Re: [PATCH v5 3/4] sample: rust_misc_device: Demonstrate additional get/set value functionality

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Dec 06, 2024 at 12:42:14PM +0000, Lee Jones wrote:
> Expand the complexity of the sample driver by providing the ability to
> get and set an integer.  The value is protected by a mutex.
> 
> Here is a simple userspace program that fully exercises the sample
> driver's capabilities.

nit, subject line should have "samples" not "sample" :)

> +    fn get_value(&self, mut writer: UserSliceWriter) -> Result<isize> {
> +        let guard = self.inner.lock();
> +        let value = guard.value;
> +
> +        // Refrain from calling write() on a locked resource
> +        drop(guard);
> +
> +        pr_info!("-> Copying data to userspace (value: {})\n", &value);
> +
> +        writer.write::<i32>(&value)?;
> +        Ok(0)
> +    }

I don't understand why you have to drop the mutex before calling
pr_info() and write (i.e. copy_to_user())?  It's a mutex, not a
spinlock, so you can hold it over that potentially-sleeping call, right?
Or is there some other reason why here?

thanks,

greg k-h