Converting stack address to shared-memory object name

[Utkarsh Rai]

On Wed, Jul 8, 2020 at 6:56 PM Gedare Bloom <gedare at rtems.org> wrote:

> On Wed, Jul 8, 2020 at 6:53 AM Sebastian Huber
> <sebastian.huber at embedded-brains.de> wrote:
> >
> > On 08/07/2020 14:43, Utkarsh Rai wrote:
> >
> > > Hello,
> > > For my GSoC project, I have to provide high-level APIs for sharing
> > > isolated stacks.
> > > The POSIX compliant high-level way of sharing stacks can be to create
> > > a shared memory object of the stack to be shared through shm_open and
> > > then mmap that to the address space of the current stack. My doubt is,
> > > shm_open() takes the path-name of the shared memory object. Since this
> > > is a high-level API, how does the user 'convert' the stack address to
> > > a shared memory object name?
> > Do we need any POSIX compatibility for this? What would you do in a
> > POSIX environment? You first get some memory, then hand it over to
> > shm_open() to get a file descriptor, then use the file descriptor in
> > mmap(), then use this for pthread_attr_setstack() and whatever?
>
> Yes, but the way to name objects is not set by posix.
>
> We need to provide our own way of translating an address into a name.
>
> > >
> > > Dr.Gedare mentioned that one way to deal with naming would be
> > > something like Mr.Sebastian has been doing with specifications. From
> > > what I could gather, it is a hierarchical way of representing
> > > objects(Though, I am not very sure if  I understand this accurately).
> > > How can something like this be implemented for naming stack-addresses?
> > I am not sure if the specification of RTEMS is helpful in this context.
>
> I should have provided a little bit more guidance. I was thinking out
> loud in yesterday's IRC meeting. My thought was more along the lines
> of looking at how UIDs/naming should be done, and that specs had to
> solve a naming problem. However the static nature of specs is not a
> great fit to this problem.
>
> Actually, what is a good model would be something like /proc or
> Linux's sysfs. An IMFS filesystem that exports task information could
> be used to name memory regions. (It could eventually supplant
> task-based statistics reporting too.)
>
> Another idea I had though, which seems to have been lost in the
> shuffle, is to look at how the object names work in RTEMS and see if
> we can add some fixed relationships, e.g., task_name # stack.
>
> I think we should start by just treating the entire task stack as a
> single named object; either it is all shared, or none of it is shared.
> This will be easier to implement and also more widely supported by
> simpler MPU/MMU hardware. Later on, we can consider extending the
> namespace with 'offsets' /taskfs/IDLE/stack/00000A28
> could be a location at byte A28 offset from the start of the stack of
> the IDLE task.
>
>
I have a few questions -

> Users would get the stack address of the stack they want to share
through pthread_attr_getstack(). Now, when they get the address they want
to share, they would pass the appropriate name of this memory-region. What
we have to provide is a mechanism to 'convert' this address to an
appropriate name. Is this the accepted way or the other way round, i.e. the
user passes a name as per a specified convention, and that name is
'converted' to a specific address?

> When you say "treating the entire task stack as a single named object"
does it mean that we assign a single name, say "task_stack" to the complete
stack address space? In that case, how do we deal we the presence of
multiple tasks that are allocated from the same pool of task stack? I
understand that on a simpler MPU/MMU hardware it would make sense to
specify names for each memory section (.txt- "text", .bss - "bss" etc.) but
in this case,  where we are sharing only selected thread-stacks, I suppose
we will have to have a way to handle 'offsets' right from the start?


> Gedare
>
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