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

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Message-ID: <Z7xh5bEyh_MII4WV@pollux>
Date: Mon, 24 Feb 2025 13:11:17 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Dave Airlie <airlied@...il.com>, Gary Guo <gary@...yguo.net>,
	Joel Fernandes <joel@...lfernandes.org>,
	Boqun Feng <boqun.feng@...il.com>,
	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
Subject: Re: [RFC PATCH 0/3] gpu: nova-core: add basic timer subdevice
 implementation

On Mon, Feb 24, 2025 at 01:07:19PM +0100, Danilo Krummrich wrote:
> CC: Gary
> 
> On Mon, Feb 24, 2025 at 10:40:00AM +0900, Alexandre Courbot wrote:
> > This inability to sleep while we are accessing registers seems very
> > constraining to me, if not dangerous. It is pretty common to have
> > functions intermingle hardware accesses with other operations that might
> > sleep, and this constraint means that in such cases the caller would
> > need to perform guard lifetime management manually:
> > 
> >   let bar_guard = bar.try_access()?;
> >   /* do something non-sleeping with bar_guard */
> >   drop(bar_guard);
> > 
> >   /* do something that might sleep */
> > 
> >   let bar_guard = bar.try_access()?;
> >   /* do something non-sleeping with bar_guard */
> >   drop(bar_guard);
> > 
> >   ...
> > 
> > Failure to drop the guard potentially introduces a race condition, which
> > will receive no compile-time warning and potentialy not even a runtime
> > one unless lockdep is enabled. This problem does not exist with the
> > equivalent C code AFAICT

Without klint [1] it is exactly the same as in C, where I have to remember to
not call into something that might sleep from atomic context.

> > which makes the Rust version actually more
> > error-prone and dangerous, the opposite of what we are trying to achieve
> > with Rust. Or am I missing something?
> 
> Generally you are right, but you have to see it from a different perspective.
> 
> What you describe is not an issue that comes from the design of the API, but is
> a limitation of Rust in the kernel. People are aware of the issue and with klint
> [1] there are solutions for that in the pipeline, see also [2] and [3].
> 
> [1] https://rust-for-linux.com/klint
> [2] https://github.com/Rust-for-Linux/klint/blob/trunk/doc/atomic_context.md
> [3] https://www.memorysafety.org/blog/gary-guo-klint-rust-tools/