[RFC,11/11] rust: sync: arc: Add lockdep integration

  Now that we have magic lock class support and a LockdepMap that can be
hooked up into arbitrary Rust types, we can integrate lockdep support
directly into the Rust Arc<T> type. This means we can catch potential
Drop codepaths that could result in a locking error, even if those
codepaths never actually execute due to the reference count being
nonzero at that point.

Signed-off-by: Asahi Lina <lina@asahilina.net>
---
 lib/Kconfig.debug       |  8 ++++++
 rust/kernel/init.rs     |  6 +++++
 rust/kernel/sync/arc.rs | 71 ++++++++++++++++++++++++++++++++++++++++++++++---
 3 files changed, 82 insertions(+), 3 deletions(-)
  

On Fri, 14 Jul 2023 18:14:03 +0900
Asahi Lina <lina@asahilina.net> wrote:

> Now that we have magic lock class support and a LockdepMap that can be
> hooked up into arbitrary Rust types, we can integrate lockdep support
> directly into the Rust Arc<T> type. This means we can catch potential
> Drop codepaths that could result in a locking error, even if those
> codepaths never actually execute due to the reference count being
> nonzero at that point.
> 
> Signed-off-by: Asahi Lina <lina@asahilina.net>
> ---
>  lib/Kconfig.debug       |  8 ++++++
>  rust/kernel/init.rs     |  6 +++++
>  rust/kernel/sync/arc.rs | 71 ++++++++++++++++++++++++++++++++++++++++++++++---
>  3 files changed, 82 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index fbc89baf7de6..ff4f06df88ee 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -3010,6 +3010,14 @@ config RUST_BUILD_ASSERT_ALLOW
>  
>  	  If unsure, say N.
>  
> +config RUST_EXTRA_LOCKDEP
> +	bool "Extra lockdep checking"
> +	depends on RUST && PROVE_LOCKING
> +	help
> +	  Enabled additional lockdep integration with certain Rust types.
> +
> +	  If unsure, say N.
> +
>  endmenu # "Rust"
>  
>  endmenu # Kernel hacking
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index f190bbd0bab1..b64a507f0a34 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -1208,6 +1208,7 @@ pub trait InPlaceInit<T>: Sized {
>      /// type.
>      ///
>      /// If `T: !Unpin` it will not be able to move afterwards.
> +    #[track_caller]
>      fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
>      where
>          E: From<AllocError>;
> @@ -1216,6 +1217,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
>      /// type.
>      ///
>      /// If `T: !Unpin` it will not be able to move afterwards.
> +    #[track_caller]
>      fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Pin<Self>>
>      where
>          Error: From<E>,
> @@ -1228,11 +1230,13 @@ fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Pin<Self>>
>      }
>  
>      /// Use the given initializer to in-place initialize a `T`.
> +    #[track_caller]
>      fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
>      where
>          E: From<AllocError>;
>  
>      /// Use the given initializer to in-place initialize a `T`.
> +    #[track_caller]
>      fn init<E>(init: impl Init<T, E>) -> error::Result<Self>
>      where
>          Error: From<E>,
> @@ -1277,6 +1281,7 @@ fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
>  
>  impl<T> InPlaceInit<T> for UniqueArc<T> {
>      #[inline]
> +    #[track_caller]
>      fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
>      where
>          E: From<AllocError>,
> @@ -1291,6 +1296,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
>      }
>  
>      #[inline]
> +    #[track_caller]
>      fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
>      where
>          E: From<AllocError>,
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index a89843cacaad..3bb73b614fd1 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -34,6 +34,9 @@
>  };
>  use macros::pin_data;
>  
> +#[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> +use crate::sync::lockdep::LockdepMap;
> +
>  mod std_vendor;
>  
>  /// A reference-counted pointer to an instance of `T`.
> @@ -127,6 +130,17 @@ pub struct Arc<T: ?Sized> {
>      _p: PhantomData<ArcInner<T>>,
>  }
>  
> +#[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> +#[pin_data]
> +#[repr(C)]
> +struct ArcInner<T: ?Sized> {
> +    refcount: Opaque<bindings::refcount_t>,
> +    lockdep_map: LockdepMap,
> +    data: T,
> +}
> +
> +// FIXME: pin_data does not work well with cfg attributes within the struct definition.
> +#[cfg(not(CONFIG_RUST_EXTRA_LOCKDEP))]
>  #[pin_data]
>  #[repr(C)]
>  struct ArcInner<T: ?Sized> {
> @@ -159,11 +173,14 @@ unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
>  
>  impl<T> Arc<T> {
>      /// Constructs a new reference counted instance of `T`.
> +    #[track_caller]
>      pub fn try_new(contents: T) -> Result<Self, AllocError> {
>          // INVARIANT: The refcount is initialised to a non-zero value.
>          let value = ArcInner {
>              // SAFETY: There are no safety requirements for this FFI call.
>              refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
> +            #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> +            lockdep_map: LockdepMap::new(),
>              data: contents,
>          };
>  
> @@ -178,6 +195,7 @@ pub fn try_new(contents: T) -> Result<Self, AllocError> {
>      ///
>      /// If `T: !Unpin` it will not be able to move afterwards.
>      #[inline]
> +    #[track_caller]
>      pub fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Self>
>      where
>          Error: From<E>,
> @@ -189,6 +207,7 @@ pub fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Self>
>      ///
>      /// This is equivalent to [`Arc<T>::pin_init`], since an [`Arc`] is always pinned.
>      #[inline]
> +    #[track_caller]
>      pub fn init<E>(init: impl Init<T, E>) -> error::Result<Self>
>      where
>          Error: From<E>,
> @@ -292,15 +311,46 @@ fn drop(&mut self) {
>          // freed/invalid memory as long as it is never dereferenced.
>          let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
>  
> +        // SAFETY: By the type invariant, there is necessarily a reference to the object.
> +        // We cannot hold the map lock across the reference decrement, as we might race
> +        // another thread. Therefore, we lock and immediately drop the guard here. This
> +        // only serves to inform lockdep of the dependency up the call stack.
> +        #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> +        unsafe { self.ptr.as_ref() }.lockdep_map.lock();

Make the intention explicit by

	drop(unsafe { self.ptr.as_ref() }.lockdep_map.lock());

and make `lock` function `must_use`.

> +
>          // INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
>          // this instance is being dropped, so the broken invariant is not observable.
>          // SAFETY: Also by the type invariant, we are allowed to decrement the refcount.
>          let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
> +
>          if is_zero {
>              // The count reached zero, we must free the memory.
> -            //
> -            // SAFETY: The pointer was initialised from the result of `Box::leak`.
> -            unsafe { Box::from_raw(self.ptr.as_ptr()) };
> +
> +            // SAFETY: If we get this far, we had the last reference to the object.
> +            // That means we are responsible for freeing it, so we can safely lock
> +            // the fake lock again. This wraps dropping the inner object, which
> +            // informs lockdep of the dependencies down the call stack.
> +            #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> +            let guard = unsafe { self.ptr.as_ref() }.lockdep_map.lock();
> +
> +            // SAFETY: The pointer was initialised from the result of `Box::leak`,
> +            // and the value is valid.
> +            unsafe { core::ptr::drop_in_place(&mut self.ptr.as_mut().data) };
> +
> +            // We need to drop the lock guard before freeing the LockdepMap itself
> +            #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> +            core::mem::drop(guard);
> +
> +            // SAFETY: The pointer was initialised from the result of `Box::leak`,
> +            // and the lockdep map is valid.
> +            #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> +            unsafe {
> +                core::ptr::drop_in_place(&mut self.ptr.as_mut().lockdep_map)
> +            };
> +
> +            // SAFETY: The pointer was initialised from the result of `Box::leak`, and
> +            // a ManuallyDrop<T> is compatible. We already dropped the contents above.
> +            unsafe { Box::from_raw(self.ptr.as_ptr() as *mut ManuallyDrop<ArcInner<T>>) };

I feel there are a lot more `as_ref/as_mut` calls than it could be.
Could you refactor the code to make a single `as_ref()` call for the
non-zero path and a single `as_mut()` call for the zero path?

>          }
>      }
>  }
> @@ -512,6 +562,7 @@ pub struct UniqueArc<T: ?Sized> {
>  
>  impl<T> UniqueArc<T> {
>      /// Tries to allocate a new [`UniqueArc`] instance.
> +    #[track_caller]
>      pub fn try_new(value: T) -> Result<Self, AllocError> {
>          Ok(Self {
>              // INVARIANT: The newly-created object has a ref-count of 1.
> @@ -520,13 +571,27 @@ pub fn try_new(value: T) -> Result<Self, AllocError> {
>      }
>  
>      /// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
> +    #[track_caller]
>      pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>, AllocError> {
>          // INVARIANT: The refcount is initialised to a non-zero value.
> +        #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> +        let inner = {
> +            let map = LockdepMap::new();
> +            Box::try_init::<AllocError>(try_init!(ArcInner {
> +                // SAFETY: There are no safety requirements for this FFI call.
> +                refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
> +                lockdep_map: map,
> +                data <- init::uninit::<T, AllocError>(),
> +            }? AllocError))?
> +        };
> +        // FIXME: try_init!() does not work with cfg attributes.
> +        #[cfg(not(CONFIG_RUST_EXTRA_LOCKDEP))]
>          let inner = Box::try_init::<AllocError>(try_init!(ArcInner {
>              // SAFETY: There are no safety requirements for this FFI call.
>              refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
>              data <- init::uninit::<T, AllocError>(),
>          }? AllocError))?;
> +
>          Ok(UniqueArc {
>              // INVARIANT: The newly-created object has a ref-count of 1.
>              // SAFETY: The pointer from the `Box` is valid.
>