linux-kernel - Re: [RFC 04/10] kmod: provide wrappers for kmod

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Date:   Tue, 10 Jan 2017 19:57:10 +0100
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Petr Mladek <pmladek@...e.com>, Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     mcgrof@...nel.org, shuah@...nel.org, Jessica Yu <jeyu@...hat.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jonathan Corbet <corbet@....net>, martin.wilck@...e.com,
        Michal Marek <mmarek@...e.com>, hare@...e.com, rwright@....com,
        Jeff Mahoney <jeffm@...e.com>, DSterba@...e.com,
        fdmanana@...e.com, neilb@...e.com,
        Guenter Roeck <linux@...ck-us.net>, rgoldwyn@...e.com,
        subashab@...eaurora.org, Heinrich Schuchardt <xypron.glpk@....de>,
        Aaron Tomlin <atomlin@...hat.com>, mbenes@...e.cz,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kselftest@...r.kernel.org,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 04/10] kmod: provide wrappers for kmod_concurrent inc/dec

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@...nel.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