linux-kernel - Re: [PATCH 3/3] rust: block: convert `block::mq` to use `Refcount`

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Message-ID: <2024100507-percolate-kinship-fc9a@gregkh>
Date: Sat, 5 Oct 2024 09:47:25 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Gary Guo <gary@...yguo.net>
Cc: Andreas Hindborg <a.hindborg@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Jens Axboe <axboe@...nel.dk>, Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mark Rutland <mark.rutland@....com>, linux-block@...r.kernel.org,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] rust: block: convert `block::mq` to use `Refcount`

On Fri, Oct 04, 2024 at 04:52:24PM +0100, Gary Guo wrote:
> There is an operation needed by `block::mq`, atomically decreasing
> refcount from 2 to 0, which is not available through refcount.h, so
> I exposed `Refcount::as_atomic` which allows accessing the refcount
> directly.

That's scary, and of course feels wrong on many levels, but:


> @@ -91,13 +95,17 @@ pub(crate) unsafe fn start_unchecked(this: &ARef<Self>) {
>      /// C `struct request`. If the operation fails, `this` is returned in the
>      /// `Err` variant.
>      fn try_set_end(this: ARef<Self>) -> Result<*mut bindings::request, ARef<Self>> {
> -        // We can race with `TagSet::tag_to_rq`
> -        if let Err(_old) = this.wrapper_ref().refcount().compare_exchange(
> -            2,
> -            0,
> -            Ordering::Relaxed,
> -            Ordering::Relaxed,
> -        ) {
> +        // To hand back the ownership, we need the current refcount to be 2.
> +        // Since we can race with `TagSet::tag_to_rq`, this needs to atomically reduce
> +        // refcount to 0. `Refcount` does not provide a way to do this, so use the underlying
> +        // atomics directly.
> +        if this
> +            .wrapper_ref()
> +            .refcount()
> +            .as_atomic()
> +            .compare_exchange(2, 0, Ordering::Relaxed, Ordering::Relaxed)
> +            .is_err()

Why not just call rust_helper_refcount_set()?  Or is the issue that you
think you might not be 2 here?  And if you HAVE to be 2, why that magic
value (i.e. why not just always be 1 and rely on normal
increment/decrement?)

I know some refcounts are odd in the kernel, but I don't see where the
block layer is caring about 2 as a refcount anywhere, what am I missing?

thanks,

greg k-h