(fmt, ##__VA_ARGS__);	\
> > > -	}					\
> > > -})
> > > +	DO_ONCE(printk(fmt, ##__VA_ARGS__));
> > 
> > It's hard to believe the overhead is worth it.
> 
> Which overhead?

The one you were proposing with xchg

Apparently the 1/3 series you submitted
didn't use it.

Given that it didn't, does the indirection
to DO_ONCE really help?

btw:

https://lkml.org/lkml/2009/5/21/300

Perhaps Alan's comment still applies:

https://lkml.org/lkml/2009/5/21/305


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