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Subject: Re: state notation code flags
From: Andrew Johnson <anj@aps.anl.gov>
To: tech-talk@aps.anl.gov
Date: Mon, 5 Oct 2009 10:16:04 -0500
Gentlemen,

On Saturday 03 October 2009 08:05:37 Mark Rivers wrote:
>
> What is it you are trying to do?  You are doing both pvPuts and looking for
> external changes to the same PV?
>
> If so, then one solution is to ignore events which correspond to the value
> that your SNL program set with pvPut, and only act on events with values
> that must have been done by some external agent.  Here is an example from
> one of my programs:
> int abort;
> assign abort to "{P}{R}Abort.VAL";
> monitor abort;
> evflag abortMon;
> sync abort abortMon;
> ...
> ss xpsTrajectoryAbort {
>     state monitorAbort {
>         when ((efTestAndClear(abortMon)) && (abort==1) &&
>               (execState==EXECUTE_STATE_EXECUTING)) {
> ...
>             abort=0;
>             pvPut(abort);
>         } state monitorAbort
>     }
> }

I believe the issue you are both discussing is a classic case of a race 
condition which is caused by doing exactly what Mark discusses.  I suspect 
it's possible that the monitor callback can occur and change the value of the 
abort variable above in between the lines abort=0 and pvPut(abort), but even 
if that's not possible it is still causing the problems that Patrick is 
seeing.  It is unfortunate that the design of the SNL language makes this 
possible, and it may require some major surgery to fix this in the 
implementation.

However there is an easy solution, which I'm pretty sure will fix Patrick's 
issue: Add a second variable pointing to the same PV which is not monitored 
and is used only to do the pvPut operations.  Copying Mark's example, I've 
added the abortOut variable and removed the test of the abort value from the 
when() condition, although it's possible that you might still want that.

int abort;
int abortOut;
assign abort to "{P}{R}Abort.VAL";
assign abortOut to "{P}{R}Abort.VAL";
monitor abort;
evflag abortMon;
sync abort abortMon;
...
ss xpsTrajectoryAbort {
    state monitorAbort {
        when (efTestAndClear(abortMon) &&
              (execState==EXECUTE_STATE_EXECUTING)) {
...
            abortOut=0;
            pvPut(abortOut);
        } state monitorAbort
    }
}

If you adopt this model, you're never changing the value of the abort variable 
yourself, and I think you'll find that the result will only set the event flag 
once, when your put value makes its way back to you via the monitor.

I suspect it would be a good idea to try and get the the SNL compiler to issue 
a warning whenever it sees someone calling pvPut() on a variable which has a 
monitor on it, since this is IMHO an unsafe practice.

- Andrew
-- 
The best FOSS code is written to be read by other humans -- Harald Welte


Replies:
RE: state notation code flags Mark Rivers
Re: state notation code flags Benjamin Franksen
References:
state notation code flags Patrick Thomas
Re: state notation code flags Patrick Thomas
RE: state notation code flags Mark Rivers

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