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

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Message-ID: <87iku6o080.fsf@kernel.org>
Date: Sat, 05 Oct 2024 20:47:59 +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`

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

> On Fri, 04 Oct 2024 20:05:41 +0200
> Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
>> 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
>>
>
> The helper LTO patch series still need some time. I'd want to be able to
> test on 32-bit archs in the meantime.

In that case I would rather just conditionally gate the `block` module
and the `rnull` driver on 64 bit arch.

BR Andreas