Subj : Re: proper order of pthread_mutex_unlock() and pthread_cond_signal()
To   : comp.programming.threads
From : el_bandido
Date : Tue Feb 01 2005 12:47 am

Hello Eric,

>     It is said that signalling with the mutex lock can
> lead to "more predictable scheduling behavior."  Consider
> a scenario where thread A signals while thread B is parked
> in cond_wait, but thread C is also interested in the same
> mutex.  If A unlocks and then signals, C may re-lock the
> mutex and perhaps change the predicate before B can awaken.
> If A signals and then unlocks this can still happen, but
> may be less likely.

Your explanation makes a lot of sense.

>     However, a program that depends so sensitively on the
> vagaries of thread scheduling is suspect to begin with:

Fully agreed.

> why should you care whether B gets its chance before or
> after C?  Or, if you do care, why aren't you doing something
> explicit about it instead of relying on fiddly implementation
> details?  If the exact ordering of B and C is unimportant
> or is being handled in some other way, then it seems to me
> that another principle should rule: Keep critical regions
> as short as is practical, that is, hold locks for the least
> time you can comfortably manage.  This principle suggests
> that unlocking and then signalling is preferable: cond_signal
> doesn't need mutex protection, hence cond_signal can be moved
> outside the critical section, hence it *should* be outside.

I think so too.

>     Neither arrangement is "right" or "wrong," but I think
> it is usually better to unlock first and signal afterward.
> Of course, if the program requires a particular ordering of
> B and C and hasn't taken other steps to enforce it, the program
> is already broken beyond the power of a simple rearrangement
> to repair it.

Thank you very much for your excellent explanation, it all makes perfect 
sense. I was confused since I read the example code in the O'Reilly book 
"Pthreads Programming" written by Nichols, Buttlar and Farrell and they 
signal first and then unlock (p80-81).

Best regards,
Roberto Nibali, ratz
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