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

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Date:   Mon, 9 May 2022 10:22:02 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        linux-ide@...r.kernel.org,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
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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 <byungchul.park@....com> 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
work.

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.

	Byungchul

> 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