linux-kernel - Re: acpi_cpu_freq

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Date:	Tue, 6 May 2008 13:20:33 -0700
From:	Greg KH <gregkh@...e.de>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 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.

> Whoops, sorry. You are right.
> 
> diff -puN drivers/base/sys.c~fix-sys-bogus-warning drivers/base/sys.c
> --- linux-2.6/drivers/base/sys.c~fix-sys-bogus-warning	2008-05-07 03:51:00.000000000 +0900
> +++ linux-2.6-hirofumi/drivers/base/sys.c	2008-05-07 04:01:14.000000000 +0900
> @@ -175,8 +175,7 @@ int sysdev_driver_register(struct sysdev
>  	}
>  
>  	/* Check whether this driver has already been added to a class. */
> -	if ((drv->entry.next != drv->entry.prev) ||
> -	    (drv->entry.next != NULL)) {
> +	if (drv->entry.next && !list_empty(&drv->entry)) {

Did you try this patch out?

I say rip the whole thing out, it was added to try to make some bugs in
upper layers more obvious, but if it can't be correct, I have no
objection to removing the thing.  Other bad things happen later on if
the developer messes this one up, this is not a 'user can cause this'
type of error at all.

thanks,

greg k-h
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