linux-ext4 - Re: [PATCH v11 14/26] locking/lockdep, cpu/hotplus: Use a weaker annotation in AP thread

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Message-ID: <20240130025836.GA49173@system.software.com>
Date: Tue, 30 Jan 2024 11:58:36 +0900
From: Byungchul Park <byungchul@...com>
To: Thomas Gleixner <tglx@...utronix.de>
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Subject: Re: [PATCH v11 14/26] locking/lockdep, cpu/hotplus: Use a weaker
 annotation in AP thread

On Fri, Jan 26, 2024 at 06:30:02PM +0100, Thomas Gleixner wrote:
> On Wed, Jan 24 2024 at 20:59, Byungchul Park wrote:
> 
> Why is lockdep in the subsystem prefix here? You are changing the CPU
> hotplug (not hotplus) code, right?
> 
> > cb92173d1f0 ("locking/lockdep, cpu/hotplug: Annotate AP thread") was
> > introduced to make lockdep_assert_cpus_held() work in AP thread.
> >
> > However, the annotation is too strong for that purpose. We don't have to
> > use more than try lock annotation for that.
> 
> This lacks a proper explanation why this is too strong.
> 
> > Furthermore, now that Dept was introduced, false positive alarms was
> > reported by that. Replaced it with try lock annotation.
> 
> I still have zero idea what this is about.

1. can track PG_locked that is a potential deadlock trigger.

   https://lore.kernel.org/lkml/1674268856-31807-1-git-send-email-byungchul.park@lge.com/

2. can track any waits/events e.g. wait_for_xxx(), dma fence and so on.

3. easy to annotate using dept_wait() on waits, dept_event() on events.

4. track read lock better way instead of the ugly way, by assinging wait
   or event annotations onto read lock and write lock. For instrance, a
   read lock is annotated as a potential waiter for its write unlock,
   and a write lock is annotated as a potential waiter for either write
   unlock or read unlock.

I'd like to remove unnecessary complexity on deadlock detection and add
additional functionality by making it do what the type of tool exactly
should do.

	Byungchul

> Thanks,
> 
>         tglx