linux-kernel - Re: [PATCH 0/3] Devres optimization with bound devices

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Message-ID: <aA0TU1Abvm3YxMgS@Mac.home>
Date: Sat, 26 Apr 2025 10:09:39 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: 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, gary@...yguo.net,
	bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
	a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu,
	linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] Devres optimization with bound devices

On Sat, Apr 26, 2025 at 03:30:38PM +0200, Danilo Krummrich wrote:
> This patch series implements a direct accessor for the data stored within
> a Devres container 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 not clear whether the data has been revoked already due to the device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.
> 

Nice! However, IIUC, if the users use Devres::new() to create a `Devres`
, they will have a `Devres` they can revoke anytime, which means you can
still revoke the `Devres` even if the device is bound.

Also if a `Devres` belongs to device A, but someone passes device B's
bound reference to `access_with()`, the compiler won't check for that,
and the `Devres` can be being revoked as the same, no? If so the
function is not safe.

Regards,
Boqun

> The patches of this series are also available in [1].
> 
> [1] https://web.git.kernel.org/pub/scm/linux/kernel/git/dakr/linux.git/log/?h=rust/devres
> 
> Danilo Krummrich (3):
>   rust: revocable: implement Revocable::access()
>   rust: devres: implement Devres::access_with()
>   samples: rust: pci: take advantage of Devres::access_with()
> 
>  rust/kernel/devres.rs           | 35 +++++++++++++++++++++++++++++++++
>  rust/kernel/revocable.rs        | 12 +++++++++++
>  samples/rust/rust_driver_pci.rs | 12 +++++------
>  3 files changed, 53 insertions(+), 6 deletions(-)
> 
> 
> base-commit: b5cb47f81a2857d270cabbbb3a9feec0e483caed
> -- 
> 2.49.0
>