Re: [PATCH v2 4/4] rust: samples: Add debugfs sample

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Apr 30, 2025 at 11:31:59PM +0000, Matthew Maurer wrote:
> Provides an example of using the Rust DebugFS bindings.
> 
> Signed-off-by: Matthew Maurer <mmaurer@google.com>

Much nicer, many thanks for this!

Some minor comments on the sample code here.  As someone coming from C
with limited Rust experience, I think I do understand it, but I think it
could use a bunch of comments to make it more "obvious" what is
happening, see below.

> +static EXAMPLE: AtomicU32 = AtomicU32::new(8);

Wait, why is this set to 8 and then you automatically set it to 10 after
you create the file?  No one is ever going to see 8 as a valid value
unless they really race to read the file, right?

> +impl kernel::Module for RustDebugFs {
> +    fn init(_this: &'static ThisModule) -> Result<Self> {
> +        let debugfs = Dir::new(c_str!("sample_debugfs"));
> +        debugfs
> +            .fmt_file(c_str!("example"), &EXAMPLE, &|example, f| {
> +                writeln!(f, "Reading atomic: {}", example.load(Ordering::Relaxed))
> +            })
> +            .keep();
> +        EXAMPLE.store(10, Ordering::Relaxed);
> +        Ok(Self { _debugfs: debugfs })
> +    }
> +}


How about this rewrite with comments added to help make things more
obvious:

impl kernel::Module for RustDebugFs {
    fn init(_this: &'static ThisModule) -> Result<Self> {

        // Create a debugfs directory in the root of the filesystem
        // called "sample_debugfs"
        let debugfs = Dir::new(c_str!("sample_debugfs"));

        // Create a single file in the directory called "example" that
        // allows to read from the EXAMPLE atomic variable, and make
        // sure it lives past the scope of this function by calling
        // .keep() on it.
        debugfs
            .fmt_file(c_str!("example"), &EXAMPLE, &|example, f| {
                writeln!(f, "Reading atomic: {}", example.load(Ordering::Relaxed))
            })
            .keep();

        // Change the value of EXAMPLE to be 10 so that will be the
        // value read from the file.  Note, the original value 8 will be
        // read if the file is read right before this is called.
        EXAMPLE.store(10, Ordering::Relaxed);

        // Create our module object and save off the pointer to the
        // debugfs directory we created.  It will be automatically
        // removed when the module is unloaded by virtue of the
        // reference count to the structure being dropped at that point
        // in time.
        Ok(Self { _debugfs: debugfs })
    }
}

thanks,

greg k-h