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

[Danilo Krummrich <dakr@kernel.org>]

On Fri, Jun 27, 2025 at 01:33:41AM +0200, Benno Lossin wrote:
> On Thu Jun 26, 2025 at 10:00 PM CEST, Danilo Krummrich wrote:
> > diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
> > index 60b86f370284..47653c14838b 100644
> > --- a/drivers/gpu/nova-core/gpu.rs
> > +++ b/drivers/gpu/nova-core/gpu.rs
> 
> > @@ -161,14 +161,14 @@ fn new(bar: &Bar0) -> Result<Spec> {
> >  pub(crate) struct Gpu {
> >      spec: Spec,
> >      /// MMIO mapping of PCI BAR 0
> > -    bar: Devres<Bar0>,
> > +    bar: Arc<Devres<Bar0>>,
> 
> Can't you store it inline, given that you return an `impl PinInit<Self>`
> below?

I could, but I already know that we'll have to share bar later on.

> >      fw: Firmware,
> >  }
> >  
> >  impl Gpu {
> >      pub(crate) fn new(
> >          pdev: &pci::Device<device::Bound>,
> > -        devres_bar: Devres<Bar0>,
> > +        devres_bar: Arc<Devres<Bar0>>,
> >      ) -> Result<impl PinInit<Self>> {
> 
> While I see this code, is it really necessary to return `Result`
> wrapping the initializer here? I think it's probably better to return
> `impl PinInit<Self, Error>` instead. (of course in a different patch/an
> issue)

I will double check, but it's rather unlikely it makes sense. There's a lot of
initialization going on in Gpu::new(), the try_pin_init! call would probably get
too crazy.

> 
> >          let bar = devres_bar.access(pdev.as_ref())?;
> >          let spec = Spec::new(bar)?;
> 
> > @@ -44,6 +49,10 @@ struct DevresInner<T: Send> {
> >  /// [`Devres`] users should make sure to simply free the corresponding backing resource in `T`'s
> >  /// [`Drop`] implementation.
> >  ///
> > +/// # Invariants
> > +///
> > +/// [`Self::inner`] is guaranteed to be initialized and is always accessed read-only.
> > +///
> 
> Let's put this section below the examples, I really ought to write the
> safety docs one day and let everyone vote on this kind of stuff...

Sure!

> >  /// # Example
> >  ///
> >  /// ```no_run
> 
> > @@ -213,44 +233,63 @@ pub fn new(dev: &Device<Bound>, data: T, flags: Flags) -> Result<Self> {
> >      /// }
> >      /// ```
> >      pub fn access<'a>(&'a self, dev: &'a Device<Bound>) -> Result<&'a T> {
> > -        if self.0.dev.as_raw() != dev.as_raw() {
> > +        if self.dev.as_raw() != dev.as_raw() {
> >              return Err(EINVAL);
> >          }
> >  
> >          // SAFETY: `dev` being the same device as the device this `Devres` has been created for
> > -        // proves that `self.0.data` hasn't been revoked and is guaranteed to not be revoked as
> > -        // long as `dev` lives; `dev` lives at least as long as `self`.
> > -        Ok(unsafe { self.0.data.access() })
> > +        // proves that `self.data` hasn't been revoked and is guaranteed to not be revoked as long
> > +        // as `dev` lives; `dev` lives at least as long as `self`.
> 
> What if the device has been unbound and a new device has been allocated
> in the exact same memory?

Unbound doesn't mean freed. Devres holds a reference of the device is was
created with, so it is impossible that it has been freed.