linux-kernel - Re: [PATCH] lockdep: Show up to three levels for a deadlock scenario

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Date:   Thu, 14 Dec 2017 13:19:41 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] lockdep: Show up to three levels for a deadlock
 scenario

On Thu, 14 Dec 2017 18:59:31 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Dec 14, 2017 at 12:38:52PM -0500, Steven Rostedt wrote:
> > 
> > Currently, when lockdep detects a possible deadlock scenario that involves 3
> > or more levels, it just shows the chain, and a CPU sequence order of the
> > first and last part of the scenario, leaving out the middle level and this
> > can take a bit of effort to understand. By adding a third level, it becomes
> > easier to see where the deadlock is.  
> 
> So is anybody actually using this? This (together with the callchain for
> #0) is always the first thing of the lockdep output I throw away.

Um, most people that post lockdep issues do (including myself). You are
unique and understand lockdep inside and out, so this doesn't help you.
I've had talks accepted on how to read lockdep output (having a talk on
how to read output shows it's not trivial at all to do so). Most people
have no idea what the lockdep output means. This is the only part
that "normal" people appear to understand from it. I've seen this
part of the output posted many times on the mailing list to discuss
deadlocks. I've been asked by many people to add this change, I just
never had time to implement it.

-- Steve