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Re: mutexes, locks and so on...
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
> "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
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