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

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Message-ID: <20171219164619.ehrsqvqajj4n6wr6@hirez.programming.kicks-ass.net>
Date:   Tue, 19 Dec 2017 17:46:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dhaval Giani <dhaval.giani@...cle.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        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 Tue, Dec 19, 2017 at 11:30:29AM -0500, Dhaval Giani wrote:
> On 2017-12-14 12:59 PM, Peter Zijlstra 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.
> > 
> 
> Yes :-). The other stuff is unreadable to people not you.

It really isn't that hard, Its mostly a question of TL;DR.

#0 is useless and should be thrown out
#1 shows where we take #1 while holding #0
..
#n shows where we take #n while holding #n-1

And the bottom callstack shows where we take #0 while holding #n. Which
gets you a nice circle in your graph, which spells deadlock.

Plenty people have shown they get this stuff.