Message-ID: <D9GUR8Y08PQ6.2ULV6V4UJAGQB@proton.me>
Date: Sat, 26 Apr 2025 20:28:30 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>, gregkh@...uxfoundation.org, rafael@...nel.org, bhelgaas@...gle.com, kwilczynski@...nel.org, zhiw@...dia.com, cjia@...dia.com, jhubbard@...dia.com, bskeggs@...dia.com, acurrid@...dia.com, joelagnelf@...dia.com, ttabi@...dia.com, acourbot@...dia.com, 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
Cc: linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] rust: devres: implement Devres::access_with()

On Sat Apr 26, 2025 at 3:30 PM CEST, Danilo Krummrich wrote:
> Implement a direct accessor for the data stored within the Devres for
> cases where we can proof that we own a reference to a Device<Bound>
> (i.e. a bound device) of the same device that was used to create the
> corresponding Devres container.
>
> Usually, when accessing the data stored within a Devres container, it is
> not clear whether the data has been revoked already due to the device
> being unbound and, hence, we have to try whether the access is possible
> and subsequently keep holding the RCU read lock for the duration of the
> access.
>
> However, when we can proof that we hold a reference to Device<Bound>
> matching the device the Devres container has been created with, we can
> guarantee that the device is not unbound for the duration of the
> lifetime of the Device<Bound> reference and, hence, it is not possible
> for the data within the Devres container to be revoked.
>
> Therefore, in this case, we can bypass the atomic check and the RCU read
> lock, which is a great optimization and simplification for drivers.
>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
> ---
>  rust/kernel/devres.rs | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
>
> diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> index 1e58f5d22044..ec2cd9cdda8b 100644
> --- a/rust/kernel/devres.rs
> +++ b/rust/kernel/devres.rs
> @@ -181,6 +181,41 @@ pub fn new_foreign_owned(dev: &Device<Bound>, data: T, flags: Flags) -> Result {
>  
>          Ok(())
>      }
> +
> +    /// Obtain `&'a T`, bypassing the [`Revocable`].
> +    ///
> +    /// This method allows to directly obtain a `&'a T`, bypassing the [`Revocable`], by presenting
> +    /// a `&'a Device<Bound>` of the same [`Device`] this [`Devres`] instance has been created with.
> +    ///
> +    /// An error is returned if `dev` does not match the same [`Device`] this [`Devres`] instance
> +    /// has been created with.
> +    ///
> +    /// # Example
> +    ///
> +    /// ```no_run

The `no_run` is not necessary, as you don't run any code, you only
define a function.

> +    /// # use kernel::{device::Core, devres::Devres, pci};
> +    ///
> +    /// fn from_core(dev: &pci::Device<Core>, devres: Devres<pci::Bar<0x4>>) -> Result<()> {
> +    ///     let bar = devres.access_with(dev.as_ref())?;
> +    ///
> +    ///     let _ = bar.read32(0x0);
> +    ///
> +    ///     // might_sleep()
> +    ///
> +    ///     bar.write32(0x42, 0x0);
> +    ///
> +    ///     Ok(())
> +    /// }

Missing '```'?

> +    pub fn access_with<'s, 'd: 's>(&'s self, dev: &'d Device<Bound>) -> Result<&'s T> {

I don't think that we need the `'d` lifetime here (if not, we should
remove it).

> +        if self.0.dev.as_raw() != dev.as_raw() {
> +            return Err(EINVAL);
> +        }
> +
> +        // SAFETY: `dev` being the same device as the device this `Devres` has been created for
> +        // proofes that `self.0.data` hasn't been revoked and is guaranteed to not be revoked as

s/proofes/proves/

---
Cheers,
Benno

> +        // long as `dev` lives; `dev` lives at least as long as `self`.
> +        Ok(unsafe { self.deref().access() })
> +    }
>  }
>  
>  impl<T> Deref for Devres<T> {

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