Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Thu, 12 Mar 1998 21:16:16 +0100

Jeff Hill wrote:
> I will still argue that a mutex semaphore should be used in favor of intLock()/intUnlock()
> in task level code. Even if the locking interval is currently quite short, you or some
> other developer who may not be as keen on these issues will invariably
> need to expand the amount of code in-between the lock calls. I hope that
> this person, despite schedule and time pressures, is conscientious and therefore is
> willing to write the additional code require to initialize and use the mutex semaphore.
> I am also concerned about what would happen to the system if for any reason
> an exception was generated (say a floating point exception or a bus error) while 
> intLock() is applied. On some architectures a floating point exception can occur
> when loading an invalid floating point number. The key difference is that intLock()
> is a global operation with side effects on the entire system, and semTake() will only 
> have side effects on tasks using the mutex semaphore. It is much easier to 
> understand (and predict the failure modes of ) a system that is made up of smaller 
> components that minimize their global side effects.

I agree that intLock/intUnlock is dangerous in the situations you
describe. On the other hand: what if you have some external SW package
that you cannot or don't want to modify, and that installs an interrupt
handler that updates your global variable. Then you will have a problem
with the semaphore solution because your SW package does not know
anything about it. I think the Symb device support should be able to
handle such a situation safely.

The argument about affecting the whole system (intLock) vs. small
components (mutex semaphore) is a good one, but mind that we are talking
about a _globally_ visible object that _any_ instance in the system can
modify, even the user from the shell. I see no other way to protect such
a variable globally but by a global mechanism.

Anyway: I'd rather have the Symb support with semaphore protection than
with no protection at all.

	Ben
-- 
The Notorious Neb Nesknarf
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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System