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Date:   Mon, 9 May 2022 10:22:02 +0900
From:   Byungchul Park <>
To:     Linus Torvalds <>
Cc:     Damien Le Moal <>,,
        Andreas Dilger <>,
        Ext4 Developers List <>,
        Ingo Molnar <>,
        Linux Kernel Mailing List <>,
        Peter Zijlstra <>,
        Will Deacon <>,
        Thomas Gleixner <>,
        Steven Rostedt <>,
        Joel Fernandes <>,
        Sasha Levin <>,
        Daniel Vetter <>,
        Chris Wilson <>,,, Tejun Heo <>,
        Theodore Ts'o <>,
        Matthew Wilcox <>,
        Dave Chinner <>,
        Amir Goldstein <>,
        "J. Bruce Fields" <>,
        Greg Kroah-Hartman <>,, Linux-MM <>,
        Andrew Morton <>,
        Michal Hocko <>,
        Minchan Kim <>,
        Johannes Weiner <>,
        Vladimir Davydov <>,,
        Jerome Glisse <>,
        Dennis Zhou <>,
        Christoph Lameter <>,
        Pekka Enberg <>,
        David Rientjes <>,
        Vlastimil Babka <>,,
        linux-block <>,, Josef Bacik <>,
        linux-fsdevel <>,
        Al Viro <>, Jan Kara <>,, Jeff Layton <>,
        Dan Williams <>,
        Christoph Hellwig <>,
        "Darrick J. Wong" <>,
        dri-devel <>,
        Dave Airlie <>,,,,
Subject: Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker)

On Wed, May 04, 2022 at 11:17:02AM -0700, Linus Torvalds wrote:
> On Wed, May 4, 2022 at 1:19 AM Byungchul Park <> wrote:
> >
> > Hi Linus and folks,
> >
> > I've been developing a tool for detecting deadlock possibilities by
> > tracking wait/event rather than lock(?) acquisition order to try to
> > cover all synchonization machanisms.
> So what is the actual status of reports these days?

I'd like to mention one important thing here. Reportability would get
stronger if the more wait-event pairs get tagged everywhere DEPT can

Everything e.g. HW-SW interface, any retry logic and so on can be a
wait-event pair if they work wait or event anyway. For example, polling
on an IO mapped read register and initiating the HW to go for the event
also can be a pair. Definitely those make DEPT more useful.


The way to use the APIs:

1. Define SDT(Simple Dependency Tracker)

   DEFINE_DEPT_SDT(my_hw_event); <- add this

2. Tag on the waits

   sdt_wait(&my_hw_event); <- add this
   ... retry logic until my hw work done ... <- the original code

3. Tag on the events

   sdt_event(&my_hw_event); <- add this
   run_my_hw(); <- the original code


These are all we should do. I believe DEPT would be a very useful tool
once all wait-event pairs get tagged by the developers in all subsystems
and device drivers.


> Last time I looked at some reports, it gave a lot of false positives
> due to mis-understanding prepare_to_sleep().
> For this all to make sense, it would need to not have false positives
> (or at least a very small number of them together with a way to sanely
> get rid of them), and also have a track record of finding things that
> lockdep doesn't.
> Maybe such reports have been sent out with the current situation, and
> I haven't seen them.
>                  Linus

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