[PATCH v2 1/4] rust: sync: add `CondVar::notify_sync`

[Alice Ryhl <aliceryhl@google.com>]

Wake up another thread synchronously.

This method behaves like `notify_one`, except that it hints to the
scheduler that the current thread is about to go to sleep, so it should
schedule the target thread on the same CPU.

This is used by Rust Binder as a performance optimization. When sending
a transaction to a different process, we usually know which thread will
handle it, so we can schedule that thread for execution next on this
CPU for better cache locality.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/sync/condvar.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/rust/kernel/sync/condvar.rs b/rust/kernel/sync/condvar.rs
index 8630faa29b78..9331eb606738 100644
--- a/rust/kernel/sync/condvar.rs
+++ b/rust/kernel/sync/condvar.rs
@@ -155,6 +155,12 @@ fn notify(&self, count: i32, flags: u32) {
         };
     }
 
+    /// Calls the kernel function to notify one thread synchronously.
+    pub fn notify_sync(&self) {
+        // SAFETY: `wait_list` points to valid memory.
+        unsafe { bindings::__wake_up_sync(self.wait_list.get(), bindings::TASK_NORMAL) };
+    }
+
     /// Wakes a single waiter up, if any.
     ///
     /// This is not 'sticky' in the sense that if no thread is waiting, the notification is lost

-- 
2.43.0.472.g3155946c3a-goog