Date:   Thu, 14 Dec 2017 14:58:41 +0900
From:   Byungchul Park <max.byungchul.park@...il.com>
To:     "Theodore Ts'o" <tytso@....edu>,
        Byungchul Park <max.byungchul.park@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, david@...morbit.com,
        willy@...radead.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>, byungchul.park@....com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        oleg@...hat.com
Subject: Re: About the try to remove cross-release feature entirely by Ingo

On Thu, Dec 14, 2017 at 12:07 PM, Theodore Ts'o <tytso@....edu> wrote:
> On Wed, Dec 13, 2017 at 04:13:07PM +0900, Byungchul Park wrote:
>>
>> Therefore, I want to say the fundamental problem
>> comes from classification, not cross-release
>> specific.
>
> You keep saying that it is "just" a matter of classificaion.

But, it's a fact.

> However, it is not obvious how to do the classification in a sane
> manner.  And this is why I keep pointing out that there is no
> documentation on how to do this, and somehow you never respond to this
> point....

I can write a document explaining what lock class is but.. I
cannot explain how to assign it perfectly since there's no right
answer. It's something we need to improve more and more.

> In the case where you have multiple unrelated subsystems that can be
> stacked in different ways, with potentially multiple instances stacked
> on top of each other, it is not at all clear to me how this problem
> should be solved.

I cannot give you a perfect solution immediately. I know, and
as you know, it's a very difficult problem to solve.

> It was said on one of these threads (perhaps by you, perhaps by
> someone else), that we can't expect the lockdep maintainers to
> understand all of the subsystems in the kernels, and so therefore it
> must be up to the subsystem maintainers to classify the locks.  I
> interpreted this as the lockdep maintainers saying, "hey, not my
> fault, it's the subsystem maintainer's fault for not properly
> classifying the locks" --- and thus dumping the responsibility in the
> subsystem maintainers' laps.

Sorry to say, making you feel like that.

Precisely speaking, the responsibility for something caused by
cross-release is on me, and the responsibility for something caused
by lockdep itselt is on lockdep.

I meant, in the current way to assign lock class automatically, it's
inevitable for someone to annotate places manually, and it can be
done best by each expert. But, anyway fundamentally I think the
responsibility is on lockdep.

> I don't know if the situation is just that lockdep is insufficiently
> documented, and with the proper tutorial, it would be obvious how to
> solve the classification problem.
>
> Or, if perhaps, there *is* no way to solve the classification problem,
> at least not in a general form.

Agree. It's a very difficult one to solve.

> For example --- suppose we have a network block device on which there
> is an btrfs file system, which is then exported via NFS.  Now all of
> the TCP locks will be used twice for two different instances, once for
> the TCP connection for the network block device, and then for the NFS
> export.
>
> How exactly are we supposed to classify the locks to make it all work?
>
> Or the loop device built on top of an ext4 file system which on a
> LVM/device mapper device.  And suppose the loop device is then layered
> with a dm-error device for regression testing, and with another ext4
> file system on top of that?

Ditto.

> How exactly are we supposed to classify the locks in that situation?
> Where's the documentation and tutorials which explain how to make this
> work, if the responsibility is going to be dumped on the subsystem
> maintainers' laps?  Or if the lockdep maintainers are expected to fix
> and classify all of these locks, are you volunteering to do this?

I have the will. I will.

> How hard is it exactly to do all of this classification work, no
> matter whose responsibility it will ultimately be?
>
> And if the answer is that it is too hard, then let me gently suggest
> to you that perhaps, if this is a case, that maybe this is a
> fundamental and fatal flaw with the cross-release and completion
> lockdep feature?

I don't understand this.

> Best regards,
>
>                                                 - Ted



-- 
Thanks,
Byungchul

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