linux-kernel - Re: [PATCH] bug: Use xchg() to update WARN_ON

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Date:	Tue, 15 Oct 2013 16:35:59 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Liu, Chuansheng" <chuansheng.liu@...el.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] bug: Use xchg() to update WARN_ON_ONCE() static
 variable

On Tue, 15 Oct 2013 13:12:59 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

 
> Also, we're now incurring an atomic op for every "call".  Presumably
> these calls are rare, but not necessarily - one can envisage uses of a
> generic ONCE() which are called at high frequency.  Should we avoid
> that with
> 
> #define ONCE() ({
> 	static int state;
> 	int ret;
> 
> 	if (likely(state))
> 		ret = 0;
> 	else
> 		ret = !xchg(&state, 1);
> 	ret;
> })

I was talking with Boris on IRC about having a shortcut if "state" is
already true. I argued against it, but that was just for the WARN()
operations because I rather add a LOCK xchg, then more branches to hot
paths.

But for a generic ONCE() function, I guess we would want the shortcut
as it may be used by something that gets hit all the time with a normal
kernel.

-- Steve
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