linux-kernel - Re: [PATCH v4 (resend)] lockdep: Allow tuning tracing capacity constants.

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Message-ID: <d1bc6d20-4150-0d64-824e-6d469a8a422e@i-love.sakura.ne.jp>
Date:   Mon, 1 Feb 2021 22:24:14 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v4 (resend)] lockdep: Allow tuning tracing capacity
 constants.

Hello, Andrew and Linus.

We are stuck because Peter cannot respond.
I think it is time to send this patch to linux-next. What do you think?

On 2021/01/20 19:18, Dmitry Vyukov wrote:
> On Wed, Jan 20, 2021 at 11:12 AM Tetsuo Handa
> <penguin-kernel@...ove.sakura.ne.jp> wrote:
>>
>> Since syzkaller continues various test cases until the kernel crashes,
>> syzkaller tends to examine more locking dependencies than normal systems.
>> As a result, syzbot is reporting that the fuzz testing was terminated
>> due to hitting upper limits lockdep can track [1] [2] [3].
>>
>> Peter Zijlstra does not want to allow tuning these limits via kernel
>> config options, for such change discourages thinking. But analysis via
>> /proc/lockdep* did not show any obvious culprit [4] [5]. It is possible
>> that many hundreds of kn->active lock instances are to some degree
>> contributing to these problems, but there is no means to verify whether
>> these instances are created for protecting same callback functions.
>> Unless Peter provides a way to make these instances per "which callback
>> functions the lock instance will call (identified by something like MD5
>> of string representations of callback functions which each lock instance
>> will protect)" than plain "serial number", I don't think that we can
>> verify the culprit.
>>
>> [1] https://syzkaller.appspot.com/bug?id=3d97ba93fb3566000c1c59691ea427370d33ea1b
>> [2] https://syzkaller.appspot.com/bug?id=381cb436fe60dc03d7fd2a092b46d7f09542a72a
>> [3] https://syzkaller.appspot.com/bug?id=a588183ac34c1437fc0785e8f220e88282e5a29f
>> [4] https://lkml.kernel.org/r/4b8f7a57-fa20-47bd-48a0-ae35d860f233@i-love.sakura.ne.jp
>> [5] https://lkml.kernel.org/r/1c351187-253b-2d49-acaf-4563c63ae7d2@i-love.sakura.ne.jp
>>
>> Reported-by: syzbot <syzbot+cd0ec5211ac07c18c049@...kaller.appspotmail.com>
>> Reported-by: syzbot <syzbot+91fd909b6e62ebe06131@...kaller.appspotmail.com>
>> Reported-by: syzbot <syzbot+62ebe501c1ce9a91f68c@...kaller.appspotmail.com>
>> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
>> Acked-by: Dmitry Vyukov <dvyukov@...gle.com>
> 
> Thanks for your persistence!
> I still support this. And assessment of lockdep stats on overflow
> seems to confirm it's just a very large lock graph triggered by
> syzkaller.
>