Synchronization primitives#

When the user wants to use a sycl::buffer simultaneously in the SYCL runtime and their own code (e.g. a multi-threaded mechanism) and wants to use manual synchronization without using a sycl::host_accessor, a std::mutex can be passed to the sycl::buffer constructor through a property.

The runtime promises to lock the mutex whenever the data is in use and unlock it when it no longer needs it:

{
  std::mutex m;
  auto shD = std::make_shared<int>(42)
  sycl::buffer b { shD, { sycl::property::buffer::use_mutex { m } } };
  {
    std::lock_guard lck { m };

    // User accesses the data
    do_something(shD);

    /* m is unlocked when lck goes out of scope, by normal end of this
      block but also if an exception is thrown for example */
  }
}

When the runtime releases the std::mutex the user is guaranteed that the data was copied back to the std::shared_pointer, unless the final data destination has been changed using the member function set_final_data().