linux-kernel - Re: [RFC PATCH 0/3] gpu: nova-core: add basic timer subdevice implementation

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Message-ID: <20250227161733.GH39591@nvidia.com>
Date: Thu, 27 Feb 2025 12:17:33 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Danilo Krummrich <dakr@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Alexandre Courbot <acourbot@...dia.com>,
	Dave Airlie <airlied@...il.com>, Gary Guo <gary@...yguo.net>,
	Joel Fernandes <joel@...lfernandes.org>,
	John Hubbard <jhubbard@...dia.com>, Ben Skeggs <bskeggs@...dia.com>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
	paulmck@...nel.org
Subject: Re: [RFC PATCH 0/3] gpu: nova-core: add basic timer subdevice
 implementation

On Thu, Feb 27, 2025 at 07:18:02AM -0800, Boqun Feng wrote:
> On Thu, Feb 27, 2025 at 10:46:18AM -0400, Jason Gunthorpe wrote:
> > On Wed, Feb 26, 2025 at 04:41:08PM -0800, Boqun Feng wrote:
> > > And if you don't store the HrTimerHandle anywhere, like you drop() it
> > > right after start a hrtimer, it will immediately stop the timer. Does
> > > this make sense?
> > 
> > Oh, I understand that, but it is not sufficient in the kernel.
> > 
> > You are making an implicit argument that something external to the
> > rust universe will hold the module alive until all rust destructors
> > are run. That is trivialy obvious in your example above.
> > 
> 
> The question in your previous email is about function pointer of hrtimer
> EAF because of module unload, are you moving to a broader topic
> here?

No

> If no, the for module unload, the argument is not implicit because in
> rust/macro/module.rs the module __exit() function is generated by Rust,
> and in that function, `assume_init_drop()` will call these
> destructors.

That is not what I mean. You can be running code in multiple threads
from multiple functions in the module those are all being protected
implicitly by external C code functions. Rust itself is not managing
module life time.

Then you are making the argument that everything created by a rust
module somehow traces its reference back to the module itself,
regardless of what thread, callback or memory was used to create it.

So all bindings for everything are expected to clean themselves up,
recursively.

That does make sense, but then it still raises questions that things
like workqueue don't seem to have the cleanup.

I still wonder why you couldn't also have these reliable reference
counts rooted on the device driver instead of only on the module.

Jason