Re: Modules loading modules?

[Paul Goyette <paul%whooppee.com@localhost>]

On Mon, 2 Aug 2010, Quentin Garnier wrote:

> > +int
> > +rmutex_tryenter(rmutex_t *rmtx)
> > +{
> > +       int rv = 1;
> > +
> > +       if (mutex_owned(&rmtx->rmtx_mtx)) {
> > +               rmtx->rmtx_recurse++;
> > +               KASSERT(rmtx->rmtx_recurse != 0);
> > +       } else if ((rv = mutex_tryenter(&rmtx->rmtx_mtx)) != 0) {
> > +               rmtx->rmtx_recurse++;
> > +               KASSERT(rmtx->rmtx_recurse != 0);
> > +       }
> > +       return rv;
> > +}
>
> I am probably not getting the idea, but I fail to see how this qualifies
> as a mutex.  My reading of this piece of code is that rmutex_enter() is
> always going to succeed immediately.

According to the mutex(9) man page:

    mutex_owned(mtx)

       For adaptive mutexes, return non-zero if the current LWP holds
       the mutex. ...

The rmutex_init() code ensures that we have an adaptive mutex.  So what 
this says is

        If _we_ already own the mutex,
                increment counter
                (rv is still initialized to 1)
        Else
        if we can acquire the mutex (updating rv)
                increment the counter

        Return


> Please correct me if I'm wrong--and I am very probably wrong--, but
> isn't the point of a recursive mutex to allow the thread initially
> taking the mutex to take it again, but not other threads?  The code
> would have to be a lot more complicated than that.
>
> All I see here is a fancy no-op, but maybe it's just me.

It would be a no-op for a recursive spin mutex.  That's why I didn't
allow them!  :)


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