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

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Message-ID: <20241005160857.537ae04b.gary@garyguo.net>
Date: Sat, 5 Oct 2024 16:08:57 +0100
From: Gary Guo <gary@...yguo.net>
To: Andreas Hindborg <a.hindborg@...nel.org>
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`

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.

Best,
Gary