Re: [boost] Re: Semaphores?

From: Victor A. Wagner, Jr. (vawjr_at_[hidden])
Date: 2002-05-26 11:21:46

At Saturday 2002/05/25 15:27, you wrote:
[deleted]

>  What is not clear to me is why some people call semaphores "BAD"
>in all caps.  I've used them since 1979 without any disasters so far.
>
>I want to emphasize that I am asking for my own education, not to stir
>up trouble.  With that in mind, I suggest that "these proceedings are
>closed."  If you would be kind enough to do me a favor and direct my
>attention to some article that has examples of the semaphore's dark
>side, please email me at jdadson_at_ix.netcom.com.

The first real-time embedded kernel I ever worked with was architected and 
implemented in 1972.  I re-implemented it in its entirety in 1973 (I worked 
for the mini-computer manufacturer for whom it had been originally 
written). It had two synchronization methods, semaphore and mailbox 
(basically a semaphore with 32bits of data added at 'signal' time to be 
given to the "waiting" thread).
for the next 18 years, those two methods were the basis for all the other 
synchronizers in the embedded real-time systems which we wrote.  I don't 
recall any instances of them being "mis-used" or being considered 
"dangerous".  We had more problems with the engineers attempting to put 
together instruction sets which wouldn't allow us to write a semaphore, 
than we had with the semaphores (or their users).  We even had "global" 
semaphores (signal by any thread/process, wait by any thread/process)
These systems were used to build systems on which people's lives depended, 
so either we were "very lucky" or maybe they're not so hazardous as people 
think.
OTOH, we had absolute control over the computers on which they executed, 
AND we had "instruction help" to ensure that we COULD gain (and hold) 
access to all the busses (even the multi-processor models) for shareable 
items (like a region of memory) for the time necessary to do the "critical" 
work.
The people who write high-level languages have apparently never worked in 
such an environment (an atomic test/set (exchange w/ memory works fine) 
which, of course, we had), because HLLs don't seem to have the operators 
necessary to describe such things(we still don't even have a plain swap 
operator, let alone atomic).  So we're stuck with people attempting to 
implement things in HLLs which are inherently not "sayable".  I believe 
this is what causes the current dilemma about "thread safety", we're trying 
to say something without a proper vocabulary.

If you do get some info on the hazards, could you please forward it to me?

To paraphrase the king of Siam:  " 'Tis a bafflement. "

Victor A. Wagner Jr.      http://rudbek.com
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