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From: mbj@spice.cs.cmu.edu (Michael Jones)
Newsgroups: comp.sys.next,comp.os.mach
Subject: Re: technique used to implement mutex_lock()
Summary: mutex_lock() answer
Message-ID: <8460@pt.cs.cmu.edu>
Date: 17 Mar 90 17:15:17 GMT
References: <429@kgw2.bwi.WEC.COM>
Distribution: na
Organization: Carnegie-Mellon University, CS/RI
Lines: 29

In article <429@kgw2.bwi.WEC.COM>, dennisg@kgw2.bwi.WEC.COM (Dennis Glatting) writes:
> 
> how is mutex_lock() implemented in the NeXT computer?
> 
> i have several tasks with several threads each.  i had a thread lock a
> mutex with mutex_lock() then, unfortunatly, died.  the mutex stayed locked.
> i noticed that the NeXT was running a bit slow so i fired up gdb of
> the task in question.  i found more than one thread in cthread_yield().
> my question is:  when you call mutex_lock() and the mutex is locked then
> what happens?  does the thread do a cthread_yield() until the mutex
> is unlocked?  if that is true then why?  it seems a waste of cpu.

mutex_lock(m) spins a few times calling mutex_try_lock(m), and then spins
alternating between mutex_try_lock(m) and cthread_yield().  mutex_try_lock(m)
is a test-and-set on the mutex.

Now to answer your real question.  No, it's not a waste of cpu to spin on a
mutex in a correctly written threads application.   A mutex provides mutual
exclusion between two threads.  It's intended to be called only when the
mutual exclusion is of a short or bounded duration.  It's not the right
primitive for unbounded waiting.  Any potentially unbounded waiting should be
done using condition_wait and condition_{signal,broadcast} (which don't spin).

If a thread dies, none of the memory resources which it logically owns will be
cleaned up.  That includes its locks.  If a thread dies, you have an incorrect
program anyway, and the fact that some locks are still held is the least of
your problems.

				-- Mike