linux-kernel - Re: [PATCH] kernel, add bug_on

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Date:	Mon, 20 Oct 2014 15:24:48 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Fabian Frederick <fabf@...net.be>, linux-doc@...r.kernel.org
Subject: Re: [PATCH] kernel, add bug_on_warn

On Mon, 20 Oct 2014 08:00:20 -0400 Prarit Bhargava <prarit@...hat.com> wrote:

> There have been several times where I have had to rebuild a kernel to
> cause a panic when hitting a WARN() in the code in order to get a crash
> dump from a system.  Sometimes this is easy to do, other times (such as
> in the case of a remote admin) it is not trivial to send new images to the
> user.
> 
> A much easier method would be a switch to change the WARN() over to a
> BUG().  This makes debugging easier in that I can now test the actual
> image the WARN() was seen on and I do not have to engage in remote
> debugging.
> 
> This patch adds a bug_on_warn kernel parameter, which calls BUG() in the
> warn_slowpath_common() path.  The function will still print out the
> location of the warning.
> 
> Successfully tested by me.

Looks nice and simple and useful.  However I suspect you're exclusively
focussed on "I want a crash dump" and things haven't been fully thought
through.

- Do you have any example WARN->BUG console output at hand?  I'd like
  to check for missing or duplicated info.

- Did you consider permitting this to be tweaked at runtime via
  /proc?  Sometimes we get pesky WARNs at boot time and having runtime
  alteration would permit the user to prevent those from tripping a
  BUG.

- Also, perhaps bug_on_warn should be single-shot: clear itself after
  it has triggered one BUG.  Because once the kernel has gone
  WARN->BUG, it's probably messed up and is likely to trigger more
  WARNs.  Also, the kernel might generate many WARNs for the same
  issue.

> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -553,6 +553,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  	bttv.pll=	See Documentation/video4linux/bttv/Insmod-options
>  	bttv.tuner=
>  
> +	bug_on_warn	BUG() instead of WARN()

There's no mention here that this feature is mainly aimed at generating
a crash dump.  How do we tell the people who aren't reading this email
thread (ie: all of humanity except you and me ;)) that this feature
even exists?  Is there crash dump documentation that we can update?


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