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STARDEV  February 2009

STARDEV February 2009

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Subject:

Re: JNIAST threads

From:

David Berry <[log in to unmask]>

Reply-To:

Starlink development <[log in to unmask]>

Date:

Fri, 20 Feb 2009 11:40:31 +0000

Content-Type:

text/plain

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2009/2/20 Mark Taylor <[log in to unmask]>:
> On Fri, 20 Feb 2009, David Berry wrote:
>
>> 2009/2/19 Mark Taylor <[log in to unmask]>:
>> > On Thu, 19 Feb 2009, David Berry wrote:
>> >
>> >> Yes, there is a point here that it's pretty easy to end up with
>> >> partially locked objects. Say a thread creates a Frame, then creates a
>> >> FrameSet containing the Frame, then unlocks the original Frame and
>> >> passes it to another thread that locks it. The FrameSet itself is then
>> >> still  locked by the first thread, but the Frame within it is locked
>> >> by the second thread. If the first thread then attempts to unlock the
>> >> FrameSet what should happen? It cannot unlock the Frame within the
>> >> FrameSet because it is locked by another thread.
>> >
>> > The semantics I expect if thread T calls astUnlock on object A
>> > is that T unilaterally relinquishes claims on A.  That is,
>> > I don't expect astUnlock to guarantee that an object is not
>> > locked by anybody, but only that it is not locked by the calling thread.
>> > Seen like this, there's no reason (I think) that astUnlock can't
>> > be implemented in such a way as to recurse through all objects
>> > contained in the one which is unlocked, and (I think) that's what
>> > I'd expect to happen.
>>
>> I can see that this approach could be a reasonable way of handling
>> objects contained within other objects, but I'm a bit hesitant about
>> applying this idea to top level objects. Surely each thread should
>> know what top-level objects it has locked for itself, and so why would
>> it want to unlock an object that is currently locked by another
>> thread, except as the result of a prgramming error?  On the other
>> hand, whilst a thread could reasonably be expected to know what
>> top-level objects it currently owns, I can see it may not necessarily
>> know much about any objects contained within those top-level objects.
>>
>> So what about reporting an error if the top-level object supplied to
>> astUnlock is currently locked by another thread, but silently ignoring
>> any sub-objects that are locked by other threads?
>
> I'm not sure that's an improvement on my suggestion (additional
> complication in the interface from distinction between top-level
> and contained objects; possibility of requiring more bookkeeping
> from calling code e.g. in try/catch/finally type patterns), however,
> I'm happy for you to implement it like this.

My concern is that from my experience with smurf, it was by no means
uncommon for me to introduce a bug in smurf that resulted in a thread
thinking it owned an AST object when in fact it didn't. Error messages
such as the one issued by astUnlock helped to identify these bug.

Our difference seems to come down to the issue of whether a thread
should be expected to know what Objects it has locked for its own use.

I could of course add an extra parameter to astUnlock indicating if an
error should be reported if the object is not currently locked by the
calling thread.

David




> Mark
>
> --
> Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
> [log in to unmask] +44-117-928-8776 http://www.star.bris.ac.uk/~mbt/
>

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