[Vm-dev] Re: [Pharo-project] BlockClosure>>ensure: implementation

Igor Stasenko siguctua at gmail.com
Wed Oct 24 02:56:34 UTC 2012

On 24 October 2012 00:57, Eliot Miranda <eliot.miranda at gmail.com> wrote:
> On Mon, Oct 22, 2012 at 2:17 AM, Clément Bera <clement.bera at inria.fr> wrote:
>> Hello,
>> I don't understand something on BlockClosure>>ensure:. Why does it use 'self valueNoContextSwitch'  and not 'self value' ? In which case is there an issue ?
> Prior to my closure implementation BlockContext>>value[:value:*] was not a context switch point.  So to preserve the threading semantics of ensure: we implemented ensure: in terms of BlockClosure>>valueNoContextSwitch, which is not a context-switch point, as opposed to BlockClosure>>value[:value:*].  The context switch points in the VM are non-primitive sends (or failing primitives, i.e. full method activation), backward branches, the Process primitives suspend, resume, signal and wait (and CrtiticalSection/Mutex enter and exit) and block evaluation.

Hmm.. your response puzzles me even more.

just simple example to show a controversy:

[ do something ] ensure: [ semaphore signal. anotherSemaphore wait ].

I think that semantics of ensure should be:
 - same as #value, but evaluate additional "ensure" block prior return
point , be it normal return or stack unwinding (non-local return or
process termination).

Anything else, like scheduler interference, which may preempt current
process which evaluates block doesn't matters because semantics is
local to process scope where given block is evaluated.

Trying to ensure "ensure" semantics even regardless of scheduling is
pointless and futile.
So, i am puzzled, what exactly "threading semantics" you trying to
preserve here.

Best regards,
Igor Stasenko.

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