ANDROID: rust_binder: add epoll support

This adds epoll integration, allowing you to get an epoll notification
when an incoming transaction arrives.

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-7-08ba9197f637@google.com/
Change-Id: I42e874205f4c7901d6914ac014c7e4deadaa2b88
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Bug: 278052745

drivers/android/binder/process.rs

@@ -125,8 +125,27 @@ impl ProcessInner {
         } else if self.is_dead {
             Err((BinderError::new_dead(), work))
         } else {
-            // There are no ready threads. Push work to process queue.
+            let sync = work.should_sync_wakeup();
+
+            // Didn't find a thread waiting for proc work; this can happen
+            // in two scenarios:
+            // 1. All threads are busy handling transactions
+            //    In that case, one of those threads should call back into
+            //    the kernel driver soon and pick up this work.
+            // 2. Threads are using the (e)poll interface, in which case
+            //    they may be blocked on the waitqueue without having been
+            //    added to waiting_threads. For this case, we just iterate
+            //    over all threads not handling transaction work, and
+            //    wake them all up. We wake all because we don't know whether
+            //    a thread that called into (e)poll is handling non-binder
+            //    work currently.
             self.work.push_back(work);
+
+            // Wake up polling threads, if any.
+            for thread in self.threads.values() {
+                thread.notify_if_poll_ready(sync);
+            }
+
             Ok(())
         }
     }
@@ -1000,11 +1019,16 @@ impl Process {
     }
 
     pub(crate) fn poll(
-        _this: ArcBorrow<'_, Process>,
-        _file: &File,
-        _table: &mut PollTable,
+        this: ArcBorrow<'_, Process>,
+        file: &File,
+        table: &mut PollTable,
     ) -> Result<u32> {
-        Err(EINVAL)
+        let thread = this.get_current_thread()?;
+        let (from_proc, mut mask) = thread.poll(file, table);
+        if mask == 0 && from_proc && !this.inner.lock().work.is_empty() {
+            mask |= bindings::POLLIN;
+        }
+        Ok(mask)
     }
 }
 

drivers/android/binder/thread.rs

@@ -9,13 +9,15 @@
 
 use kernel::{
     bindings,
+    file::File,
     list::{
         AtomicListArcTracker, HasListLinks, List, ListArc, ListArcSafe, ListItem, ListLinks,
         TryNewListArc,
     },
     prelude::*,
     security,
-    sync::{Arc, CondVar, SpinLock},
+    sync::poll::{PollCondVar, PollTable},
+    sync::{Arc, SpinLock},
     types::Either,
     uaccess::{UserSlice, UserSliceWriter},
 };
@@ -76,6 +78,7 @@ const LOOPER_EXITED: u32 = 0x04;
 const LOOPER_INVALID: u32 = 0x08;
 const LOOPER_WAITING: u32 = 0x10;
 const LOOPER_WAITING_PROC: u32 = 0x20;
+const LOOPER_POLL: u32 = 0x40;
 
 impl InnerThread {
     fn new() -> Result<Self> {
@@ -162,6 +165,15 @@ impl InnerThread {
     fn should_use_process_work_queue(&self) -> bool {
         !self.process_work_list && self.is_looper()
     }
+
+    fn poll(&mut self) -> u32 {
+        self.looper_flags |= LOOPER_POLL;
+        if self.process_work_list || self.looper_need_return {
+            bindings::POLLIN
+        } else {
+            0
+        }
+    }
 }
 
 /// This represents a thread that's used with binder.
@@ -172,7 +184,7 @@ pub(crate) struct Thread {
     #[pin]
     inner: SpinLock<InnerThread>,
     #[pin]
-    work_condvar: CondVar,
+    work_condvar: PollCondVar,
     /// Used to insert this thread into the process' `ready_threads` list.
     ///
     /// INVARIANT: May never be used for any other list than the `self.process.ready_threads`.
@@ -204,7 +216,7 @@ impl Thread {
             id,
             process,
             inner <- kernel::new_spinlock!(inner, "Thread::inner"),
-            work_condvar <- kernel::new_condvar!("Thread::work_condvar"),
+            work_condvar <- kernel::new_poll_condvar!("Thread::work_condvar"),
             links <- ListLinks::new(),
             links_track <- AtomicListArcTracker::new(),
         }))
@@ -623,6 +635,12 @@ impl Thread {
         ret
     }
 
+    pub(crate) fn poll(&self, file: &File, table: &mut PollTable) -> (bool, u32) {
+        table.register_wait(file, &self.work_condvar);
+        let mut inner = self.inner.lock();
+        (inner.should_use_process_work_queue(), inner.poll())
+    }
+
     /// Make the call to `get_work` or `get_work_local` return immediately, if any.
     pub(crate) fn exit_looper(&self) {
         let mut inner = self.inner.lock();
@@ -637,6 +655,22 @@ impl Thread {
         }
     }
 
+    pub(crate) fn notify_if_poll_ready(&self, sync: bool) {
+        // Determine if we need to notify. This requires the lock.
+        let inner = self.inner.lock();
+        let notify = inner.looper_flags & LOOPER_POLL != 0 && inner.should_use_process_work_queue();
+        drop(inner);
+
+        // Now that the lock is no longer held, notify the waiters if we have to.
+        if notify {
+            if sync {
+                self.work_condvar.notify_sync();
+            } else {
+                self.work_condvar.notify_one();
+            }
+        }
+    }
+
     pub(crate) fn release(self: &Arc<Self>) {
         self.inner.lock().is_dead = true;