Experimental Physics and Industrial Control System

On Friday 21 May 2010, Andrew Johnson wrote:
> On Thursday 20 May 2010 17:53:04 Benjamin Franksen wrote:
> > This bug must have been in the sequencer for a long, long time,
> > probably right from the beginning.
> 
> I'm not convinced it's quite as bad as you think; I suspect it derives
>  from R3.14.x when Jeff introduced the ca_enable_preemptive_callback
>  option.  Before this time the CA library never got a chance to process
>  network messages unless you called it using ca_pend_event() or
>  ca_pend_io() so the callbacks would only run when the thread was inside
>  the CA library.

Unfortunately, pre-3.14 versions are affected, too, see below.

> Hmm, that doesn't necessarily work with multiple state sets though, I
>  wonder if CA supported sharing CA contexts back then?  

It did, this is what the ca_import routine was for.

You are right that if you use one single task and 
ca_disable_preemptive_callback (or, equivalently, the old 
ca_task_initialize), then there is no problem. However, even the old 1.9.x 
sequencer releases use an extra auxiliary task to initialize CA, and a 
separate task for each state set taht calls ca_import. The auxiliary task is 
the only place where ca_pend_event is called. This means all callbacks will 
be executed in the task context of the auxiliary task, and (I checked it) 
there are no locks taken when the callback copies the data into the program 
variable.

So, the only difference is that in the old versions it is the sequencer's 
own task, while in later versions it is a (hidden) CA internal task.

Cheers
Ben

Replies:

Re: Race conditions in SNL programs Dirk Zimoch

References:

Race conditions in SNL programs Benjamin Franksen

Re: Race conditions in SNL programs Andrew Johnson

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