Re: lock ownership enforcement in OO design

[Andrew Johnson <anj@aps.anl.gov>]

In general I agree with Kay; you have a working design, and any change 
now would require spending time deleting all the guard parameters and 
changing the tests.  I'm not that keen on the idea of using macros to 
remove the guards either -- nobody actually builds EPICS with NDEBUG 
defined that I know of, and I wouldn't be to surprised if some things 
actually stop working when built like that - I've fixed more than one 
routine in the last few years that was doing processing inside an 
assert() statement.

Eric Norum wrote:
> Andrew and I discussed some of the technical issues.
> I see no showstoppers, even on multi-CPU, multi-cache, machines -- 
> thought I don't think that my crushing logic has completely convinced 
> Andrew yet.....

I have don't like going beyond what is strictly guaranteed by the 
threading primitive libraries that we're using in portable code, and I 
think it's fair to say that we'd be outside what Posix promises.

pthread_mutex_lock() provides a read barrier (think cache-invalidate); 
pthread_mutex_unlock() provides a write barrier (cache-flush).  The 
guaranteed synchronization requirements are that a write barrier must 
follow the last write to the protected data, and a read barrier precede 
the first read.  Attempting to read before the read barrier could give 
you anything, and that's what we'd be doing by comparing the 
mutex.threadId with ours without.

> Another benefit would be that it would be useful to have the task id of 
> a mutex holder exposed as a API-accessible value when trying to diagnose 
> deadlocks.

Careful, that task id might not be visible from threads across a CPU 
boundary; if we offer the thread ID as a mutex method we'd risk this not 
working in some circumstances (or we'd have to have a mutex to protect 
the mutex...).  Just because you only intend that API for debugging 
doesn't mean that it won't get used in regular code...

- Andrew
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