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

[Luis R. Rodriguez]

On Wed, Jan 11, 2017 at 09:08:57PM +0100, Luis R. Rodriguez wrote:
> On Tue, Jan 10, 2017 at 07:57:10PM +0100, Luis R. Rodriguez wrote:
> > 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@xxxxxxxxxx> 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@xxxxxxxxxxxxx
> > > > 
> > > > 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.
> 
> Spoke too soon, kref_read() is not upstream yet either, so I can hold conversion
> over until Peter's work is merged. Peter please Cc me on those patches if possible
> :D

All the needed kref stuff is upstream now, however, kref is overkill for
kmod_concurrent given this is just a counter, it is not used to release
any object, and kref_put() requires such mechanism. The lightweight
refcount_t is much more appropriate here so will use that and respin
this series, finally.

  Luis