linux-kernel - Re: [PATCH] lockdep: Introduce CONFIG_LOCKDEP

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Message-ID: <37b60b14-4eb6-36b3-1195-97c2d27b7ed8@i-love.sakura.ne.jp>
Date:   Tue, 4 Aug 2020 11:36:09 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lockdep: Introduce CONFIG_LOCKDEP_LARGE

Hello, Peter, Ingo and Will.

(Q1) Can we change the capacity using kernel config?

(Q2) If we can change the capacity, is it OK to specify these constants
     independently? (In other words, is there inter-dependency among
     these constants?)

(Q3) Do you think that we can extend lockdep to be used as a tool for auditing
     locks held in kernel space and rebuilding lock dependency map in user space?

On 2020/07/25 14:23, Tetsuo Handa wrote:
>> Also somebody may use it to _reduce_ size of the table for a smaller kernel.
> 
> Maybe. But my feeling is that it is very rare that the kernel actually deadlocks
> as soon as lockdep warned the possibility of deadlock.
> 
> Since syzbot runs many instances in parallel, a lot of CPU resource is spent for
> checking the same dependency tree. However, the possibility of deadlock can be
> warned for only locks held within each kernel boot, and it is impossible to hold
> all locks with one kernel boot.
> 
> Then, it might be nice if lockdep can audit only "which lock was held from which
> context and what backtrace" and export that log like KCOV data (instead of evaluating
> the possibility of deadlock), and rebuild the whole dependency (and evaluate the
> possibility of deadlock) across multiple kernel boots in userspace.