linux-kernel - Re: [PATCH v5 4/5] rust: block: convert `block::mq` to use `Refcount`

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Message-Id: <DCDVM56I5WPT.2L24NI3SBBIHU@kernel.org>
Date: Thu, 28 Aug 2025 09:18:40 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Gary Guo" <gary@...yguo.net>
Cc: "Gary Guo" <gary@...nel.org>, "Miguel Ojeda" <ojeda@...nel.org>, "Alex
 Gaynor" <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>,
 "Will Deacon" <will@...nel.org>, "Peter Zijlstra" <peterz@...radead.org>,
 "Mark Rutland" <mark.rutland@....com>, "Tamir Duberstein"
 <tamird@...il.com>, "Francesco Zardi" <frazar00@...il.com>, "Antonio
 Hickey" <contact@...oniohickey.com>, <rust-for-linux@...r.kernel.org>,
 "David Gow" <davidgow@...gle.com>, <linux-block@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 4/5] rust: block: convert `block::mq` to use
 `Refcount`

On Wed Aug 27, 2025 at 9:51 PM CEST, Gary Guo wrote:
> On Tue, 12 Aug 2025 10:17:44 +0200
> "Benno Lossin" <lossin@...nel.org> wrote:
>> On Thu Jul 24, 2025 at 1:32 AM CEST, Gary Guo wrote:
>> > @@ -34,6 +36,18 @@ fn as_ptr(&self) -> *mut bindings::refcount_t {
>> >          self.0.get()
>> >      }
>> >  
>> > +    /// Get the underlying atomic counter that backs the refcount.
>> > +    ///
>> > +    /// NOTE: This will be changed to LKMM atomic in the future.  
>> 
>> Can we discourage using this function a bit more in the docs? At least
>> point people to try other ways before reaching for this, since it allows
>> overflowing & doesn't warn on saturate etc.
>
> Would this additional doc comment be good enough for you?
>
> /// NOTE: usage of this function is discouraged unless there is no way
> /// to achieve the desired result using APIs in `refcount.h`. If an API
> /// in `refcount.h` does not currently contain a binding, please
> /// consider adding a binding for it instead.

I'd like to stress that the atomic doesn't have the same protections as
the refcount type, how about:

    /// NOTE: usage of this function is discouraged as it can circumvent the protections offered by
    /// `refcount.h`. If there is no way to achieve the result using APIs in `refcount.h`, then this
    /// function can be used. Otherwise consider adding a binding for the required API.

---
Cheers,
Benno