Re: mutexes, locks and so on...

[Andrew Doran <ad%homeworld.netbsd.org@localhost>]

On Wed, Nov 17, 2010 at 07:26:04AM -0500, Thor Lancelot Simon wrote:

> On Wed, Nov 17, 2010 at 12:30:59PM +0100, Johnny Billquist wrote:
> >
> > There are obviously a lot of calls to mutex_enter and mutex_exit with  
> > spin mutexes...
> 
> Indeed there are.  I don't underestand what Matt said about this.  When
> I was converting /dev/crypto to mutexes I am pretty sure it was explained
> to me that the _spin forms of the calls were simply optimizations for when
> the programmer knew the mutex to be a spin mutex due to the IPL at which
> it was declared.  That is, I had a a pile of driver code full of mutex_enter
> and mutex_exit which were on a high-IPL mutex and I simply converted it all
> to mutex_spin_enter and mutex_spin_exit with qsubst.
> 
> Perhaps the rule is that if you take the mutex with spin_enter you must
> release it with spin_exit.  That would make some sense.  But I'd love some
> clarity on this point.

You can release it with either call, mutex_spin_ is just a way to avoid
additional atomic operations.  The ususal case is adaptive mutex, but
stuff like the dispatcher/scheduler makes use of spin mutexes exclusively
and the fast path versions were invented for that. (Because you can measure
the effect with benchmarks :-).

Is Solaris -- the API we copied -- part of the motivation for allowing
one call to lock/unlock both types of mutex must have been to hide the
structure of the machine.  But for us we hide the machine at a different
level and having say mutex_enter handle both types of lock is really a
convenience thing so locks can be passed about by pointer.

> > Also a fair number of calls to mutex_spin_exit with the spin lock not  
> > appearing to be held (I actually set and clear a bit to indicate if the  
> > mutex is held, in addition to bumping the IPL).
> 
> This *does* seem like a bug.
> 
> -- 
>   Thor Lancelot Simon                                      
> tls%rek.tjls.com@localhost
> 
>   "We cannot usually in social life pursue a single value or a single moral
>    aim, untroubled by the need to compromise with others."      - H.L.A. Hart