Re: [PATCH v7 08/16] rust: add devres abstraction

Danilo Krummrich
2025-01-02T10:30:11Z

On Tue, Dec 24, 2024 at 09:53:23PM +0000, Gary Guo wrote:
> On Thu, 19 Dec 2024 18:04:10 +0100
> Danilo Krummrich <dakr@kernel.org> wrote:
>
> > Add a Rust abstraction for the kernel's devres (device resource
> > management) implementation.
> >
> > The Devres type acts as a container to manage the lifetime and
> > accessibility of device bound resources. Therefore it registers a
> > devres callback and revokes access to the resource on invocation.
> >
> > Users of the Devres abstraction can simply free the corresponding
> > resources in their Drop implementation, which is invoked when either the
> > Devres instance goes out of scope or the devres callback leads to the
> > resource being revoked, which implies a call to drop_in_place().
> >
> > Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> > ---
> > MAINTAINERS | 1 +
> > rust/helpers/device.c | 10 +++
> > rust/helpers/helpers.c | 1 +
> > rust/kernel/devres.rs | 178 +++++++++++++++++++++++++++++++++++++++++
> > rust/kernel/lib.rs | 1 +
> > 5 files changed, 191 insertions(+)
> > create mode 100644 rust/helpers/device.c
> > create mode 100644 rust/kernel/devres.rs
> >
> > <snip>
> >
> > +pub struct Devres<T>(Arc<DevresInner<T>>);
> > +
> > +impl<T> DevresInner<T> {
> > + fn new(dev: &Device, data: T, flags: Flags) -> Result<Arc<DevresInner<T>>> {
> > + let inner = Arc::pin_init(
> > + pin_init!( DevresInner {
> > + data <- Revocable::new(data),
> > + }),
> > + flags,
> > + )?;
> > +
> > + // Convert `Arc<DevresInner>` into a raw pointer and make devres own this reference until
> > + // `Self::devres_callback` is called.
> > + let data = inner.clone().into_raw();
> > +
> > + // SAFETY: `devm_add_action` guarantees to call `Self::devres_callback` once `dev` is
> > + // detached.
> > + let ret = unsafe {
> > + bindings::devm_add_action(dev.as_raw(), Some(Self::devres_callback), data as _)
> > + };
> > +
> > + if ret != 0 {
> > + // SAFETY: We just created another reference to `inner` in order to pass it to
> > + // `bindings::devm_add_action`. If `bindings::devm_add_action` fails, we have to drop
> > + // this reference accordingly.
> > + let _ = unsafe { Arc::from_raw(data) };
> > + return Err(Error::from_errno(ret));
> > + }
> > +
> > + Ok(inner)
> > + }
> > +
> > + #[allow(clippy::missing_safety_doc)]
> > + unsafe extern "C" fn devres_callback(ptr: *mut kernel::ffi::c_void) {
> > + let ptr = ptr as *mut DevresInner<T>;
> > + // Devres owned this memory; now that we received the callback, drop the `Arc` and hence the
> > + // reference.
> > + // SAFETY: Safe, since we leaked an `Arc` reference to devm_add_action() in
> > + // `DevresInner::new`.
> > + let inner = unsafe { Arc::from_raw(ptr) };
> > +
> > + inner.data.revoke();
> > + }
> > +}
> > +
> > +impl<T> Devres<T> {
> > + /// Creates a new [`Devres`] instance of the given `data`. The `data` encapsulated within the
> > + /// returned `Devres` instance' `data` will be revoked once the device is detached.
> > + pub fn new(dev: &Device, data: T, flags: Flags) -> Result<Self> {
> > + let inner = DevresInner::new(dev, data, flags)?;
> > +
> > + Ok(Devres(inner))
> > + }
> > +
> > + /// Same as [`Devres::new`], but does not return a `Devres` instance. Instead the given `data`
> > + /// is owned by devres and will be revoked / dropped, once the device is detached.
> > + pub fn new_foreign_owned(dev: &Device, data: T, flags: Flags) -> Result {
> > + let _ = DevresInner::new(dev, data, flags)?;
> > +
> > + Ok(())
> > + }
> > +}
> > +
> > +impl<T> Deref for Devres<T> {
> > + type Target = Revocable<T>;
> > +
> > + fn deref(&self) -> &Self::Target {
> > + &self.0.data
> > + }
> > +}
> > +
> > +impl<T> Drop for Devres<T> {
> > + fn drop(&mut self) {
> > + // Revoke the data, such that it gets dropped already and the actual resource is freed.
> > + //
> > + // `DevresInner` has to stay alive until the devres callback has been called. This is
> > + // necessary since we don't know when `Devres` is dropped and calling
> > + // `devm_remove_action()` instead could race with `devres_release_all()`.
>
> IIUC, the outcome of that race is the `WARN` if
> devres_release_all takes the spinlock first and has already remvoed the
> action?

Yes, this was one issue. But I think there was another when you have a class
`Registration` that is owned by a `Devres`, which holds private data that is
encapsulated in a `Devres` too.

I have this case in Nova where the DRM device' private data holds the PCI bar,
and the DRM device registration has a reference of the corresponding DRM device.

But maybe this also was something else. I will double check and if I can confirm
that the WARN_ON() in devm_remove_action() is the only issue, we can certainly
change this.

>
> Could you do a custom devres_release here that mimick
> `devm_remove_action` but omit the `WARN`? This way it allows the memory
> behind DevresInner to be freed early without keeping it allocated until
> the end of device lifetime.
>
> > + //
> > + // SAFETY: When `drop` runs, it's guaranteed that nobody is accessing the revocable data
> > + // anymore, hence it is safe not to wait for the grace period to finish.
> > + unsafe { self.revoke_nosync() };
> > + }
> > +}
> > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> > index 6c836ab73771..2b61bf99d1ee 100644
> > --- a/rust/kernel/lib.rs
> > +++ b/rust/kernel/lib.rs
> > @@ -41,6 +41,7 @@
> > pub mod cred;
> > pub mod device;
> > pub mod device_id;
> > +pub mod devres;
> > pub mod driver;
> > pub mod error;
> > #[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
>