linux-kernel - Re: [PATCH v2 1/4] rust: device: implement Device::parent()

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Message-ID: <Z9yowSwzNQqPzc0r@cassiopeiae>
Date: Fri, 21 Mar 2025 00:46:09 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: bhelgaas@...gle.com, 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, linux-pci@...r.kernel.org,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] rust: device: implement Device::parent()

On Thu, Mar 20, 2025 at 10:44:38PM +0000, Benno Lossin wrote:
> On Thu Mar 20, 2025 at 11:27 PM CET, Danilo Krummrich wrote:
> > Device::parent() returns a reference to the device' parent device, if
> > any.
> >
> > Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> > Signed-off-by: Danilo Krummrich <dakr@...nel.org>
> > ---
> >  rust/kernel/device.rs | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> >
> > diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> > index 21b343a1dc4d..f6bdc2646028 100644
> > --- a/rust/kernel/device.rs
> > +++ b/rust/kernel/device.rs
> > @@ -65,6 +65,21 @@ pub(crate) fn as_raw(&self) -> *mut bindings::device {
> >          self.0.get()
> >      }
> >  
> > +    /// Returns a reference to the parent device, if any.
> > +    pub fn parent<'a>(&self) -> Option<&'a Self> {
> > +        // SAFETY:
> > +        // - By the type invariant `self.as_raw()` is always valid.
> > +        // - The parent device is only ever set at device creation.
> > +        let parent = unsafe { (*self.as_raw()).parent };
> > +
> > +        if parent.is_null() {
> > +            None
> > +        } else {
> > +            // SAFETY: Since `parent` is not NULL, it must be a valid pointer to a `struct device`.
> > +            Some(unsafe { Self::as_ref(parent) })
> 
> Why is this valid for `'static`? Since you declare the lifetime `'a`
> independently from the elided one on `&self`, the user can set it to
> `'static`.

Good catch -- this is indeed a problem when the &Device comes from an
ARef<Device>, rather than Device::as_ref(), which is what I had in mind
originally.