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

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Message-ID: <20241004194330.651c1db8.gary@garyguo.net>
Date: Fri, 4 Oct 2024 19:43:30 +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 20:34:01 +0200
Andreas Hindborg <a.hindborg@...nel.org> wrote:

> Hi Gary,
> 
> "Gary Guo" <gary@...yguo.net> writes:
> 
> [...]
> 
> >  // SAFETY: All instances of `Request<T>` are reference counted. This
> >  // implementation of `AlwaysRefCounted` ensure that increments to the ref count
> >  // keeps the object alive in memory at least until a matching reference count
> >  // decrement is executed.
> >  unsafe impl<T: Operations> AlwaysRefCounted for Request<T> {
> >      fn inc_ref(&self) {
> > -        let refcount = &self.wrapper_ref().refcount();
> > -
> > -        #[cfg_attr(not(CONFIG_DEBUG_MISC), allow(unused_variables))]
> > -        let updated = atomic_relaxed_op_unless(refcount, |x| x + 1, 0);
> > -
> > -        #[cfg(CONFIG_DEBUG_MISC)]
> > -        if !updated {
> > -            panic!("Request refcount zero on clone")
> > -        }
> > +        self.wrapper_ref().refcount().inc();  
> 
> What happened to the debug code?
> 
> 
> BR Andreas
> 

`refcount_inc` will WARN and saturate the refcount when trying to
increment from zero. This is true regardless of config.

I've already captured this in `Refcount::inc` documentation.

Best,
Gary