Help in figuring out score objects in RTEMS and ticket #3131

[Joel Sherrill]

On Wed, Jun 10, 2020 at 7:34 PM Utkarsh Rai <utkarsh.rai60 at gmail.com> wrote:

> Hello,
> For my GSoC project, user-configurable thread stack protection, I need to
> track, allocate, and manipulate attributes of shared as well as protected
> stacks. Dr.Gedare suggested that tracking them with score objects would be
> a good idea. He also indicated a good place to start and understand
> score objects would be through this ticket.
> <https://devel.rtems.org/ticket/3131>
>
> From the information that I could find in docs, my understanding of score
> objects is that they are a type of directive that
> helps introduce modularity to RTEMS as various types of RTEMS objects like
> message queues, semaphores, etc. can use the same set of directives for
> allocation/deallocation and control of their object types.
>

It is probably easier for you to think about the score as a collection of
core classes that are reused across the public APIs. They do have strong
modularity but they also ensure that the correct and optimized behavior is
present across both the Classic and POSIX APIs. For example, a mutex can be
optimized and made computer science correct just once and both the POSIX
and Classic APIs benefit. Whether you call this layering, modularity,
packaging, or classes is OK.

If you look at the Doxygen, you will see there is even a bit of inheritance.


> Some of the examples of the implementation that I could find used
> _Object_Iniitialize_information()
> to initialize the object type, then _Object_Allocate()/Free()
> allocate/de-allocate the object. Object_Id is used to control the object
> type. My confusion is, how do we use object IDs to refer to and control the
> allocated objects?
>

Every public API with an ID uses the score Object Handler to convert that
ID into an instance to an object (base class) which is cast to an instance
of the proper API type. An object has a name, ID, and a chain node. Any
object can be put on lists. This is important because that's how the
inactive set of each class of objects is managed. Various things are put on
FIFO lists.

The Chain handler which defines the Node structure and the Red-Black Tree
are the foundation of many RTEMS data structures/algorithms.


>
> I also have some confusion in the above-mentioned ticket, it says-
> 'The mmap_h handler should construct a mapping object. A destructor is
> currently missing. Maybe the mmap_h handler should use a flag to deal with
> construction and destruction.'
>  I am unclear as to how the mmap_h handler should precisely look like and
> where should it be defined?
>

I don't think this will be useful for your project. This is only used to
provide a BSP specific mechanism to attach memory. This is primarily used
in paravirtualized environments to allow the BSP to map memory shared with
other address spaces. Without a hypervisor or host under RTEMS, this isn't
going to be used. Although I think Gedare has at least one use case beyond
this for it.

I think the stack allocator/deallocator hooks will be more useful.


> I would be grateful if you can provide some help in figuring this out.
>
> Regards,
> Utkarsh
>
> _______________________________________________
> devel mailing list
> devel at rtems.org
> http://lists.rtems.org/mailman/listinfo/devel
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.rtems.org/pipermail/devel/attachments/20200615/6f45ad76/attachment-0001.html>