[PATCH v3 0/2] Avoid synchronize_rcu() for every thread drop in Rust Binder

[PATCH v3 0/2] Avoid synchronize_rcu() for every thread drop in Rust Binder

[Alice Ryhl]

Right now Rust Binder calls synchronize_rcu() more often than is
necessary. Most processes do not use epoll at all, so they don't require
rcu here. Back in Kangrejos I came up with a way to avoid this. Idea is
to move the value that needs rcu to a separate allocation that's easy to
kfree_rcu(). We pay the allocation only when the proc uses epoll using
an "upgrade" strategy - most processes don't.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v3:
- This series was almost entirely rewritten to use a different
  implementation strategy. By moving the PollCondVar to the process we
  can avoid the upgrade logic entirely.
- Link to v2: https://lore.kernel.org/r/20260213-upgrade-poll-v2-0-984a0fb184fb@google.com

Changes in v2:
- Change how Rust Binder handles the lock class key.
- Rebase.
- Link to v1: https://lore.kernel.org/r/20260117-upgrade-poll-v1-0-179437b7bd49@google.com

---
Alice Ryhl (2):
      rust: poll: use kfree_rcu() for PollCondVar
      rust_binder: move (e)poll wait queue to Process

 drivers/android/binder/process.rs     | 51 +++++++++++++++++------
 drivers/android/binder/thread.rs      | 78 +++++++++++++++++------------------
 drivers/android/binder/transaction.rs |  4 ++
 rust/kernel/sync/poll.rs              | 73 +++++++++++++++++++++++++++++++-
 4 files changed, 154 insertions(+), 52 deletions(-)
---
base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
change-id: 20260117-upgrade-poll-37ee2a7a79dd

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>

[PATCH v3 1/2] rust: poll: use kfree_rcu() for PollCondVar

[Alice Ryhl]

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>
---
 rust/kernel/sync/poll.rs | 73 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 72 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync/poll.rs b/rust/kernel/sync/poll.rs
index 0ec985d560c8..684dfa242b1a 100644
--- a/rust/kernel/sync/poll.rs
+++ b/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 @@ pub fn register_wait(&self, file: &File, cv: &PollCondVar) {
 ///
 /// [`CondVar`]: crate::sync::CondVar
 #[pin_data(PinnedDrop)]
+#[repr(transparent)]
 pub struct PollCondVar {
     #[pin]
     inner: CondVar,
@@ -104,3 +111,67 @@ fn drop(self: Pin<&mut Self>) {
         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>()) };
+    }
+}

-- 
2.54.0.563.g4f69b47b94-goog

[PATCH v3 2/2] rust_binder: move (e)poll wait queue to Process

[Alice Ryhl]

Most processes do not use Rust Binder with epoll, so avoid paying the
synchronize_rcu() cost in drop for those that don't need it. For those
that do, we also manage to replace synchronize_rcu() with kfree_rcu(),
though we introduce an extra allocation.

In case the last ref to an Arc<Thread> is dropped outside of
deferred_release(), this also ensures that synchronize_rcu() is not
called in destructor of Arc<Thread> in other places. Theoretically that
could lead to jank by making other syscalls slow, which would be
problematic.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 drivers/android/binder/process.rs     | 51 +++++++++++++++++------
 drivers/android/binder/thread.rs      | 78 +++++++++++++++++------------------
 drivers/android/binder/transaction.rs |  4 ++
 3 files changed, 82 insertions(+), 51 deletions(-)

diff --git a/drivers/android/binder/process.rs b/drivers/android/binder/process.rs
index 820cbd541435..29fdfc964685 100644
--- a/drivers/android/binder/process.rs
+++ b/drivers/android/binder/process.rs
@@ -30,6 +30,7 @@
     sync::{
         aref::ARef,
         lock::{spinlock::SpinLockBackend, Guard},
+        poll::PollCondVarBox,
         Arc, ArcBorrow, CondVar, CondVarTimeoutResult, Mutex, SpinLock, UniqueArc,
     },
     task::Task,
@@ -135,6 +136,7 @@ pub(crate) struct ProcessInner {
     pub(crate) binderfs_file: Option<BinderfsProcFile>,
     /// Check for oneway spam
     oneway_spam_detection_enabled: bool,
+    poll: Option<PollCondVarBox>,
 }
 
 impl ProcessInner {
@@ -158,6 +160,7 @@ fn new() -> Self {
             async_recv: false,
             binderfs_file: None,
             oneway_spam_detection_enabled: false,
+            poll: None,
         }
     }
 
@@ -174,19 +177,23 @@ pub(crate) fn push_work(
         &mut self,
         work: DLArc<dyn DeliverToRead>,
     ) -> Result<(), (BinderError, DLArc<dyn DeliverToRead>)> {
+        let sync = work.should_sync_wakeup();
+
         // Try to find a ready thread to which to push the work.
         if let Some(thread) = self.ready_threads.pop_front() {
             // Push to thread while holding state lock. This prevents the thread from giving up
             // (for example, because of a signal) when we're about to deliver work.
-            match thread.push_work(work) {
+            match thread.push_work_inner(work, sync) {
                 PushWorkRes::Ok => Ok(()),
+                PushWorkRes::OkNotifyPoll => {
+                    self.notify_poll(sync);
+                    Ok(())
+                }
                 PushWorkRes::FailedDead(work) => Err((BinderError::new_dead(), work)),
             }
         } else if self.is_dead {
             Err((BinderError::new_dead(), work))
         } else {
-            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
@@ -194,22 +201,26 @@ pub(crate) fn push_work(
             //    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.
+            //    added to waiting_threads. For this case, we wake it up
+            //    directly.
             self.work.push_back(work);
 
             // Wake up polling threads, if any.
-            for thread in self.threads.values() {
-                thread.notify_if_poll_ready(sync);
-            }
+            self.notify_poll(sync);
 
             Ok(())
         }
     }
 
+    pub(crate) fn notify_poll(&self, sync: bool) {
+        if let Some(poll) = &self.poll {
+            if sync {
+                poll.notify_sync();
+            }
+            poll.notify_all();
+        }
+    }
+
     pub(crate) fn remove_node(&mut self, ptr: u64) {
         self.nodes.remove(&ptr);
     }
@@ -1322,11 +1333,15 @@ fn deferred_flush(&self) {
 
     fn deferred_release(self: Arc<Self>) {
         let is_manager = {
+            // These variables contain values to be dropped after releasing spinlock.
+            let _poll;
+
             let mut inner = self.inner.lock();
             inner.is_dead = true;
             inner.is_frozen = IsFrozen::No;
             inner.sync_recv = false;
             inner.async_recv = false;
+            _poll = inner.poll.take();
             inner.is_manager
         };
 
@@ -1696,7 +1711,19 @@ pub(crate) fn poll(
         table: PollTable<'_>,
     ) -> Result<u32> {
         let thread = this.get_current_thread()?;
-        let (from_proc, mut mask) = thread.poll(file, table);
+        {
+            let mut inner = this.inner.lock();
+            let poll = if let Some(poll) = &inner.poll {
+                &**poll
+            } else {
+                drop(inner);
+                let poll = PollCondVarBox::new(c"Process::poll", kernel::static_lock_class!())?;
+                inner = this.inner.lock();
+                &**inner.poll.get_or_insert(poll)
+            };
+            table.register_wait(file, poll);
+        }
+        let (from_proc, mut mask) = thread.poll()?;
         if mask == 0 && from_proc && !this.inner.lock().work.is_empty() {
             mask |= bindings::POLLIN;
         }
diff --git a/drivers/android/binder/thread.rs b/drivers/android/binder/thread.rs
index 97d5f31e8fe3..425c50298706 100644
--- a/drivers/android/binder/thread.rs
+++ b/drivers/android/binder/thread.rs
@@ -9,15 +9,14 @@
 
 use kernel::{
     bindings,
-    fs::{File, LocalFile},
+    fs::LocalFile,
     list::{AtomicTracker, List, ListArc, ListLinks, TryNewListArc},
     prelude::*,
     security,
     seq_file::SeqFile,
     seq_print,
     sync::atomic::{ordering::Relaxed, Atomic},
-    sync::poll::{PollCondVar, PollTable},
-    sync::{aref::ARef, Arc, SpinLock},
+    sync::{aref::ARef, Arc, CondVar, SpinLock},
     task::Task,
     uaccess::{UserPtr, UserSlice, UserSliceReader},
     uapi,
@@ -225,8 +224,10 @@ fn claim_next(&mut self, size: usize) -> Result<usize> {
     }
 }
 
+#[must_use]
 pub(crate) enum PushWorkRes {
     Ok,
+    OkNotifyPoll,
     FailedDead(DLArc<dyn DeliverToRead>),
 }
 
@@ -234,6 +235,7 @@ impl PushWorkRes {
     fn is_ok(&self) -> bool {
         match self {
             PushWorkRes::Ok => true,
+            PushWorkRes::OkNotifyPoll => true,
             PushWorkRes::FailedDead(_) => false,
         }
     }
@@ -310,27 +312,32 @@ fn pop_work(&mut self) -> Option<DLArc<dyn DeliverToRead>> {
 
     fn push_work(&mut self, work: DLArc<dyn DeliverToRead>) -> PushWorkRes {
         if self.is_dead {
-            PushWorkRes::FailedDead(work)
+            return PushWorkRes::FailedDead(work);
+        }
+        self.work_list.push_back(work);
+        self.process_work_list = true;
+        if self.looper_flags & LOOPER_POLL != 0 {
+            PushWorkRes::OkNotifyPoll
         } else {
-            self.work_list.push_back(work);
-            self.process_work_list = true;
             PushWorkRes::Ok
         }
     }
 
-    fn push_reply_work(&mut self, code: u32) {
+    fn push_reply_work(&mut self, code: u32) -> PushWorkRes {
         if let Ok(work) = ListArc::try_from_arc(self.reply_work.clone()) {
             work.set_error_code(code);
-            self.push_work(work);
+            self.push_work(work)
         } else {
             pr_warn!("Thread reply work is already in use.");
+            PushWorkRes::Ok
         }
     }
 
     fn push_return_work(&mut self, reply: u32) {
         if let Ok(work) = ListArc::try_from_arc(self.return_work.clone()) {
             work.set_error_code(reply);
-            self.push_work(work);
+            // Not notifying: Reply to current thread.
+            let _ = self.push_work(work);
         } else {
             pr_warn!("Thread return work is already in use.");
         }
@@ -422,7 +429,7 @@ pub(crate) struct Thread {
     #[pin]
     inner: SpinLock<InnerThread>,
     #[pin]
-    work_condvar: PollCondVar,
+    work_condvar: CondVar,
     /// 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`.
@@ -453,7 +460,7 @@ pub(crate) fn new(id: i32, process: Arc<Process>) -> Result<Arc<Self>> {
                 process,
                 task: ARef::from(&**kernel::current!()),
                 inner <- kernel::new_spinlock!(inner, "Thread::inner"),
-                work_condvar <- kernel::new_poll_condvar!("Thread::work_condvar"),
+                work_condvar <- kernel::new_condvar!("Thread::work_condvar"),
                 links <- ListLinks::new(),
                 links_track <- AtomicTracker::new(),
             }),
@@ -617,7 +624,14 @@ fn get_work(self: &Arc<Self>, wait: bool) -> Result<Option<DLArc<dyn DeliverToRe
     /// Returns whether the item was successfully pushed. This can only fail if the thread is dead.
     pub(crate) fn push_work(&self, work: DLArc<dyn DeliverToRead>) -> PushWorkRes {
         let sync = work.should_sync_wakeup();
+        self.push_work_inner(work, sync)
+    }
 
+    pub(crate) fn push_work_inner(
+        &self,
+        work: DLArc<dyn DeliverToRead>,
+        sync: bool,
+    ) -> PushWorkRes {
         let res = self.inner.lock().push_work(work);
 
         if res.is_ok() {
@@ -636,7 +650,8 @@ pub(crate) fn push_work(&self, work: DLArc<dyn DeliverToRead>) -> PushWorkRes {
     pub(crate) fn push_work_if_looper(&self, work: DLArc<dyn DeliverToRead>) -> BinderResult {
         let mut inner = self.inner.lock();
         if inner.is_looper() && !inner.is_dead {
-            inner.push_work(work);
+            // Not notifying: Reply to current thread.
+            let _ = inner.push_work(work);
             Ok(())
         } else {
             drop(inner);
@@ -1142,7 +1157,7 @@ fn deliver_single_reply(
             transaction.set_outstanding(&mut self.process.inner.lock());
         }
 
-        {
+        let ret = {
             let mut inner = self.inner.lock();
             if !inner.pop_transaction_replied(transaction) {
                 return false;
@@ -1153,15 +1168,16 @@ fn deliver_single_reply(
             }
 
             match reply {
-                Ok(work) => {
-                    inner.push_work(work);
-                }
+                Ok(work) => inner.push_work(work),
                 Err(code) => inner.push_reply_work(code),
             }
-        }
+        };
 
         // Notify the thread now that we've released the inner lock.
         self.work_condvar.notify_sync();
+        if matches!(ret, PushWorkRes::OkNotifyPoll) {
+            self.process.inner.lock().notify_poll(true);
+        }
         false
     }
 
@@ -1319,7 +1335,8 @@ fn reply_inner(self: &Arc<Self>, info: &mut TransactionInfo) -> BinderResult {
             let process = orig.from.process.clone();
             let allow_fds = orig.flags & TF_ACCEPT_FDS != 0;
             let reply = Transaction::new_reply(self, process, info, allow_fds)?;
-            self.inner.lock().push_work(completion);
+            // Not notifying: Reply to current thread.
+            let _ = self.inner.lock().push_work(completion);
             orig.from.deliver_reply(Ok(reply), &orig);
             Ok(())
         })()
@@ -1351,7 +1368,8 @@ fn oneway_transaction_inner(self: &Arc<Self>, info: &mut TransactionInfo) -> Bin
         };
         let list_completion = DTRWrap::arc_try_new(DeliverCode::new(code))?;
         let completion = list_completion.clone_arc();
-        self.inner.lock().push_work(list_completion);
+        // Not notifying: Reply to current thread.
+        let _ = self.inner.lock().push_work(list_completion);
         match transaction.submit(info) {
             Ok(()) => Ok(()),
             Err(err) => {
@@ -1551,10 +1569,9 @@ pub(crate) fn write_read(self: &Arc<Self>, data: UserSlice, wait: bool) -> Resul
         ret
     }
 
-    pub(crate) fn poll(&self, file: &File, table: PollTable<'_>) -> (bool, u32) {
-        table.register_wait(file, &self.work_condvar);
+    pub(crate) fn poll(&self) -> Result<(bool, u32)> {
         let mut inner = self.inner.lock();
-        (inner.should_use_process_work_queue(), inner.poll())
+        Ok((inner.should_use_process_work_queue(), inner.poll()))
     }
 
     /// Make the call to `get_work` or `get_work_local` return immediately, if any.
@@ -1571,26 +1588,9 @@ pub(crate) fn exit_looper(&self) {
         }
     }
 
-    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;
 
-        //self.work_condvar.clear();
         self.unwind_transaction_stack();
 
         // Cancel all pending work items.
diff --git a/drivers/android/binder/transaction.rs b/drivers/android/binder/transaction.rs
index 47d5e4d88b07..3c9620da3d25 100644
--- a/drivers/android/binder/transaction.rs
+++ b/drivers/android/binder/transaction.rs
@@ -322,6 +322,10 @@ pub(crate) fn submit(self: DLArc<Self>, info: &mut TransactionInfo) -> BinderRes
             crate::trace::trace_transaction(false, &self, Some(&thread.task));
             match thread.push_work(self) {
                 PushWorkRes::Ok => Ok(()),
+                PushWorkRes::OkNotifyPoll => {
+                    process_inner.notify_poll(true);
+                    Ok(())
+                }
                 PushWorkRes::FailedDead(me) => Err((BinderError::new_dead(), me)),
             }
         } else {

-- 
2.54.0.563.g4f69b47b94-goog