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

[Danilo Krummrich <dakr@kernel.org>]

On Thu, Jul 03, 2025 at 02:50:48PM +0200, Greg Kroah-Hartman wrote:
> On Thu, Jul 03, 2025 at 02:29:31PM +0200, 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);
> > 
> > Pin-Init API
> > ------------
> > 
> > 	#[pin_data]
> > 	struct Process {
> > 	    task: ARef<Task>,
> > 	    #[pin]
> > 	    inner: File<SpinLock<ProcessInner>>,
> > 	}
> > 	
> > 	pub(crate) struct ProcessInner {
> > 	    threads: RBTree<i32, Arc<Thread>>,
> > 	    max_threads: u32,
> > 	}
> > 
> > Here Process does not need to be within an Arc and no separate file instances
> > need to be kept around, that happens already within the constructor of Process:
> > 
> > 	pin_init!(Process {
> > 	   inner <- dir.create_file("process_inner", ...),
> > 	   [...]
> > 	})
> > 
> > The file itself has a reference to SpinLock<ProcessInner>, hence we can access:
> > 
> > 	let guard = inner.lock();
> > 
> > 	read_or_write(guard.threads)
> > 	read_or_write(guard.max_threads)
> > 
> > The difference is that with the ForeignOwnable API it was possible to have
> > separate files for threads and max_threads.
> > 
> > While with the pin-init one we either have to have a single file exposing
> > ProcessInner (which is what I did above) or protect threads and max_threads
> > with separate locks (of course max_threads could also just be an atomic).
> > 
> > (If you like I can sketch up this case as well.)
> > 
> > At a first glance this seems like an undesirable limitation, but I argue that
> > this is a good thing.
> > 
> > The reason I think so is what I also explained in [1], but let me adjust it a
> > bit for this reply:
> > 
> > threads and max_threads being protected by the same lock means that they are in
> > a certain relationship to each other. Meaning that they only really make sense
> > looking at them atomically.
> > 
> > So I argue it does not make sense to expose those values to userspace through
> > separate files.
> > 
> > For instance:
> > 
> > 	$ cat max_threads && cat threads
> > 	$ 5
> > 	$ 10
> > 
> > This way you may read 5 max_threads, but 10 actual threads, because things may
> > have changed in between the two cat commands.
> > 
> > However, if instead, they are exposed through a single file, we get an atomic
> > view of them, such that the semantic relationship between them is preserved.
> > 
> > For instance:
> > 
> > 	$ cat process_info
> > 	$ threads: 2
> > 	$ max_threads: 10
> 
> I think you mean to write:
>  	$ cat process_info
>  	threads: 2
>  	max_threads: 10
> 
> right?

Yes, indeed. :)