robot/include/asio/detail/impl/io_uring_service.ipp

//
// detail/impl/io_uring_service.ipp
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2023 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#ifndef ASIO_DETAIL_IMPL_IO_URING_SERVICE_IPP
#define ASIO_DETAIL_IMPL_IO_URING_SERVICE_IPP

#if defined(_MSC_VER) && (_MSC_VER >= 1200)
# pragma once
#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)

#include "asio/detail/config.hpp"

#if defined(ASIO_HAS_IO_URING)

#include <cstddef>
#include <sys/eventfd.h>
#include "asio/detail/io_uring_service.hpp"
#include "asio/detail/reactor_op.hpp"
#include "asio/detail/scheduler.hpp"
#include "asio/detail/throw_error.hpp"
#include "asio/error.hpp"

#include "asio/detail/push_options.hpp"

namespace asio {
namespace detail {

io_uring_service::io_uring_service(asio::execution_context& ctx)
  : execution_context_service_base<io_uring_service>(ctx),
    scheduler_(use_service<scheduler>(ctx)),
    mutex_(ASIO_CONCURRENCY_HINT_IS_LOCKING(
          REACTOR_REGISTRATION, scheduler_.concurrency_hint())),
    outstanding_work_(0),
    submit_sqes_op_(this),
    pending_sqes_(0),
    pending_submit_sqes_op_(false),
    shutdown_(false),
    timeout_(),
    registration_mutex_(mutex_.enabled()),
    reactor_(use_service<reactor>(ctx)),
    reactor_data_(),
    event_fd_(-1)
{
  reactor_.init_task();
  init_ring();
  register_with_reactor();
}

io_uring_service::~io_uring_service()
{
  if (ring_.ring_fd != -1)
    ::io_uring_queue_exit(&ring_);
  if (event_fd_ != -1)
    ::close(event_fd_);
}

void io_uring_service::shutdown()
{
  mutex::scoped_lock lock(mutex_);
  shutdown_ = true;
  lock.unlock();

  op_queue<operation> ops;

  // Cancel all outstanding operations.
  while (io_object* io_obj = registered_io_objects_.first())
  {
    for (int i = 0; i < max_ops; ++i)
    {
      if (!io_obj->queues_[i].op_queue_.empty())
      {
        ops.push(io_obj->queues_[i].op_queue_);
        if (::io_uring_sqe* sqe = get_sqe())
          ::io_uring_prep_cancel(sqe, &io_obj->queues_[i], 0);
      }
    }
    io_obj->shutdown_ = true;
    registered_io_objects_.free(io_obj);
  }

  // Cancel the timeout operation.
  if (::io_uring_sqe* sqe = get_sqe())
    ::io_uring_prep_cancel(sqe, &timeout_, IOSQE_IO_DRAIN);
  submit_sqes();

  // Wait for all completions to come back.
  for (; outstanding_work_ > 0; --outstanding_work_)
  {
    ::io_uring_cqe* cqe = 0;
    if (::io_uring_wait_cqe(&ring_, &cqe) != 0)
      break;
  }

  timer_queues_.get_all_timers(ops);

  scheduler_.abandon_operations(ops);
}

void io_uring_service::notify_fork(
    asio::execution_context::fork_event fork_ev)
{
  switch (fork_ev)
  {
  case asio::execution_context::fork_prepare:
    {
      // Cancel all outstanding operations. They will be restarted
      // after the fork completes.
      mutex::scoped_lock registration_lock(registration_mutex_);
      for (io_object* io_obj = registered_io_objects_.first();
          io_obj != 0; io_obj = io_obj->next_)
      {
        mutex::scoped_lock io_object_lock(io_obj->mutex_);
        for (int i = 0; i < max_ops; ++i)
        {
          if (!io_obj->queues_[i].op_queue_.empty()
              && !io_obj->queues_[i].cancel_requested_)
          {
            mutex::scoped_lock lock(mutex_);
            if (::io_uring_sqe* sqe = get_sqe())
              ::io_uring_prep_cancel(sqe, &io_obj->queues_[i], 0);
          }
        }
      }

      // Cancel the timeout operation.
      {
        mutex::scoped_lock lock(mutex_);
        if (::io_uring_sqe* sqe = get_sqe())
          ::io_uring_prep_cancel(sqe, &timeout_, IOSQE_IO_DRAIN);
        submit_sqes();
      }

      // Wait for all completions to come back, and post all completed I/O
      // queues to the scheduler. Note that some operations may have already
      // completed, or were explicitly cancelled. All others will be
      // automatically restarted.
      op_queue<operation> ops;
      for (; outstanding_work_ > 0; --outstanding_work_)
      {
        ::io_uring_cqe* cqe = 0;
        if (::io_uring_wait_cqe(&ring_, &cqe) != 0)
          break;
        if (void* ptr = ::io_uring_cqe_get_data(cqe))
        {
          if (ptr != this && ptr != &timer_queues_ && ptr != &timeout_)
          {
            io_queue* io_q = static_cast<io_queue*>(ptr);
            io_q->set_result(cqe->res);
            ops.push(io_q);
          }
        }
      }
      scheduler_.post_deferred_completions(ops);

      // Restart and eventfd operation.
      register_with_reactor();
    }
    break;

  case asio::execution_context::fork_parent:
    // Restart the timeout and eventfd operations.
    update_timeout();
    register_with_reactor();
    break;

  case asio::execution_context::fork_child:
    {
      // The child process gets a new io_uring instance.
      ::io_uring_queue_exit(&ring_);
      init_ring();
      register_with_reactor();
    }
    break;
  default:
    break;
  }
}

void io_uring_service::init_task()
{
  scheduler_.init_task();
}

void io_uring_service::register_io_object(
    io_uring_service::per_io_object_data& io_obj)
{
  io_obj = allocate_io_object();

  mutex::scoped_lock io_object_lock(io_obj->mutex_);

  io_obj->service_ = this;
  io_obj->shutdown_ = false;
  for (int i = 0; i < max_ops; ++i)
  {
    io_obj->queues_[i].io_object_ = io_obj;
    io_obj->queues_[i].cancel_requested_ = false;
  }
}

void io_uring_service::register_internal_io_object(
    io_uring_service::per_io_object_data& io_obj,
    int op_type, io_uring_operation* op)
{
  io_obj = allocate_io_object();

  mutex::scoped_lock io_object_lock(io_obj->mutex_);

  io_obj->service_ = this;
  io_obj->shutdown_ = false;
  for (int i = 0; i < max_ops; ++i)
  {
    io_obj->queues_[i].io_object_ = io_obj;
    io_obj->queues_[i].cancel_requested_ = false;
  }

  io_obj->queues_[op_type].op_queue_.push(op);
  io_object_lock.unlock();
  mutex::scoped_lock lock(mutex_);
  if (::io_uring_sqe* sqe = get_sqe())
  {
    op->prepare(sqe);
    ::io_uring_sqe_set_data(sqe, &io_obj->queues_[op_type]);
    post_submit_sqes_op(lock);
  }
  else
  {
    asio::error_code ec(ENOBUFS,
        asio::error::get_system_category());
    asio::detail::throw_error(ec, "io_uring_get_sqe");
  }
}

void io_uring_service::register_buffers(const ::iovec* v, unsigned n)
{
  int result = ::io_uring_register_buffers(&ring_, v, n);
  if (result < 0)
  {
    asio::error_code ec(-result,
        asio::error::get_system_category());
    asio::detail::throw_error(ec, "io_uring_register_buffers");
  }
}

void io_uring_service::unregister_buffers()
{
  (void)::io_uring_unregister_buffers(&ring_);
}

void io_uring_service::start_op(int op_type,
    io_uring_service::per_io_object_data& io_obj,
    io_uring_operation* op, bool is_continuation)
{
  if (!io_obj)
  {
    op->ec_ = asio::error::bad_descriptor;
    post_immediate_completion(op, is_continuation);
    return;
  }

  mutex::scoped_lock io_object_lock(io_obj->mutex_);

  if (io_obj->shutdown_)
  {
    io_object_lock.unlock();
    post_immediate_completion(op, is_continuation);
    return;
  }

  if (io_obj->queues_[op_type].op_queue_.empty())
  {
    if (op->perform(false))
    {
      io_object_lock.unlock();
      scheduler_.post_immediate_completion(op, is_continuation);
    }
    else
    {
      io_obj->queues_[op_type].op_queue_.push(op);
      io_object_lock.unlock();
      mutex::scoped_lock lock(mutex_);
      if (::io_uring_sqe* sqe = get_sqe())
      {
        op->prepare(sqe);
        ::io_uring_sqe_set_data(sqe, &io_obj->queues_[op_type]);
        scheduler_.work_started();
        post_submit_sqes_op(lock);
      }
      else
      {
        lock.unlock();
        io_obj->queues_[op_type].set_result(-ENOBUFS);
        post_immediate_completion(&io_obj->queues_[op_type], is_continuation);
      }
    }
  }
  else
  {
    io_obj->queues_[op_type].op_queue_.push(op);
    scheduler_.work_started();
  }
}

void io_uring_service::cancel_ops(io_uring_service::per_io_object_data& io_obj)
{
  if (!io_obj)
    return;

  mutex::scoped_lock io_object_lock(io_obj->mutex_);
  op_queue<operation> ops;
  do_cancel_ops(io_obj, ops);
  io_object_lock.unlock();
  scheduler_.post_deferred_completions(ops);
}

void io_uring_service::cancel_ops_by_key(
    io_uring_service::per_io_object_data& io_obj,
    int op_type, void* cancellation_key)
{
  if (!io_obj)
    return;

  mutex::scoped_lock io_object_lock(io_obj->mutex_);

  bool first = true;
  op_queue<operation> ops;
  op_queue<io_uring_operation> other_ops;
  while (io_uring_operation* op = io_obj->queues_[op_type].op_queue_.front())
  {
    io_obj->queues_[op_type].op_queue_.pop();
    if (op->cancellation_key_ == cancellation_key)
    {
      if (first)
      {
        other_ops.push(op);
        if (!io_obj->queues_[op_type].cancel_requested_)
        {
          io_obj->queues_[op_type].cancel_requested_ = true;
          mutex::scoped_lock lock(mutex_);
          if (::io_uring_sqe* sqe = get_sqe())
          {
            ::io_uring_prep_cancel(sqe, &io_obj->queues_[op_type], 0);
            submit_sqes();
          }
        }
      }
      else
      {
        op->ec_ = asio::error::operation_aborted;
        ops.push(op);
      }
    }
    else
      other_ops.push(op);
    first = false;
  }
  io_obj->queues_[op_type].op_queue_.push(other_ops);

  io_object_lock.unlock();

  scheduler_.post_deferred_completions(ops);
}

void io_uring_service::deregister_io_object(
    io_uring_service::per_io_object_data& io_obj)
{
  if (!io_obj)
    return;

  mutex::scoped_lock io_object_lock(io_obj->mutex_);
  if (!io_obj->shutdown_)
  {
    op_queue<operation> ops;
    bool pending_cancelled_ops = do_cancel_ops(io_obj, ops);
    io_obj->shutdown_ = true;
    io_object_lock.unlock();
    scheduler_.post_deferred_completions(ops);
    if (pending_cancelled_ops)
    {
      // There are still pending operations. Prevent cleanup_io_object from
      // freeing the I/O object and let the last operation to complete free it.
      io_obj = 0;
    }
    else
    {
      // Leave io_obj set so that it will be freed by the subsequent call to
      // cleanup_io_object.
    }
  }
  else
  {
    // We are shutting down, so prevent cleanup_io_object from freeing
    // the I/O object and let the destructor free it instead.
    io_obj = 0;
  }
}

void io_uring_service::cleanup_io_object(
    io_uring_service::per_io_object_data& io_obj)
{
  if (io_obj)
  {
    free_io_object(io_obj);
    io_obj = 0;
  }
}

void io_uring_service::run(long usec, op_queue<operation>& ops)
{
  __kernel_timespec ts;
  int local_ops = 0;

  if (usec > 0)
  {
    ts.tv_sec = usec / 1000000;
    ts.tv_nsec = (usec % 1000000) * 1000;
    mutex::scoped_lock lock(mutex_);
    if (::io_uring_sqe* sqe = get_sqe())
    {
      ++local_ops;
      ::io_uring_prep_timeout(sqe, &ts, 0, 0);
      ::io_uring_sqe_set_data(sqe, &ts);
      submit_sqes();
    }
  }

  ::io_uring_cqe* cqe = 0;
  int result = (usec == 0)
    ? ::io_uring_peek_cqe(&ring_, &cqe)
    : ::io_uring_wait_cqe(&ring_, &cqe);

  if (local_ops > 0)
  {
    if (result != 0 || ::io_uring_cqe_get_data(cqe) != &ts)
    {
      mutex::scoped_lock lock(mutex_);
      if (::io_uring_sqe* sqe = get_sqe())
      {
        ++local_ops;
        ::io_uring_prep_timeout_remove(sqe, reinterpret_cast<__u64>(&ts), 0);
        ::io_uring_sqe_set_data(sqe, &ts);
        submit_sqes();
      }
    }
  }

  bool check_timers = false;
  int count = 0;
  while (result == 0 || local_ops > 0)
  {
    if (result == 0)
    {
      if (void* ptr = ::io_uring_cqe_get_data(cqe))
      {
        if (ptr == this)
        {
          // The io_uring service was interrupted.
        }
        else if (ptr == &timer_queues_)
        {
          check_timers = true;
        }
        else if (ptr == &timeout_)
        {
          check_timers = true;
          timeout_.tv_sec = 0;
          timeout_.tv_nsec = 0;
        }
        else if (ptr == &ts)
        {
          --local_ops;
        }
        else
        {
          io_queue* io_q = static_cast<io_queue*>(ptr);
          io_q->set_result(cqe->res);
          ops.push(io_q);
        }
      }
      ::io_uring_cqe_seen(&ring_, cqe);
      ++count;
    }
    result = (count < complete_batch_size || local_ops > 0)
      ? ::io_uring_peek_cqe(&ring_, &cqe) : -EAGAIN;
  }

  decrement(outstanding_work_, count);

  if (check_timers)
  {
    mutex::scoped_lock lock(mutex_);
    timer_queues_.get_ready_timers(ops);
    if (timeout_.tv_sec == 0 && timeout_.tv_nsec == 0)
    {
      timeout_ = get_timeout();
      if (::io_uring_sqe* sqe = get_sqe())
      {
        ::io_uring_prep_timeout(sqe, &timeout_, 0, 0);
        ::io_uring_sqe_set_data(sqe, &timeout_);
        push_submit_sqes_op(ops);
      }
    }
  }
}

void io_uring_service::interrupt()
{
  mutex::scoped_lock lock(mutex_);
  if (::io_uring_sqe* sqe = get_sqe())
  {
    ::io_uring_prep_nop(sqe);
    ::io_uring_sqe_set_data(sqe, this);
  }
  submit_sqes();
}

void io_uring_service::init_ring()
{
  int result = ::io_uring_queue_init(ring_size, &ring_, 0);
  if (result < 0)
  {
    ring_.ring_fd = -1;
    asio::error_code ec(-result,
        asio::error::get_system_category());
    asio::detail::throw_error(ec, "io_uring_queue_init");
  }

#if !defined(ASIO_HAS_IO_URING_AS_DEFAULT)
  event_fd_ = ::eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
  if (event_fd_ < 0)
  {
    asio::error_code ec(-result,
        asio::error::get_system_category());
    ::io_uring_queue_exit(&ring_);
    asio::detail::throw_error(ec, "eventfd");
  }

  result = ::io_uring_register_eventfd(&ring_, event_fd_);
  if (result < 0)
  {
    ::close(event_fd_);
    ::io_uring_queue_exit(&ring_);
    asio::error_code ec(-result,
        asio::error::get_system_category());
    asio::detail::throw_error(ec, "io_uring_queue_init");
  }
#endif // !defined(ASIO_HAS_IO_URING_AS_DEFAULT)
}

#if !defined(ASIO_HAS_IO_URING_AS_DEFAULT)
class io_uring_service::event_fd_read_op :
  public reactor_op
{
public:
  event_fd_read_op(io_uring_service* s)
    : reactor_op(asio::error_code(),
        &event_fd_read_op::do_perform, event_fd_read_op::do_complete),
      service_(s)
  {
  }

  static status do_perform(reactor_op* base)
  {
    event_fd_read_op* o(static_cast<event_fd_read_op*>(base));

    for (;;)
    {
      // Only perform one read. The kernel maintains an atomic counter.
      uint64_t counter(0);
      errno = 0;
      int bytes_read = ::read(o->service_->event_fd_,
          &counter, sizeof(uint64_t));
      if (bytes_read < 0 && errno == EINTR)
        continue;
      break;
    }

    op_queue<operation> ops;
    o->service_->run(0, ops);
    o->service_->scheduler_.post_deferred_completions(ops);

    return not_done;
  }

  static void do_complete(void* /*owner*/, operation* base,
      const asio::error_code& /*ec*/,
      std::size_t /*bytes_transferred*/)
  {
    event_fd_read_op* o(static_cast<event_fd_read_op*>(base));
    delete o;
  }

private:
  io_uring_service* service_;
};
#endif // !defined(ASIO_HAS_IO_URING_AS_DEFAULT)

void io_uring_service::register_with_reactor()
{
#if !defined(ASIO_HAS_IO_URING_AS_DEFAULT)
  reactor_.register_internal_descriptor(reactor::read_op,
      event_fd_, reactor_data_, new event_fd_read_op(this));
#endif // !defined(ASIO_HAS_IO_URING_AS_DEFAULT)
}

io_uring_service::io_object* io_uring_service::allocate_io_object()
{
  mutex::scoped_lock registration_lock(registration_mutex_);
  return registered_io_objects_.alloc(
      ASIO_CONCURRENCY_HINT_IS_LOCKING(
        REACTOR_IO, scheduler_.concurrency_hint()));
}

void io_uring_service::free_io_object(io_uring_service::io_object* io_obj)
{
  mutex::scoped_lock registration_lock(registration_mutex_);
  registered_io_objects_.free(io_obj);
}

bool io_uring_service::do_cancel_ops(
    per_io_object_data& io_obj, op_queue<operation>& ops)
{
  bool cancel_op = false;

  for (int i = 0; i < max_ops; ++i)
  {
    if (io_uring_operation* first_op = io_obj->queues_[i].op_queue_.front())
    {
      cancel_op = true;
      io_obj->queues_[i].op_queue_.pop();
      while (io_uring_operation* op = io_obj->queues_[i].op_queue_.front())
      {
        op->ec_ = asio::error::operation_aborted;
        io_obj->queues_[i].op_queue_.pop();
        ops.push(op);
      }
      io_obj->queues_[i].op_queue_.push(first_op);
    }
  }

  if (cancel_op)
  {
    mutex::scoped_lock lock(mutex_);
    for (int i = 0; i < max_ops; ++i)
    {
      if (!io_obj->queues_[i].op_queue_.empty()
          && !io_obj->queues_[i].cancel_requested_)
      {
        io_obj->queues_[i].cancel_requested_ = true;
        if (::io_uring_sqe* sqe = get_sqe())
          ::io_uring_prep_cancel(sqe, &io_obj->queues_[i], 0);
      }
    }
    submit_sqes();
  }

  return cancel_op;
}

void io_uring_service::do_add_timer_queue(timer_queue_base& queue)
{
  mutex::scoped_lock lock(mutex_);
  timer_queues_.insert(&queue);
}

void io_uring_service::do_remove_timer_queue(timer_queue_base& queue)
{
  mutex::scoped_lock lock(mutex_);
  timer_queues_.erase(&queue);
}

void io_uring_service::update_timeout()
{
  if (::io_uring_sqe* sqe = get_sqe())
  {
    ::io_uring_prep_timeout_remove(sqe, reinterpret_cast<__u64>(&timeout_), 0);
    ::io_uring_sqe_set_data(sqe, &timer_queues_);
  }
}

__kernel_timespec io_uring_service::get_timeout() const
{
  __kernel_timespec ts;
  long usec = timer_queues_.wait_duration_usec(5 * 60 * 1000 * 1000);
  ts.tv_sec = usec / 1000000;
  ts.tv_nsec = usec ? (usec % 1000000) * 1000 : 1;
  return ts;
}

::io_uring_sqe* io_uring_service::get_sqe()
{
  ::io_uring_sqe* sqe = ::io_uring_get_sqe(&ring_);
  if (!sqe)
  {
    submit_sqes();
    sqe = ::io_uring_get_sqe(&ring_);
  }
  if (sqe)
  {
    ::io_uring_sqe_set_data(sqe, 0);
    ++pending_sqes_;
  }
  return sqe;
}

void io_uring_service::submit_sqes()
{
  if (pending_sqes_ != 0)
  {
    int result = ::io_uring_submit(&ring_);
    if (result > 0)
    {
      pending_sqes_ -= result;
      increment(outstanding_work_, result);
    }
  }
}

void io_uring_service::post_submit_sqes_op(mutex::scoped_lock& lock)
{
  if (pending_sqes_ >= submit_batch_size)
  {
    submit_sqes();
  }
  else if (pending_sqes_ != 0 && !pending_submit_sqes_op_)
  {
    pending_submit_sqes_op_ = true;
    lock.unlock();
    scheduler_.post_immediate_completion(&submit_sqes_op_, false);
  }
}

void io_uring_service::push_submit_sqes_op(op_queue<operation>& ops)
{
  if (pending_sqes_ != 0 && !pending_submit_sqes_op_)
  {
    pending_submit_sqes_op_ = true;
    ops.push(&submit_sqes_op_);
    scheduler_.compensating_work_started();
  }
}

io_uring_service::submit_sqes_op::submit_sqes_op(io_uring_service* s)
  : operation(&io_uring_service::submit_sqes_op::do_complete),
    service_(s)
{
}

void io_uring_service::submit_sqes_op::do_complete(void* owner, operation* base,
    const asio::error_code& /*ec*/, std::size_t /*bytes_transferred*/)
{
  if (owner)
  {
    submit_sqes_op* o = static_cast<submit_sqes_op*>(base);
    mutex::scoped_lock lock(o->service_->mutex_);
    o->service_->submit_sqes();
    if (o->service_->pending_sqes_ != 0)
      o->service_->scheduler_.post_immediate_completion(o, true);
    else
      o->service_->pending_submit_sqes_op_ = false;
  }
}

io_uring_service::io_queue::io_queue()
  : operation(&io_uring_service::io_queue::do_complete)
{
}

struct io_uring_service::perform_io_cleanup_on_block_exit
{
  explicit perform_io_cleanup_on_block_exit(io_uring_service* s)
    : service_(s), io_object_to_free_(0), first_op_(0)
  {
  }

  ~perform_io_cleanup_on_block_exit()
  {
    if (io_object_to_free_)
    {
      mutex::scoped_lock lock(service_->mutex_);
      service_->free_io_object(io_object_to_free_);
    }

    if (first_op_)
    {
      // Post the remaining completed operations for invocation.
      if (!ops_.empty())
        service_->scheduler_.post_deferred_completions(ops_);

      // A user-initiated operation has completed, but there's no need to
      // explicitly call work_finished() here. Instead, we'll take advantage of
      // the fact that the scheduler will call work_finished() once we return.
    }
    else
    {
      // No user-initiated operations have completed, so we need to compensate
      // for the work_finished() call that the scheduler will make once this
      // operation returns.
      service_->scheduler_.compensating_work_started();
    }
  }

  io_uring_service* service_;
  io_object* io_object_to_free_;
  op_queue<operation> ops_;
  operation* first_op_;
};

operation* io_uring_service::io_queue::perform_io(int result)
{
  perform_io_cleanup_on_block_exit io_cleanup(io_object_->service_);
  mutex::scoped_lock io_object_lock(io_object_->mutex_);

  if (result != -ECANCELED || cancel_requested_)
  {
    if (io_uring_operation* op = op_queue_.front())
    {
      if (result < 0)
      {
        op->ec_.assign(-result, asio::error::get_system_category());
        op->bytes_transferred_ = 0;
      }
      else
      {
        op->ec_.assign(0, op->ec_.category());
        op->bytes_transferred_ = static_cast<std::size_t>(result);
      }
    }

    while (io_uring_operation* op = op_queue_.front())
    {
      if (op->perform(io_cleanup.ops_.empty()))
      {
        op_queue_.pop();
        io_cleanup.ops_.push(op);
      }
      else
        break;
    }
  }

  cancel_requested_ = false;

  if (!op_queue_.empty())
  {
    io_uring_service* service = io_object_->service_;
    mutex::scoped_lock lock(service->mutex_);
    if (::io_uring_sqe* sqe = service->get_sqe())
    {
      op_queue_.front()->prepare(sqe);
      ::io_uring_sqe_set_data(sqe, this);
      service->post_submit_sqes_op(lock);
    }
    else
    {
      lock.unlock();
      while (io_uring_operation* op = op_queue_.front())
      {
        op->ec_ = asio::error::no_buffer_space;
        op_queue_.pop();
        io_cleanup.ops_.push(op);
      }
    }
  }

  // The last operation to complete on a shut down object must free it.
  if (io_object_->shutdown_)
  {
    io_cleanup.io_object_to_free_ = io_object_;
    for (int i = 0; i < max_ops; ++i)
      if (!io_object_->queues_[i].op_queue_.empty())
        io_cleanup.io_object_to_free_ = 0;
  }

  // The first operation will be returned for completion now. The others will
  // be posted for later by the io_cleanup object's destructor.
  io_cleanup.first_op_ = io_cleanup.ops_.front();
  io_cleanup.ops_.pop();
  return io_cleanup.first_op_;
}

void io_uring_service::io_queue::do_complete(void* owner, operation* base,
    const asio::error_code& ec, std::size_t bytes_transferred)
{
  if (owner)
  {
    io_queue* io_q = static_cast<io_queue*>(base);
    int result = static_cast<int>(bytes_transferred);
    if (operation* op = io_q->perform_io(result))
    {
      op->complete(owner, ec, 0);
    }
  }
}

io_uring_service::io_object::io_object(bool locking)
  : mutex_(locking)
{
}

} // namespace detail
} // namespace asio

#include "asio/detail/pop_options.hpp"

#endif // defined(ASIO_HAS_IO_URING)

#endif // ASIO_DETAIL_IMPL_IO_URING_SERVICE_IPP