Date: Tue, 15 Oct 2013 16:25:40 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "H. Peter Anvin" <hpa@linux.intel.com>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Liu, Chuansheng" <chuansheng.liu@intel.com>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH] bug: Use xchg() to update WARN_ON_ONCE() static
 variable
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	<20131015201816.GA3269@localhost.localdomain>
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On Tue, 15 Oct 2013 22:18:48 +0200
Frederic Weisbecker <fweisbec@gmail.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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