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Date: Wed, 13 Nov 2019 10:44:46 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Subject: Re: [PATCH 00/50] Add log level to show_stack()
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On Wed, 6 Nov 2019 23:25:13 +0000
Russell King - ARM Linux admin <linux@armlinux.org.uk> wrote:

> On Wed, Nov 06, 2019 at 09:34:40PM +0100, Peter Zijlstra wrote:
> > I suppose I'm surprised there are backtraces that are not important.
> > Either badness happened and it needs printing, or the user asked for it
> > and it needs printing.  
> 
> Or utterly meaningless.
> 
> > Perhaps we should be removing backtraces if they're not important
> > instead of allowing to print them as lower loglevels?  
> 
> Definitely!  WARN_ON() is well overused - and as is typical, used
> without much thought.  Bound to happen after Linus got shirty about
> BUG_ON() being over used.  Everyone just grabbed the next nearest thing
> to assert().
> 
> As a kind of example, I've recently come across one WARN_ON() in a
> driver subsystem (that shall remain nameless at the moment) which very
> likely has multiple different devices on a platform.  The WARN_ON()
> triggers as a result of a problem with the hardware, but because it's a
> WARN_ON(), you've no idea which device has a problem.  The backtrace is
> mostly meaningless.  So you know that a problem has occurred, but the
> kernel prints *useless* backtrace to let you know, and totally omits
> the *useful* information.
> 

I would like to bring up a topic for the next maintainers summit
(although I may not even be there), that we define a clear use of
WARN_ON(). I use it only if the code does something I do not expect it
to do, and is considered a bug in the code if it triggers. But it
appears that some drivers use it for "oh I didn't realize this hardware
does something I didn't expect". And is ignored when the warn on is
triggered and reported, with "you have buggy hardware" but my hardware
appears to work just fine!

-- Steve