linux-kernel - Re: [2/3] 2.6.22-rc2: known regressions v2

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Date:	Thu, 24 May 2007 21:37:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Christoph Lameter <clameter@....com>,
	Michal Piotrowski <michal.k.k.piotrowski@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Cherwin R. Nooitmeer" <cherwin@...il.com>,
	linux-pcmcia@...ts.infradead.org,
	Robert de Rooy <robert.de.rooy@...il.com>,
	Alan Cox <alan@...hat.com>, Tejun Heo <htejun@...il.com>,
	sparclinux@...r.kernel.org, David Miller <davem@...emloft.net>,
	Mikael Pettersson <mikpe@...uu.se>,
	linux1394-devel@...ts.sourceforge.net,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Kristian Høgsberg <krh@...planet.net>,
	linux-pm@...ts.linux-foundation.org,
	"Rafael J. Wysocki" <rjw@...k.pl>, Pavel Machek <pavel@....cz>,
	Marcus Better <marcus@...ter.se>,
	Andrey Borzenkov <arvidjaar@...l.ru>,
	linux-usb-devel@...ts.sourceforge.net,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [2/3] 2.6.22-rc2: known regressions v2


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > Looks like this is in DRM code:
> > 
> > BUG: at include/linux/slub_def.h:88 kmalloc_index()
> 
> I'm going to change that "BUG:" to "WARNING:".
> 
> I know some people disagreed with it (ie Ingo), but I think that's 
> total and utter bullshit.
> 
> It's a warning. Right now that "BUG:" message makes people all scared 
> about something that is not fatal at all, just a note that something 
> hasn't been converted, but is expected to work absolutely fine.
> 
> Calling it a bug is idiotic.

i very much agree that this kmalloc_index() one shouldnt be called a 
"BUG: ", but if you look at the majority of WARN_ON() instances they are 
checks for clear, serious kernel bugs. Very often we use WARN_ON() not 
to signal that it's just a harmless warning, but because we do not want 
to bring the system down via a BUG_ON(). The API is misnamed for sure, 
but still, the purpose and current practice is clear: to signal kernel 
bugs.

To quantify this a bit more objectively i just did a "grep WARN_ON 
kernel/*.c" and randomly picked 10 out of the 113 WARN_ON()'s:

 kernel/cpu.c:			WARN_ON(1);
 kernel/exit.c:	WARN_ON(atomic_read(&tsk->fs_excl));
 kernel/fork.c:	WARN_ON(!(tsk->exit_state & (EXIT_DEAD | EXIT_ZOMBIE)));
 kernel/futex.c:	WARN_ON(!pi_state);
 kernel/hrtimer.c:		WARN_ON_ONCE(timer->cb_mode ==  HRTIMER_CB_IRQSAFE_NO_SOFTIRQ);
 kernel/lockdep.c:		WARN_ON(1);
 kernel/mutex-debug.c:	DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
 kernel/rtmutex.c:	WARN_ON(rt_mutex_is_locked(lock));
 kernel/sched.c:	if (DEBUG_LOCKS_WARN_ON((preempt_count() < 0)))
 kernel/softirq.c:       WARN_ON_ONCE(in_irq());

and reviewed each instance, each and every one of these warnings is a 
serious kernel bug that i would not 'warn' about, but what i'd like to 
see reported ASAP. [ In fact i added 5 of these WARN_ON()s :-/ ] But 
maybe that's just me?

now regarding the naming of this API, i'd very much agree to do this 
rename:

   WARN_ON  =>  BUG_ON
   BUG_ON   =>  CRASH_ON

and make WARN_ON() print a "WARNING: ".

and i signalled this in the original discussion too a few months ago 
when i opposed the watering-down of the WARN_ON printk.

OTOH i dont feel that strongly about all this :-)

	Ingo
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