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

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Date: Mon, 29 Jan 2024 13:20:52 +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?

I will fix the typo ;( Thank you.

I referred to the commit cb92173d1f047. I will remove the prefix if the
way is more desirable.

> > 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.

rwsem_acquire() implies:

   1. might be a waiter on contention of the lock.
   2. enter to the critical section of the lock.

All we need in here is to act 2, not 1. That's why I suggested trylock
version of annotation for that purpose.

Now that dept partially replies on lockdep annotaions for the waiters
and events, dept is interpeting rwsem_acquire() as a potential waiter
and reports a deadlock by the wait.

Of course, the first priority should be not to change the current
behavior. I think the change from non-trylock to trylock for the
annotation won't. Or am I missing something?

	Byungchul

> > 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.
> 
> Thanks,
> 
>         tglx