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

[Petr Mladek <pmladek@suse.com>]

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:
> > > kmod_concurrent is used as an atomic counter for enabling
> > > the allowed limit of modprobe calls, provide wrappers for it
> > > to enable this to be expanded on more easily. This will be done
> > > later.
> > >
> > > Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
> > > ---
> > >  kernel/kmod.c | 27 +++++++++++++++++++++------
> > >  1 file changed, 21 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/kernel/kmod.c b/kernel/kmod.c
> > > index cb6f7ca7b8a5..049d7eabda38 100644
> > > --- a/kernel/kmod.c
> > > +++ b/kernel/kmod.c
> > > @@ -108,6 +111,20 @@ static int call_modprobe(char *module_name, int wait)
> > >         return -ENOMEM;
> > >  }
> > >
> > > +static int kmod_umh_threads_get(void)
> > > +{
> > > +       atomic_inc(&kmod_concurrent);

This approach might actually cause false failures. If we
are on the limit and more processes do this increment
in parallel, it makes the number bigger that it should be.

> > > +       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.

Best Regards,
Petr