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

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Message-ID: <20131015162540.5275728a@gandalf.local.home>
Date:	Tue, 15 Oct 2013 16:25:40 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
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>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"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 22:18:48 +0200
Frederic Weisbecker <fweisbec@...il.com> wrote:


> How about just updating __warned without a cmpxchg. It's not that critical
> if the update is not seen immediately to other CPUs. OTOH it's critical
> that's it is visible immediately to the current CPU

Well, I didn't use cmpxchg() I used xchg() which is actually quite
faster.

> 
> I mean some warrning can be hard to reproduce and happen to some users
> while staying for several kernel releases. If it's repetitive, the xchg
> might impact the performance.

But do we care about that? A WARN_ON() means the kernel (or hardware)
is buggy. It should be fixed.

But Andrew's ONCE() request is something we would want to avoid the
xchg() every time.

> 
> I may be overly paranoid, but I think barrier() (so that at least
> we don't recurse locally) alone would be better.

Heh, Boris is giving me the same argument on IRC ;-)

-- Steve
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