Re: [PATCH v8 4/6] rust: debugfs: Support arbitrary owned backing for File

["Benno Lossin" <lossin@kernel.org>]

On Thu Jul 3, 2025 at 2:29 PM CEST, Danilo Krummrich wrote:
> On Thu, Jul 03, 2025 at 01:41:53PM +0200, Greg Kroah-Hartman wrote:
>> Yes, we need to be able to have a debugfs file callback handle a mutable
>> structure in order to lock things correctly.  We also need to have it be
>> mutable so that it can MODIFY the value (everyone seems to forget that
>> debugfs allows that...)
>
> Well, that's possible with both approaches. Data behind a lock becomes mutable
> once you grabbed the lock. That's the same in both cases.
>
> The difference is that with the pin-init approach I propose you can't have what
> Alice sketched up. And I think it's even desirable that you can't do it.
>
> Let's me explain the difference on a simplified example, based on Alice'
> example.
>
> ForeignOwnable API
> ------------------
>
> 	#[pin_data]
> 	struct Process {
> 	    task: ARef<Task>,
> 	    #[pin]
> 	    inner: SpinLock<ProcessInner>,
> 	}
> 	
> 	pub(crate) struct ProcessInner {
> 	    threads: RBTree<i32, Arc<Thread>>,
> 	    max_threads: u32,
> 	}
>
> Here we have to create an Arc<Process> (let's call it process) and create files
> from it.
>
> 	let file_threads = dir.create_file("threads", process);
> 	let file_max_threads = dir.create_file("max_threads", process);
>
> In the file system callback of both of these, we now have an Arc<Process>, hence
> we can access:
>
> 	let guard = process.inner.lock();
>
> 	read_or_write(guard.max_threads);
>
> and in the other file:
>
> 	let guard = process.inner.lock();
>
> 	read_or_write(guard.max_threads);

Where do you actually specify this callback? At the moment, the API asks
for `Display` and thus it can only read values?

---
Cheers,
Benno