Re: [PATCH v2] rust_binder: add TF_DEFER_COMPLETE flag for avoiding userspace roundtrip

On Wed, Jul 22, 2026 at 09:09:22PM +0000, Alice Ryhl wrote:
> Outgoing transactions are able to send a message and wait for its reply
> in a single ioctl. Why not avoid a userspace roundtrip by applying the
> same logic for replying to incoming messages and waiting for the next
> incoming message?
> 
> Generally, when you send a reply using BC_REPLY, the kernel sends
> BR_TRANSACTION_COMPLETE as a reply to BC_REPLY right away. The
> BR_TRANSACTION_COMPLETE command indicates that it's safe for userspace
> to free any resources associated with this message (such as embedded fds
> or Binder nodes). However, the BR_TRANSACTION_COMPLETE message is
> problematic because after BC_REPLY is issued, there will be a pending
> message for userspace. The kernel will refuse to sleep for incoming
> messages in this scenario.
> 
> The way this is handled for outgoing transaction is through a mechanism
> known as deferred delivery of BR_TRANSACTION_COMPLETE. The idea is that
> when you send an outgoing transaction, then we do not return to
> userspace right away if BR_TRANSACTION_COMPLETE is the only pending
> message. This patch adds a new flag called TF_DEFER_COMPLETE that lets
> userspace opt-in to the same deferred delivery mechanism for
> BR_TRANSACTION_COMPLETE when using BC_REPLY.
> 
> Given this new uapi, we can adjust sendReply in userspace libbinder
> so that it writes the BC_REPLY command into mOut but does not flush the
> buffer to the kernel. Then, userspace simply continues running until it
> returns all the way out to the top-level joinThreadPool() loop, which
> calls into the kernel to get the next incoming transaction. At this
> point, mOut is flushed, sending the reply. The same ioctl then proceeds
> to sleep for an incoming message.
> 
> Userspace only actually specifies TF_DEFER_COMPLETE when the Parcel does
> not contain fds or refcounts on binder objects. This is because
> otherwise said fd or binder node will not be freed until the binder
> thread receives another incoming transaction, which could be a long
> time. In the case of fds, this is especially important because delaying
> fclose() can result in processes hanging because they read from a pipe
> that isn't being closed due to fclose() not getting called. Note that
> even if TF_DEFER_COMPLETE is not specified for this transaction, it can
> still be useful to defer the BC_REPLY command, as it can still avoid a
> userspace roundtrip when a new incoming transaction is available right
> away.

The processing of a deferred COMPLETE doesn't change right? It doesn't
matter if the kernel rejects / ignores the new flag, userspace will
still follow the same path. 100% backward-compatible then.

> 
> Observing the cuttlefish logs while booting with this change shows that
> there were 4297 opportunities for this optimization to kick in (that is,
> boot invoked BC_REPLY 4297 times). Out of those, 3441 binder ioctls sent
> and received a transaction in the same ioctl. This indicates that we
> successfully eliminated a syscall on the server side for 80% of incoming
> transactions. Generally, this means that a server is now able to handle
> incoming messages using one syscall per incoming message (for each
> incoming transaction, the syscall handles one BC_FREE_BUFFER and
> BC_REPLY command, and then waits for the next incoming transaction).
> 
> Signed-off-by: Alice Ryhl <[email protected]>
> ---
> Changes in v2:
> - Return deferred thread work instead of the process global work if
>   there is work in the process global list.
> - Link to v1: https://lore.kernel.org/r/[email protected]
> ---
>  drivers/android/binder/defs.rs      |  3 +-
>  drivers/android/binder/process.rs   |  8 ++++++
>  drivers/android/binder/thread.rs    | 55 +++++++++++++++++++++++++++++++------
>  include/uapi/linux/android/binder.h |  1 +
>  4 files changed, 57 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/android/binder/defs.rs b/drivers/android/binder/defs.rs
> index 8ac9bdd7a499..cc4becd6e168 100644
> --- a/drivers/android/binder/defs.rs
> +++ b/drivers/android/binder/defs.rs
> @@ -77,7 +77,8 @@ macro_rules! pub_no_prefix {
>      TF_ONE_WAY,
>      TF_ACCEPT_FDS,
>      TF_CLEAR_BUF,
> -    TF_UPDATE_TXN
> +    TF_UPDATE_TXN,
> +    TF_DEFER_COMPLETE,
>  );
>  
>  pub(crate) use uapi::{
> diff --git a/drivers/android/binder/process.rs b/drivers/android/binder/process.rs
> index 1778628d8acd..4f23a7cf7352 100644
> --- a/drivers/android/binder/process.rs
> +++ b/drivers/android/binder/process.rs
> @@ -686,8 +686,16 @@ pub(crate) fn get_work(&self) -> Option<DLArc<dyn DeliverToRead>> {
>      pub(crate) fn get_work_or_register<'a>(
>          &'a self,
>          thread: &'a Arc<Thread>,
> +        thread_has_deferred_work: bool,
>      ) -> GetWorkOrRegister<'a> {
>          let mut inner = self.inner.lock();
> +
> +        if thread_has_deferred_work && !inner.work.is_empty() {
> +            if let Some(work) = thread.pop_work_even_if_deferred() {
> +                return GetWorkOrRegister::Work(work);
> +            }
> +        }
> +
>          // Try to get work from the process queue.
>          if let Some(work) = inner.work.pop_front() {
>              return GetWorkOrRegister::Work(work);
> diff --git a/drivers/android/binder/thread.rs b/drivers/android/binder/thread.rs
> index a51821dde0ad..c4d67b9ef39b 100644
> --- a/drivers/android/binder/thread.rs
> +++ b/drivers/android/binder/thread.rs
> @@ -572,9 +572,21 @@ fn get_work_local(self: &Arc<Self>, wait: bool) -> Result<Option<DLArc<dyn Deliv
>      // mangled symbol names.
>      #[export_name = "rust_binder_wait"]
>      fn get_work(self: &Arc<Self>, wait: bool) -> Result<Option<DLArc<dyn DeliverToRead>>> {
> +        let thread_has_deferred_work;
> +
>          // Try to get work from the thread's work queue, using only a local lock.
>          {
>              let mut inner = self.inner.lock();
> +
> +            // The process_work_list boolean is used to make us go to sleep even if there is work
> +            // in the thread todo-list, but it doesn't apply to the process todo-list. Furthermore,
> +            // work in the thread todo-list must still be delivered before the process list.
> +            //
> +            // Thus, in some scenarios we must return the thread work now even if we were requested
> +            // to wait. Adjust `process_work_list` to `true` accordingly.
> +            inner.process_work_list |= inner.looper_need_return;
> +            inner.process_work_list |= !wait;
> +
>              if let Some(work) = inner.pop_work() {
>                  return Ok(Some(work));
>              }
> @@ -582,18 +594,26 @@ fn get_work(self: &Arc<Self>, wait: bool) -> Result<Option<DLArc<dyn DeliverToRe
>                  drop(inner);
>                  return Ok(self.process.get_work());
>              }
> +
> +            // Note that if the thread list is empty, then the call to `pop_work()` has changed
> +            // `process_work_list` back to `false` even if we set it to `true` above.
> +            thread_has_deferred_work = inner.process_work_list;

I might be getting this wrong, but for the new TF_DEFER_COMPLETE case,
we have !process_work_list and !work_list.is_empty(). Then pop_work()
does not touch process_work_list because it's already false.

So we set thread_has_deferred_work to false? Maybe this was meant to be:
	thread_has_deferred_work = !inner.work_list.is_empty()

>          }
>  
>          // If the caller doesn't want to wait, try to grab work from the process queue.
>          //
>          // We know nothing will have been queued directly to the thread queue because it is not in
> -        // a transaction and it is not in the process' ready list.
> +        // a transaction and it is not in the process' ready list. We also know the thread list has
> +        // no deferred work due to the `inner.process_work_list |= !wait` call above.
>          if !wait {
>              return self.process.get_work().ok_or(EAGAIN).map(Some);
>          }
>  
>          // Get work from the process queue. If none is available, atomically register as ready.
> -        let reg = match self.process.get_work_or_register(self) {
> +        let reg = match self
> +            .process
> +            .get_work_or_register(self, thread_has_deferred_work)
> +        {
>              GetWorkOrRegister::Work(work) => return Ok(Some(work)),
>              GetWorkOrRegister::Register(reg) => reg,
>          };
> @@ -609,14 +629,18 @@ fn get_work(self: &Arc<Self>, wait: bool) -> Result<Option<DLArc<dyn DeliverToRe
>              inner.looper_flags &= !(LOOPER_WAITING | LOOPER_WAITING_PROC);
>  
>              if signal_pending || inner.looper_need_return {
> -                // We need to return now. We need to pull the thread off the list of ready threads
> -                // (by dropping `reg`), then check the state again after it's off the list to
> -                // ensure that something was not queued in the meantime. If something has been
> -                // queued, we just return it (instead of the error).
> +                // We need to return now.
> +                //
> +                // We need to pull the thread off the list of ready threads (by dropping `reg`),
> +                // then check the state again after it's off the list to ensure that something was
> +                // not queued in the meantime. If something has been queued (or if there is
> +                // deferred work), we just return it (instead of the error).
>                  drop(inner);
>                  drop(reg);
>  
> -                let res = match self.inner.lock().pop_work() {
> +                inner = self.inner.lock();
> +                inner.process_work_list = true;
> +                let res = match inner.pop_work() {
>                      Some(work) => Ok(Some(work)),
>                      None if signal_pending => Err(EINTR),
>                      None => Ok(None),
> @@ -674,6 +698,12 @@ pub(crate) fn push_return_work(&self, reply: u32) {
>          self.inner.lock().push_return_work(reply);
>      }
>  
> +    pub(crate) fn pop_work_even_if_deferred(&self) -> Option<DLArc<dyn DeliverToRead>> {
> +        let mut thread_inner = self.inner.lock();
> +        thread_inner.process_work_list = true;
> +        thread_inner.pop_work()
> +    }
> +
>      fn translate_object(
>          &self,
>          obj_index: usize,
> @@ -1398,8 +1428,15 @@ 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)?;
> -            // Not notifying: Reply to current thread.
> -            let _ = self.inner.lock().push_work(completion);
> +            {
> +                // This performs a deferred push so that `read` can wait for the next incoming
> +                // transaction without a userspace roundtrip.
> +                let mut inner = self.inner.lock();
> +                inner.push_work_deferred(completion);
> +                // However, if `TF_DEFER_COMPLETE` is not set, then set `process_work_list` to make
> +                // the push non-deferred. This forces a userspace roundtrip.
> +                inner.process_work_list |= info.flags & TF_DEFER_COMPLETE == 0;

If there is already a push_work() and a push_work_deferred() why use
process_work_list directly? Is it to avoid an if/else?

> +            }
>              orig.from.deliver_reply(Ok(reply), &orig, None);
>              Ok(())
>          })()
> diff --git a/include/uapi/linux/android/binder.h b/include/uapi/linux/android/binder.h
> index 701cad36de43..96e5b0184a1b 100644
> --- a/include/uapi/linux/android/binder.h
> +++ b/include/uapi/linux/android/binder.h
> @@ -296,6 +296,7 @@ enum transaction_flags {
>  	TF_ACCEPT_FDS	= 0x10,	/* allow replies with file descriptors */
>  	TF_CLEAR_BUF	= 0x20,	/* clear buffer on txn complete */
>  	TF_UPDATE_TXN	= 0x40,	/* update the outdated pending async txn */
> +	TF_DEFER_COMPLETE = 0x80,	/* defer transaction complete to userspace */
>  };
>  
>  struct binder_transaction_data {
> 
> ---
> base-commit: 2cedf2272f1bb42471e646868ac572cc5752bd91
> change-id: 20260715-defer-complete-f1dea9af13a8
> 
> Best regards,
> -- 
> Alice Ryhl <[email protected]>
>