linux-kernel - Re: [RFC] DEPT(DEPendency Tracker) with DLM(Distributed Lock Manager)

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Message-ID: <CAK-6q+j3QJCB6XERbJfEkvro=Kucq0PQHrCyrZ+LxDq5yHx+=g@mail.gmail.com>
Date: Wed, 28 May 2025 08:00:02 -0400
From: Alexander Aring <aahringo@...hat.com>
To: Byungchul Park <byungchul@...com>
Cc: kernel_team@...ynix.com, linux-kernel@...r.kernel.org, 
	gfs2 <gfs2@...ts.linux.dev>
Subject: Re: [RFC] DEPT(DEPendency Tracker) with DLM(Distributed Lock Manager)

Hi,

On Sun, May 25, 2025 at 8:13 PM Alexander Aring <aahringo@...hat.com> wrote:
>
> Hi,
>
> On Thu, May 22, 2025 at 1:28 AM Byungchul Park <byungchul@...com> wrote:
> >
> > On Thu, May 22, 2025 at 02:24:53PM +0900, Byungchul Park wrote:
> > > Hi Alexander,
> > >
> > > We briefly talked about dept with DLM in an external channel.  However,
> > > it'd be great to discuss what to aim and how to make it in more detail,
> > > in this mailing list.
> > >
> > > It's worth noting that dept doesn't track dependencies beyond different
> > > contexts to avoid adding false dependencies by any chance, which means
> > > though dept checks the dependency sanity *globally*, when it comes to
> > > creating dependencies, it happens only within e.g. each single system
> > > call context, each single irq context, each worker context, and so on,
> > > with its unique context id assigned to each independent context.
> > >
> > > In order for dept to work on DLM, we need a way to assign a unique
> > > context id to each interesting context in DLM's point of view, and let
> > > dept know the id.  Once making it done, I think dept can work on DLM
> > > perfectly.
> >
> > Plus, we need a way to share the global dependency graph used by dept
> > between nodes too.
> >
>
> Having everything simulated and having nodes separated as
> net-namespaces in one Linux kernel instance is I think at first
> simpler to do and will show the "proof of concepts".
> Sharing data between nodes is then just some memory area that is not
> separated by per "struct net" context.

Alternatively the master node of the lock (this node knows everything
about the lock operations being done including the nodes that are
waiting to get the lock granted) can be used to detect cycles, we
already do that for some simple cases when converting locks directly
[0]. Maybe this is already enough to have all the information, but it
is not just a "wait_event()" mechanism, there needs to be some other
API to use DEPT for this case?

- Alex

[0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/dlm/lock.c?h=v6.15#n2163