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Re: kmod: provide wrappers for kmod_concurrent inc/dec

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+++ 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@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.
>
>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

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Re: [RFC 04/10] kmod: provide wrappers for kmod_concurrent inc/dec Petr Mladek <pmladek@suse.com> - 2016-12-15 13:50 +0100
  Re: [RFC 04/10] kmod: provide wrappers for kmod_concurrent inc/dec "Luis R. Rodriguez" <mcgrof@kernel.org> - 2016-12-16 09:10 +0100
    Re: kmod: provide wrappers for kmod_concurrent inc/dec Jessica Yu <jeyu@redhat.com> - 2016-12-22 05:50 +0100

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