Re: [PATCH] rust: devres: fix race condition due to nesting

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Feb 05, 2026 at 11:25:15PM +0100, Danilo Krummrich wrote:
> Commit f5d3ef25d238 ("rust: devres: get rid of Devres' inner Arc") did
> attempt to optimize away the internal reference count of Devres.
> 
> However, without an internal reference count, we can't support cases
> where Devres is indirectly nested, resulting into a deadlock.
> 
> Such indirect nesting easily happens in the following way:
> 
> A registration object (which is guarded by devres) hold a reference
> count of an object that holds a device resource guarded by devres
> itself.
> 
> For instance a drm::Registration holds a reference of a drm::Device. The
> drm::Device itself holds a device resource in its private data.
> 
> When the drm::Registration is dropped by devres, and it happens that it
> did hold the last reference count of the drm::Device, it also drops the
> device resource, which is guarded by devres itself.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

> +        // Take additional reference count for `devm_add_action()`.
> +        core::mem::forget(data.clone());

I'd feel better if you called .clone() prior to devm_add_action(). That
way, even if devm somehow runs the callback before we get to this call
to clone, the refcount has already been incremented.

I know it's not really a problem because of the &Device<Bound> argument.

Alice