linux-ext4 - Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker)

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Date:   Mon, 9 May 2022 16:47:12 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     Hyeonggon Yoo <42.hyeyoo@...il.com>, torvalds@...ux-foundation.org,
        damien.lemoal@...nsource.wdc.com, linux-ide@...r.kernel.org,
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Subject: Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker)

On Mon, 9 May 2022 09:16:37 +0900
Byungchul Park <byungchul.park@....com> wrote:

> CASE 2.
> 
>    lock L with depth n
>    lock A
>    lock_nested L' with depth n + 1
>    ...
>    unlock L'
>    unlock A
>    unlock L
> 
> This case is allowed by Lockdep.
> This case is *NOT* allowed by DEPT cuz it's a *DEADLOCK*.
> 
> ---
> 
> The following scenario would explain why CASE 2 is problematic.
> 
>    THREAD X			THREAD Y
> 
>    lock L with depth n
> 				lock L' with depth n
>    lock A
> 				lock A
>    lock_nested L' with depth n + 1

I'm confused by what exactly you are saying is a deadlock above.

Are you saying that lock A and L' are inversed? If so, lockdep had better
detect that regardless of L. A nested lock associates the the nesting with
the same type of lock. That is, in lockdep nested tells lockdep not to
trigger on the L and L' but it will not ignore that A was taken.

-- Steve



> 				lock_nested L'' with depth n + 1
>    ...				...
>    unlock L'			unlock L''
>    unlock A			unlock A
>    unlock L			unlock L'