Re: [Corelinux-develop] Questions (again)

[Hans - D. <dul...@eg...>]

On Fri, 25 Aug 2000, Frank V. Castellucci wrote:

> Date: Fri, 25 Aug 2000 06:58:07 -0400
> From: Frank V. Castellucci <fr...@co...>
> Reply-To: cor...@li...
> To: cor...@li...
> Subject: Re: [Corelinux-develop] Questions (again)
> 
> Hans - Dulimarta wrote:
> > 
> > I'm implementing EventSemaphore in the following setup:
> > 
> > Assuming we have three operations: down(), up(), and zero().
> 
> Are these the actual interface method names? While I don't feel the

No they are not. These are the Dijkstra primitives.

Operation | Dijkstra | IPC Semaphore
----------+----------+----------------------
down(s)   |   P(s)   | semop (..., -1, ...)
up()      |   V(s)   | semop (..., -1, ...)
zero()    |   ----   | semop (...,  0, ...) 

> requirements document quite addresses is, the semantics of an Event are
> things more like hold/trigger/waitFor
> 
> > When an EvSem is created, its value is initialized to 1 indicating the
> > expected event has not occured.
> 
> I assume this is 'up', which I would consider 'hold'.

I use setValue(1) in the code.

> 
> > Any callers who are waiting for the event to occur will do a "lock"
> > by calling zero(). When the event actually takes place, the semaphore will
> > be released via the down() operation.
> 
> Terminology shift, callers 'waitFor' events no? and the owner 'triggers'
> them (to trigger an Event)?

But the interface of EventSemaphore does not have waitFor() function
member. I could find release(), lockWait(), lockTimeOut(), and
lockNoWait(). I assume the interface design is completed for
EventSemaphore so, I was trying to implement the semantic of hold,
trigger, and waitFor using those four member functions with the following
mapping:

creator/owner   hold    --> init. sem to 1  : SemComm::setValue(1)
event observer  waitFor --> lock<XYZ>()     : SemComm::setZero ()
creator/owner   trigger --> release()       : SemComm::setLock ()

SemComm is SemaphoreCommon class.

The last two lines might look weird. Usually lock<XYZ> calls setLock() and
release() calls setUnlock(), but the users of EventSemaphore class
know the interface and semantic of the interface, they do not aware of of
these interfaces are implemented.

> 
> > The requirement document (SRS ?) for says that the Many-to-One semaphores
> > will automatically lock upon creation and there is no option to create in
> > unlock. When I look at the code, lock is similar to down() semaphore
> > operation which implies that the creator will be block in the middle of
> > the EventSemaphore ctor.
> > 
> > In the context of the above three operations, how can the creator perform
> > a release operation to indicate that the expected event has occured?
> 
> There are two things to consider here:
> 
> 1. As the SemaphoreGroup instantiates the creation of the Semaphore (in
> this case EventSemaphore), it can construct and then call 'hold'.
>

Is it correct if I interpret 'hold' as initializing the semaphore to 1,
hence automatically bringing the semaphore to a 'lock' state as mentioned
in the SRS.
 
> 2. The owner of a semaphore is not in a blocked state, the Linux kernel
> semaphore has a state change, but the owner is not blocked. Only those
> callers 'waiting for' an event are blocked.
> 

I am confused here. Does 'hold' also mean claiming ownership?  Who are
'owner' and 'creator/instantiator'? Are these two entities the different?
If they are different, how can the creator pass ownership to the
owner?

> Hope this helps.

Yes, it does a lot. But maybe more questions to come... :-)

> 
> > --
> > Hans Dulimarta, Ph.D.                  dul...@co...
> > P: 517-432-7589             http://www.egr.msu.edu/~dulimart
> > F: 760-281-7691             http://corelinux.sourceforge.net
> > Elec. & Comp. Engg., Mich. State Univ., E. Lansing, MI 48824
> > 
> > _______________________________________________
> > Corelinux-develop mailing list
> > Cor...@li...
> > http://lists.sourceforge.net/mailman/listinfo/corelinux-develop
> 
> 

-- 
Hans Dulimarta, Ph.D.                  dul...@co...
P: 517-432-7589             http://www.egr.msu.edu/~dulimart   
F: 760-281-7691             http://corelinux.sourceforge.net
Elec. & Comp. Engg., Mich. State Univ., E. Lansing, MI 48824