rust: poll: use kfree_rcu() for PollCondVar

Rust Binder currently uses PollCondVar, but it calls synchronize_rcu()
in the destructor, which we would like to avoid. Add a variation of
PollCondVar that kfree_rcu() instead.

One could avoid the `rcu` field and allocate the rcu_head on drop using
a fallback to synchronize_rcu() on ENOMEM. However, I'd prefer to avoid
the potential for synchronize_rcu(), and Binder will only use this for a
small fraction of processes, so even if it changes which kmalloc bucket
it falls into, the extra memory is not a problem.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Author: Darksonn

rust/kernel/sync/poll.rs

@@ -5,12 +5,18 @@
 //! Utilities for working with `struct poll_table`.
 
 use crate::{
+    alloc::AllocError,
     bindings,
     fs::File,
     prelude::*,
     sync::{CondVar, LockClassKey},
+    types::Opaque, //
+};
+use core::{
+    marker::PhantomData,
+    mem::ManuallyDrop,
+    ops::Deref, //
 };
-use core::{marker::PhantomData, ops::Deref};
 
 /// Creates a [`PollCondVar`] initialiser with the given name and a newly-created lock class.
 #[macro_export]
@@ -66,6 +72,7 @@ impl<'a> PollTable<'a> {
 ///
 /// [`CondVar`]: crate::sync::CondVar
 #[pin_data(PinnedDrop)]
+#[repr(transparent)]
 pub struct PollCondVar {
     #[pin]
     inner: CondVar,
@@ -104,3 +111,67 @@ impl PinnedDrop for PollCondVar {
         unsafe { bindings::synchronize_rcu() };
     }
 }
+
+/// A [`KBox<PollCondVar>`] that uses `kfree_rcu`.
+///
+/// [`KBox<PollCondVar>`]: PollCondVar
+pub struct PollCondVarBox {
+    inner: ManuallyDrop<Pin<KBox<PollCondVarBoxInner>>>,
+}
+
+#[pin_data]
+#[repr(C)]
+struct PollCondVarBoxInner {
+    #[pin]
+    inner: PollCondVar,
+    rcu: Opaque<bindings::callback_head>,
+}
+
+// SAFETY: PollCondVar is Send
+unsafe impl Send for PollCondVarBoxInner {}
+// SAFETY: PollCondVar is Sync
+unsafe impl Sync for PollCondVarBoxInner {}
+
+impl PollCondVarBox {
+    /// Constructs a new boxed [`PollCondVar`].
+    pub fn new(name: &'static CStr, key: Pin<&'static LockClassKey>) -> Result<Self, AllocError> {
+        let b = KBox::pin_init(
+            pin_init!(PollCondVarBoxInner {
+                inner <- PollCondVar::new(name, key),
+                rcu: Opaque::uninit(),
+            }),
+            GFP_KERNEL,
+        )
+        .map_err(|_| AllocError)?;
+
+        Ok(PollCondVarBox {
+            inner: ManuallyDrop::new(b),
+        })
+    }
+}
+
+impl Deref for PollCondVarBox {
+    type Target = PollCondVar;
+    fn deref(&self) -> &PollCondVar {
+        &self.inner.inner
+    }
+}
+
+impl Drop for PollCondVarBox {
+    #[inline]
+    fn drop(&mut self) {
+        // SAFETY: ManuallyDrop::take ok because not already taken.
+        let boxed = unsafe { ManuallyDrop::take(&mut self.inner) };
+
+        // SAFETY: The code below frees the box without calling the actual destructor of the type,
+        // but it's okay because it re-implements the destructor using `kfree_rcu()` in place of
+        // `synchronize_rcu()`.
+        let ptr = KBox::into_raw(unsafe { Pin::into_inner_unchecked(boxed) });
+
+        // SAFETY: The pointer points at a valid `wait_queue_head`.
+        unsafe { bindings::__wake_up_pollfree((*ptr).inner.inner.wait_queue_head.get()) };
+
+        // SAFETY: This was allocated using `KBox::pin_init`, so it can be freed with `kvfree`.
+        unsafe { bindings::kvfree_call_rcu((*ptr).rcu.get(), ptr.cast::<ffi::c_void>()) };
+    }
+}