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

[Danilo Krummrich <dakr@kernel.org>]

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, 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/