Re: [RFC 04/10] kmod: provide wrappers for kmod_concurrent inc/dec

["Luis R. Rodriguez" <mcgrof@kernel.org>]

On Fri, Dec 16, 2016 at 09:05:00AM +0100, Luis R. Rodriguez wrote:
> On Thu, Dec 15, 2016 at 01:46:25PM +0100, Petr Mladek wrote:
> > On Thu 2016-12-08 22:08:59, Luis R. Rodriguez wrote:
> > > On Thu, Dec 08, 2016 at 12:29:42PM -0800, Kees Cook wrote:
> > > > On Thu, Dec 8, 2016 at 11:48 AM, Luis R. Rodriguez <mcgrof@kernel.org> wrote:
> > > > > +       if (atomic_read(&kmod_concurrent) < max_modprobes)
> > > > > +               return 0;
> > > > > +       atomic_dec(&kmod_concurrent);
> > > > > +       return -ENOMEM;
> > > > > +}
> > > > > +
> > > > > +static void kmod_umh_threads_put(void)
> > > > > +{
> > > > > +       atomic_dec(&kmod_concurrent);
> > > > > +}
> > > > 
> > > > Can you use a kref here instead? We're trying to kill raw use of
> > > > atomic_t for reference counting...
> > > 
> > > That's a much broader functional change than I was looking for, but I am up for
> > > it. Can you describe the benefit of using kref you expect or why this is an
> > > ongoing crusade? Since its a larger functional change how about doing this
> > > change later, and we can test impact with the tress test driver. In theory if
> > > there are benefits can't we add a test case to prove the gains?
> > 
> > Kees probably refers to the kref improvements that Peter Zijlstra
> > is working on, see
> > https://lkml.kernel.org/r/20161114174446.832175072@infradead.org
> > 
> > The advantage is that the new refcount API handles over and
> > underflow.
> > 
> > Another advantage is that it increments/decrements the value
> > only when it is safe. It uses cmpxchg to make sure that
> > the checks are valid.
> 
> Great thanks, will look into that.

OK I've done the conversion now, the only thing is linux-next as of today lacks
KREF_INIT() so I've open coded it for now. Once Peter's changes get merged the
only thing we'dneed is to change the open code line to KREF_INIT().

I'll annotate this as Suggested-by Kees and Petr, I did this as a separate atomic
step after this to make it easier for review.

  Luis