[NOT FOR MERGE] [RFC PATCH v2 4/4] rust: poll: use kfree_rcu() for PollCondVar

[Boqun Feng <boqun@kernel.org>]

From: Alice Ryhl <aliceryhl@google.com>

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>
[boqun: Use RcuFreeSafe]
Signed-off-by: Boqun Feng <boqun@kernel.org>
---
 rust/kernel/sync/poll.rs | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/rust/kernel/sync/poll.rs b/rust/kernel/sync/poll.rs
index 5aa0ce9ba01b..830a4763a5c1 100644
--- a/rust/kernel/sync/poll.rs
+++ b/rust/kernel/sync/poll.rs
@@ -9,12 +9,19 @@
     fs::File,
     prelude::*,
     sync::{
-        rcu::synchronize_rcu,
+        rcu::{
+            synchronize_rcu, //
+            RcuFreeSafe,
+        },
         CondVar,
         LockClassKey, //
     }, //
 };
-use core::{marker::PhantomData, ops::Deref};
+
+use core::{
+    marker::PhantomData,
+    ops::Deref, //
+};
 
 /// Creates a [`PollCondVar`] initialiser with the given name and a newly-created lock class.
 #[macro_export]
@@ -70,6 +77,7 @@ pub fn register_wait(&self, file: &File, cv: &PollCondVar) {
 ///
 /// [`CondVar`]: crate::sync::CondVar
 #[pin_data(PinnedDrop)]
+#[repr(transparent)]
 pub struct PollCondVar {
     #[pin]
     inner: CondVar,
@@ -97,12 +105,20 @@ fn deref(&self) -> &CondVar {
 impl PinnedDrop for PollCondVar {
     #[inline]
     fn drop(self: Pin<&mut Self>) {
+        self.drop_before_gp();
+
+        // Wait for epoll items to be properly removed.
+        synchronize_rcu();
+    }
+}
+
+// SAFETY: __wake_up_pollfree() guarantees all the epoll items on the wait list will be gone after
+// one grace period.
+unsafe impl RcuFreeSafe for PollCondVar {
+    fn drop_before_gp(self: Pin<&mut Self>) {
         // Clear anything registered using `register_wait`.
         //
         // SAFETY: The pointer points at a valid `wait_queue_head`.
         unsafe { bindings::__wake_up_pollfree(self.inner.wait_queue_head.get()) };
-
-        // Wait for epoll items to be properly removed.
-        synchronize_rcu();
     }
 }
-- 
2.50.1 (Apple Git-155)