Re: mutexes, locks and so on...

[Thor Lancelot Simon <tls%panix.com@localhost>]

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.

> 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