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Date: Wed, 4 Oct 2006 10:28:12 -0700
From: Andrew Morton <akpm@osdl.org>
To: "Ananiev, Leonid I" <leonid.i.ananiev@intel.com>
Cc: <tim.c.chen@linux.intel.com>, "Jeremy Fitzhardinge" <jeremy@goop.org>,
       <herbert@gondor.apana.org.au>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression
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On Wed, 4 Oct 2006 20:57:57 +0400
"Ananiev, Leonid I" <leonid.i.ananiev@intel.com> wrote:

> >Guys.  Please.  Help us out here.  None of this makes sense, and it's
> > possible that we have an underlying problem in there which we need to
> know
> > about.
>  This is explantion:
> 
> The static variable __warn_once was "never" read (until there is no bug)
> before patch "Let WARN_ON/WARN_ON_ONCE return the condition"
> http://kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commi
> t;h=684f978347deb42d180373ac4c427f82ef963171
>  in WARN_ON_ONCE's line 
> - if (unlikely((condition) && __warn_once)) { \
> because 'condition' is false. There was no cache miss as a result.
> 
> Cache miss for __warn_once is happened in new lines
> + if (likely(__warn_once)) \
> + if (WARN_ON(__ret_warn_once)) \
> 

That's one cache miss.  One.  For the remainder of the benchmark,
__warn_once is in cache and there are no more misses.  That's how caches
work ;)

But it appears this isn't happening.  Why?