Re: Fwd: Sequencer bug?

[Tim Mooney <mooney@aps.anl.gov>]

On 2/25/2010 7:43 AM, Benjamin Franksen wrote:

> It remains to draw practical conclusions for how to avoid such race
> conditions in SNL code. Should we generally be wary of pvPut and monitor to
> the same state variable? Could the compiler warn us about that? Or is this
> too restrictive?

Great analysis.  Thanks for that.

Wrong take-home message, though.  If you had written this in C,
your first thoughts would probably have been something like this:

"Hmmm... two tasks; shared resource; better implement a mutex"

and your life would have been less eventful.  (Sorry for the pun.
Here's another: this is a very old tale with a context switch.)

I've stabbed myself with exactly this blade more times than I can
count, in every programming language I've ever used.

I think the take-home messages are simply that SNL is not magic,
and that multitasking event-driven software is hard to get right.

Compilers can't really help with stuff like this, because you
could have the same problem by writing to X and monitoring some
other PV, Y, that gets changed as a result of the write to X.
What's really pernicious is the case in which X modifies Y
via a CA link, or via the reply to a serial device.  In these
cases, the race condition is moved out in time so you almost
never can find it in testing.

--
Tim Mooney (mooney@aps.anl.gov) (630)252-5417
Beamline Controls & Data Acquisition Group
Advanced Photon Source, Argonne National Lab.