linux-kernel - Re: [PATCH v4 3/5] rust: devres: get rid of Devres' inner Arc

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Message-Id: <DAWUWCQCW7WD.29U1POJFXTLXS@kernel.org>
Date: Fri, 27 Jun 2025 01:33:41 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Danilo Krummrich" <dakr@...nel.org>, <gregkh@...uxfoundation.org>,
 <rafael@...nel.org>, <ojeda@...nel.org>, <alex.gaynor@...il.com>,
 <boqun.feng@...il.com>, <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>,
 <a.hindborg@...nel.org>, <aliceryhl@...gle.com>, <tmgross@...ch.edu>,
 <david.m.ertman@...el.com>, <ira.weiny@...el.com>, <leon@...nel.org>,
 <kwilczynski@...nel.org>, <bhelgaas@...gle.com>
Cc: <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v4 3/5] rust: devres: get rid of Devres' inner Arc

On Thu Jun 26, 2025 at 10:00 PM CEST, Danilo Krummrich wrote:
> diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
> index 60b86f370284..47653c14838b 100644
> --- a/drivers/gpu/nova-core/gpu.rs
> +++ b/drivers/gpu/nova-core/gpu.rs

> @@ -161,14 +161,14 @@ fn new(bar: &Bar0) -> Result<Spec> {
>  pub(crate) struct Gpu {
>      spec: Spec,
>      /// MMIO mapping of PCI BAR 0
> -    bar: Devres<Bar0>,
> +    bar: Arc<Devres<Bar0>>,

Can't you store it inline, given that you return an `impl PinInit<Self>`
below?

>      fw: Firmware,
>  }
>  
>  impl Gpu {
>      pub(crate) fn new(
>          pdev: &pci::Device<device::Bound>,
> -        devres_bar: Devres<Bar0>,
> +        devres_bar: Arc<Devres<Bar0>>,
>      ) -> Result<impl PinInit<Self>> {

While I see this code, is it really necessary to return `Result`
wrapping the initializer here? I think it's probably better to return
`impl PinInit<Self, Error>` instead. (of course in a different patch/an
issue)

>          let bar = devres_bar.access(pdev.as_ref())?;
>          let spec = Spec::new(bar)?;

> @@ -44,6 +49,10 @@ struct DevresInner<T: Send> {
>  /// [`Devres`] users should make sure to simply free the corresponding backing resource in `T`'s
>  /// [`Drop`] implementation.
>  ///
> +/// # Invariants
> +///
> +/// [`Self::inner`] is guaranteed to be initialized and is always accessed read-only.
> +///

Let's put this section below the examples, I really ought to write the
safety docs one day and let everyone vote on this kind of stuff...

>  /// # Example
>  ///
>  /// ```no_run

> @@ -213,44 +233,63 @@ pub fn new(dev: &Device<Bound>, data: T, flags: Flags) -> Result<Self> {
>      /// }
>      /// ```
>      pub fn access<'a>(&'a self, dev: &'a Device<Bound>) -> Result<&'a T> {
> -        if self.0.dev.as_raw() != dev.as_raw() {
> +        if self.dev.as_raw() != dev.as_raw() {
>              return Err(EINVAL);
>          }
>  
>          // SAFETY: `dev` being the same device as the device this `Devres` has been created for
> -        // proves that `self.0.data` hasn't been revoked and is guaranteed to not be revoked as
> -        // long as `dev` lives; `dev` lives at least as long as `self`.
> -        Ok(unsafe { self.0.data.access() })
> +        // proves that `self.data` hasn't been revoked and is guaranteed to not be revoked as long
> +        // as `dev` lives; `dev` lives at least as long as `self`.

What if the device has been unbound and a new device has been allocated
in the exact same memory?

---
Cheers,
Benno

> +        Ok(unsafe { self.data().access() })
>      }