linux-kernel - Re: [lockdep] b09be676e0 BUG: unable to handle kernel NULL pointer dereference at 000001f2

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Date:   Tue, 10 Oct 2017 14:48:01 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Fengguang Wu <fengguang.wu@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        LKP <lkp@...org>, Josh Poimboeuf <jpoimboe@...hat.com>,
        kernel-team@....com
Subject: Re: [lockdep] b09be676e0 BUG: unable to handle kernel NULL pointer
 dereference at 000001f2

On Tue, Oct 03, 2017 at 09:57:02AM -0700, Linus Torvalds wrote:
> On Tue, Oct 3, 2017 at 9:54 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Can we consider just reverting the crossrelease thing?
> >
> > The apparent stack corruption really worries me [...]
> 
> Side note: I also think the thing is just broken.
> 
> Any actual cross-releaser should be way more annotated than just "set
> cross to 1" in the lockdep map.
> 
> The place where the release is done should simply be special.
> 
> Because we should *not* encourage the whole "acquire by one context,
> release by another" as being something normal and "just set the flag
> to let lockdep know".

Could you explain it more? Please let me apply what you point out. Now,
I don't understand your intention.

> So that commit is apparently buggy, but I think it might be more
> fundamentally the wrong model too.

It would be appriciated if you let me know what is buggy wrt
crossrelease and the model, then I will do my best to fix it.

But I believe the model crossrelease uses is what lockdep should have
adopted before.

Anyway, I might be wrong. It would be appriciated if you tell me why you
think so.

Thank you.