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

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Message-ID: <87ldz3pwui.fsf@kernel.org>
Date: Fri, 04 Oct 2024 20:05:41 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Gary Guo" <gary@...yguo.net>
Cc: "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`

Hi Gary,

"Gary Guo" <gary@...yguo.net> writes:

> Currently there's a custom reference counting in `block::mq`, which uses
> `AtomicU64` Rust atomics, and this type doesn't exist on some 32-bit
> architectures. We cannot just change it to use 32-bit atomics, because
> doing so will make it vulnerable to refcount overflow. So switch it to
> use the kernel refcount `kernel::sync::Refcount` instead.
>
> 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.

I would rather wait with this patch until the helper LTO patches land
upstream. Or at least let me run the benchmarks to see the effect of not
inlining these refcount operations.

Best regards,
Andreas