Re: [PATCH 2/3] rust: devres: implement Devres::access_with()

[Danilo Krummrich <dakr@kernel.org>]

On Sat, Apr 26, 2025 at 08:28:30PM +0000, Benno Lossin wrote:
> On Sat Apr 26, 2025 at 3:30 PM CEST, Danilo Krummrich wrote:
> > Implement a direct accessor for the data stored within the Devres 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 not clear whether the data has been revoked already due to the device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.
> >
> > Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> > ---
> >  rust/kernel/devres.rs | 35 +++++++++++++++++++++++++++++++++++
> >  1 file changed, 35 insertions(+)
> >
> > diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> > index 1e58f5d22044..ec2cd9cdda8b 100644
> > --- a/rust/kernel/devres.rs
> > +++ b/rust/kernel/devres.rs
> > @@ -181,6 +181,41 @@ pub fn new_foreign_owned(dev: &Device<Bound>, data: T, flags: Flags) -> Result {
> >  
> >          Ok(())
> >      }
> > +
> > +    /// Obtain `&'a T`, bypassing the [`Revocable`].
> > +    ///
> > +    /// This method allows to directly obtain a `&'a T`, bypassing the [`Revocable`], by presenting
> > +    /// a `&'a Device<Bound>` of the same [`Device`] this [`Devres`] instance has been created with.
> > +    ///
> > +    /// An error is returned if `dev` does not match the same [`Device`] this [`Devres`] instance
> > +    /// has been created with.
> > +    ///
> > +    /// # Example
> > +    ///
> > +    /// ```no_run
> 
> The `no_run` is not necessary, as you don't run any code, you only
> define a function.

Yet, I'd like to keep it to make it obvious that this test isn't supposed to
run.

> > +    /// # use kernel::{device::Core, devres::Devres, pci};
> > +    ///
> > +    /// fn from_core(dev: &pci::Device<Core>, devres: Devres<pci::Bar<0x4>>) -> Result<()> {
> > +    ///     let bar = devres.access_with(dev.as_ref())?;
> > +    ///
> > +    ///     let _ = bar.read32(0x0);
> > +    ///
> > +    ///     // might_sleep()
> > +    ///
> > +    ///     bar.write32(0x42, 0x0);
> > +    ///
> > +    ///     Ok(())
> > +    /// }
> 
> Missing '```'?

Good catch -- interestingly the doctest did compile anyways.

> 
> > +    pub fn access_with<'s, 'd: 's>(&'s self, dev: &'d Device<Bound>) -> Result<&'s T> {
> 
> I don't think that we need the `'d` lifetime here (if not, we should
> remove it).

If the returned reference out-lives dev it can become invalid, since it means
that the device could subsequently be unbound. Hence, I think we indeed need to
require that the returned reference cannot out-live dev.