Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression

Andrew Morton
2006-10-04T17:28:24Z

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?