linux-kernel - Re: [PATCH 3/4] scripts/gdb: Add rb tree iterating utilities

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Message-ID: <88d66467-1d0d-da23-3cee-f09e680d8078@kernel.org>
Date:   Wed, 27 Mar 2019 10:37:44 +0000
From:   Kieran Bingham <kbingham@...nel.org>
To:     Stephen Boyd <swboyd@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Douglas Anderson <dianders@...omium.org>,
        Nikolay Borisov <n.borisov.lkml@...il.com>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Jackie Liu <liuyun01@...inos.cn>
Subject: Re: [PATCH 3/4] scripts/gdb: Add rb tree iterating utilities

Hi Stephen,

On 26/03/2019 17:05, Stephen Boyd wrote:
> Quoting Kieran Bingham (2019-03-26 01:52:10)
>> Hi Stephen,
>>
>> On 25/03/2019 18:45, Stephen Boyd wrote:
>>> Implement gdb functions for rb_first(), rb_last(), rb_next(), and
>>> rb_prev(). These can be useful to iterate through the kernel's red-black
>>> trees.
>>
>> I definitely approve of getting data-structure helpers into scripts/gdb,
>> as it will greatly assist debug options but my last attempt to do this
>> was with the radix-tree which I had to give up on as the internals were
>> changing rapidly and caused continuous breakage to the helpers.
> 
> Thanks for the background on radix-tree. I haven't looked at that yet,
> but I suppose I'll want to have that too at some point.

Sure, it will be useful to get going again, if you get round to it -
feel free to either dig out my old patches from the list, or
git-history. (I believe they actually made it into the kernel but I had
to revert them because of the breakage, and no time to continue that
development).

Or of course - start from scratch might also be a good option :D

>> Do you foresee any similar issue here? Or is the corresponding RB code
>> in the kernel fairly 'stable'?
>>
>>
>> Please could we make sure whomever maintains the RBTree code is aware of
>> the python implementation?
>>
>> That said, MAINTAINERS doesn't actually seem to list any ownership over
>> the rb-tree code, and get_maintainers.pl [0] seems to be pointing at
>> Andrew as the probable route in for that code so perhaps that's already
>> in place :D
> 
> I don't think that the rb tree implementation is going to change. It
> feels similar to the list API. I suppose this problem of keeping things
> in sync is a more general problem than just data-structures changing.
> The only solution I can offer is to have more testing and usage of these
> scripts. Unless gdb can "simulate" or run arbitrary code for us then I
> think we're stuck reimplementing kernel internal code in gdb scripts so
> that we can get debug info out.

I agree - RB seems a lot more stable than the radix-tree was back when I
tried to mirror that implementation.

I would hope at some point we could get some automated tests going for
scripts/gdb as we see more and more functionality.

Everything should be automatable using GDB hooked up to QEmu.

GDB can 'run' arbitrary functions - but it's not a good idea as we won't
know the state of the target, and of course in the case of crash-dump
examination - the target won't even exist.

Anyway, I'm glad this is all useful to you - let us know if there's
anything we can do to help. Myself and Jan are trying to take care of
scripts/gdb - but there's not a lot of active development of new
features currently - so I'm very pleased to see your contributions !

--
Kieran