linux-kernel - Re: acpi_cpu_freq

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Date:	Tue, 6 May 2008 14:38:01 -0700
From:	Greg KH <gregkh@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Len Brown <lenb@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-acpi <linux-acpi@...r.kernel.org>
Subject: Re: acpi_cpu_freq_init warning...

On Tue, May 06, 2008 at 01:27:37PM -0700, Linus Torvalds wrote:
> 
> 
> On Tue, 6 May 2008, Greg KH wrote:
> 
> > On Wed, May 07, 2008 at 04:02:53AM +0900, OGAWA Hirofumi wrote:
> > > Linus Torvalds <torvalds@...ux-foundation.org> writes:
> > > 
> > > > On Tue, 6 May 2008, OGAWA Hirofumi wrote:
> > > >> -	if ((drv->entry.next != drv->entry.prev) ||
> > > >> +	if ((drv->entry.next != drv->entry.prev) &&
> > > >>  	    (drv->entry.next != NULL)) {
> > > >
> > > > Umm. That code still makes no sense.
> > > >
> > > > The "drv->entry.next == drv->entry.prev" condition will trigger under 
> > > > *three* different circumstances:
> > > >
> > > >  - next/prev == NULL (uninitialized). Checked for by the explicit check 
> > > >    against NULL.
> > > >
> > > >  - list empty (both next/prev point back to itself), which I assume the 
> > > >    check was *meant* for.
> > > >
> > > >  - list has only *one* entry, when next/prev both point to the list head.
> > > >
> > > > and I'm pretty damn sure that whoever wrote that code didn't mean that 
> > > > last one, but who knows..
> > > >
> > > > The fact is, looking at next/prev this way is a sure way to have bugs.
> > > >
> > > > What is that PoS *trying* to test for? I assume it is meant to test for
> > > >
> > > > 	/* Is the list initialized and non-empty? */
> > > > 	if (drv->entry.next && !list_empty(&drv->entry)) {
> > > > 		...
> > > >
> > > > and dammit, just doing it that way is shorter and simpler.
> > 
> > But I don't think that will work as others have pointed out, this
> > structure's list field isn't initialized yet.
> 
> Umm. And what do you think the test for drv->entry.next is there for?

Doh, nevermind, I don't know what I was thinking, this should be fine.

I think I need a nap...

greg k-h
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