linux-kernel - Re: [PATCH] base firmware: Fix BUG from sysfs attributes change in commit a2db6842873c8e5a70652f278d469128cb52db70

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Date:	Sun, 14 Mar 2010 07:59:52 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Larry Finger <Larry.Finger@...inger.net>,
	WANG Cong <xiyou.wangcong@...il.com>,
	Greg Kroah-Hartman <gregkh@...e.de>, Tejun Heo <tj@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] base firmware: Fix BUG from sysfs attributes change in
 commit a2db6842873c8e5a70652f278d469128cb52db70


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

> On Sat, 13 Mar 2010, Eric W. Biederman wrote:
> > 
> > It also only affects those fairly rare lockdep users as well, and the only
> > affect is to throw a nasty warning message.  Isn't lockdep all about throwing
> > nasty warning messages?
> 
> Hmm. The report has that "BUG: " message in it (and in the subject line), 
> but you're right - it ends up being just a warning, not actually a real 
> BUG() (which is a machine killer).
> 
> So yeah - it's not as bad as I thought. Sorry.
> 
> [ And that "BUG:" in turn seems to be due to Ingo for some reason wanting 
>   to confuse BUG_ON() messages (which have that "BUG: " prefix thing) with 
>   whatever warning conditions he adds.
> 
>   Our warnings used to have that bug too (see commit 8f53b6fcc4: "Don't 
>   call a warnign a bug. It's a warning.").
> 
>   Ingo: can we agree to not put "BUG: " messages in warnings, ok? It may 
>   be a bug (lower-case) that triggers them, but that whole "BUG()" thing 
>   has it's own semantics with rather more serious consequences than some 
>   warning that lets things continue.

Sure - will change those too over to the "INFO: " pattern we've been using for 
some time. All new warnings that come via our trees use 'INFO: ', the 'BUG: ' 
ones are there for historic reasons.

There's a few that are external to lockdep and are likely fatal conditions:

	printk(  "[ BUG: bad unlock balance detected! ]\n");
	printk(  "[ BUG: bad contention detected! ]\n");
	printk(  "[ BUG: held lock freed! ]\n");
	printk(  "[ BUG: lock held at task exit time! ]\n");

(these things often tend to cause hangs/crashes later on.)

and then there's a few that are mostly internal to lockdep, and should never 
be fatal:

		printk("BUG: MAX_STACK_TRACE_ENTRIES too low!\n");
		printk("BUG: MAX_LOCKDEP_KEYS too low!\n");
		printk("BUG: MAX_LOCKDEP_ENTRIES too low!\n");
		printk("BUG: MAX_LOCKDEP_CHAINS too low!\n");
		printk("BUG: key %p not in .data!\n", key);
		printk("BUG: MAX_LOCKDEP_SUBCLASSES too low!\n");
		printk("BUG: MAX_LOCK_DEPTH too low!\n");

[ there's rare exceptions - i've seen 'BUG: key' + real crash on a few occasions,
  when the warning was caused by memory corruption. But typically the warning 
  is not fatal, and this is what matters to the severity of the message. ]

So i'm wondering whether we should/could keep those first four with a 'BUG: ' 
message, as lockdep wont crash the machine in the BUG() fashion. The other 7 
should definitely be less alarming messages.

	Ingo
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