On Monday 12 March 2007 19:27, Julian Elischer wrote:
> John Baldwin wrote:
> > On Monday 12 March 2007 16:03, Pawel Jakub Dawidek wrote:
> >> On Mon, Mar 12, 2007 at 03:35:21PM -0400, John Baldwin wrote:
> >>> On Monday 12 March 2007 14:56, Pawel Jakub Dawidek wrote:
> >>>> On Mon, Mar 12, 2007 at 11:16:23AM -0400, John Baldwin wrote:
> >>>>> On Saturday 10 March 2007 15:52, Pawel Jakub Dawidek wrote:
> >>>>>> What about something like this:
> >>>>>>
> >>>>>> #define cv_wait(cv, lock) do {
> >>>>>> switch (LO_CLASSINDEX((struct lock_object *)(lock))) {
> >>>>> The problem with a cast is you use type checking. Might as well do
> > this:
> >>>>> #define cv_wait(cv, lock) _cv_wait((cv), (struct lock_object
> >>>>> *)(lock))
> >>>> This will skip type checking and my version only cast to provide type
> >>>> checking, so when you pass some random variable it will give you an
> >>>> error.
> >>> Not really, you may pass some garbage and the LO_CLASSINDEX turns out to
> > be a
> >>> mutex. :) You only get a runtime error, not a compile-time one.
> >>> Type-checking by the compiler is nice because you get compile-time
errors.
> >> I'll get compile-time error, because cv_wait_mtx() takes
> >> 'struct condvar *' and 'struct mtx *' as arguments. So even if some
> >> garbage returns 1, which turns out to be a mutex, call to cv_wait_mtx()
> >> will generate compile-time error.
> >
> > Err, no, actually, yours will always give compile errors actually. Keep
in
> > mind that LO_CLASSINDEX() is a run-time check. This:
> >
> > #define cv_wait(cv, lock) do {
> > switch (LO_CLASSINDEX((struct lock_object *)(lock))) {
> > case 1:
> > cv_wait_mtx(cv, lock);
> > break;
> > case 2:
> > cv_wait_sx(cv, lock);
> > break;
> > case 3:
> > cv_wait_rw(cv, lock);
> > break;
> > default:
> > panic("Invalid lock.");
> > }
> > } while (0)
> >
> > Will try to pass 'lock' to three different functions, at least 2 of which
will
> > trigger compile errors. :) The kernel won't choose which one to run until
> > runtime though. The key is that I want a compile error, not a panic(). :)
>
> I've been asking for awhile that for example spin and sleep mutexes should
> be different types so that we could catch those problems at compile time.
That is on my todo list actually. Stephan and I talked at BSDCan 06 about
various alternative strategies for spin locks.
--
John Baldwin
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