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

[Benno Lossin]

On Thu Jun 26, 2025 at 10:00 PM CEST, Danilo Krummrich wrote:
> So far Devres uses an inner memory allocation and reference count, i.e.
> an inner Arc, in order to ensure that the devres callback can't run into
> a use-after-free in case where the Devres object is dropped while the
> devres callback runs concurrently.
>
> Instead, use a completion in order to avoid a potential UAF: In
> Devres::drop(), if we detect that we can't remove the devres action
> anymore, we wait for the completion that is completed from the devres
> callback. If, in turn, we were able to successfully remove the devres
> action, we can just go ahead.
>
> This, again, allows us to get rid of the internal Arc, and instead let
> Devres consume an `impl PinInit<T, E>` in order to return an
> `impl PinInit<Devres<T>, E>`, which enables us to get away with less
> memory allocations.
>
> Additionally, having the resulting explicit synchronization in
> Devres::drop() prevents potential subtle undesired side effects of the
> devres callback dropping the final Arc reference asynchronously within
> the devres callback.
>
> Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>

With the invariants section moved:

Reviewed-by: Benno Lossin <lossin@xxxxxxxxxx>

---
Cheers,
Benno

> ---
>  drivers/gpu/nova-core/driver.rs |   7 +-
>  drivers/gpu/nova-core/gpu.rs    |   6 +-
>  rust/kernel/devres.rs           | 213 +++++++++++++++++++-------------
>  rust/kernel/pci.rs              |  20 +--
>  samples/rust/rust_driver_pci.rs |  19 +--
>  5 files changed, 154 insertions(+), 111 deletions(-)