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:
> > > 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@xxxxxxxxxx>
> > > ---
> > > 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@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.
Best Regards,
Petr
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