linux-kernel - Re: kmod: provide wrappers for kmod

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Date:   Wed, 21 Dec 2016 20:48:06 -0800
From:   Jessica Yu <jeyu@...hat.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Petr Mladek <pmladek@...e.com>, Kees Cook <keescook@...omium.org>,
        shuah@...nel.org, 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: kmod: provide wrappers for kmod_concurrent inc/dec

+++ Luis R. Rodriguez [16/12/16 09:05 +0100]:
>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:
>> > > > 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@...nel.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.
>
>This approach is *exactly* what the existing code does :P
>I just provided wrappers. I agree with the old approach though,
>reason is it acts as a lock in for the bump. 

I think what Petr meant was that we could run into false failures when multiple
atomic increments happen between the first increment and the subsequent
atomic_read.

Say max_modprobes is 64 -

       atomic_inc(&kmod_concurrent); // thread 1: kmod_concurrent is 63
            atomic_inc(&kmod_concurrent); // thread 2: kmod_concurrent is 64
                 atomic_inc(&kmod_concurrent); // thread 3: kmod_concurrent is 65
       if (atomic_read(&kmod_concurrent) < max_modprobes) // if all threads read 65 here, then all will error out
               return 0;                                  // when the first two should have succeeded (false failures)
       atomic_dec(&kmod_concurrent);
       return -ENOMEM;

But yeah, I think this issue was already in the existing kmod code..

Jessica