linux-kernel - Re: [RFC] Are you good with Lockdep?

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Message-ID: <20201123104538.GA9464@X58A-UD3R>
Date:   Mon, 23 Nov 2020 19:45:38 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Byungchul Park <max.byungchul.park@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
        will@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        alexander.levin@...rosoft.com,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Chris Wilson <chris@...is-wilson.co.uk>, duyuyang@...il.com,
        johannes.berg@...el.com, Tejun Heo <tj@...nel.org>,
        Theodore Ts'o <tytso@....edu>, willy@...radead.org,
        david@...morbit.com, Amir Goldstein <amir73il@...il.com>,
        bfields@...ldses.org, gregkh@...uxfoundation.org,
        kernel-team <kernel-team@....com>
Subject: Re: [RFC] Are you good with Lockdep?

On Mon, Nov 16, 2020 at 06:05:47PM +0900, Byungchul Park wrote:
> On Thu, Nov 12, 2020 at 11:58:44PM +0900, Byungchul Park wrote:
> > > > FYI, roughly Lockdep is doing:
> > > >
> > > >    1. Dependency check
> > > >    2. Lock usage correctness check (including RCU)
> > > >    3. IRQ related usage correctness check with IRQFLAGS
> > > >
> > > > 2 and 3 should be there forever which is subtle and have gotten matured.
> > > > But 1 is not. I've been talking about 1. But again, it's not about
> > > > replacing it right away but having both for a while. I'm gonna try my
> > > > best to make it better.
> > >
> > > And I believe lockdep does handle 1. Perhaps show some tangible use case
> > > that you want to cover that you do not believe that lockdep can handle. If
> > > lockdep cannot handle it, it will show us where lockdep is lacking. If it
> > > can handle it, it will educate you on other ways that lockdep can be
> > > helpful in your development ;-)
> 
> 1) OK. Lockdep might work with trylock well.
> 2) Definitely Lockdep cannot do what Cross-release was doing.
> 3) For readlock handling, let me be back later and give you examples. I
>    need check current Lockdep code first. But I have to all-stop what
>    I'm doing at the moment because of a very big personal issue, which
>    is a sad thing.

I just found that Boqun Feng has made a lot of changes into Lockdep
recently to support tracking recursive read locks, while I was checking
how the current Lockdep deals with read locks.

I need to read the code more.. I'll add my opinion on it once I see
how it works. Before that, I'd like to share my approach so that you
guys can see what means to track *wait* and *event*, how simply the tool
can work and what exactly a deadlock detection tool should do. Let me
add my patches onto this thread right away.

I understand you all don't want to replace such a stable tool but I
hope you to see *the* right way to track things for that purpose.
Again, I do never touch any other functions of Lockdep including all
great efforts that have been made but dependency tracking.

Thanks,
Byungchul