Re: [PATCH 1/2] pid: tighten pidmap spinlock critical section by removing kfree()

[Oleg Nesterov <oleg@redhat.com>]

On 11/23, Pekka Enberg wrote:
> (Adding some CC's.)
>
> On Sat, Nov 21, 2009 at 2:16 PM, André Goddard Rosa
> <andre.goddard@gmail.com> wrote:
> > Avoid calling kfree() under pidmap spinlock, calling it afterwards.
> >
> > Normally kfree() is very fast, but sometimes it can be slow, so avoid
> > calling it under the spinlock if we can.

kfree() is called when we race with another process which also
finds map->page == NULL, allocs the new page and takes pidmap_lock
before us. This is extremely unlikely case, right?

> > @@ -141,11 +141,12 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
> >                         * installing it:
> >                         */
> >                        spin_lock_irq(&pidmap_lock);
> > -                       if (map->page)
> > -                               kfree(page);
> > -                       else
> > +                       if (!map->page) {
> >                                map->page = page;
> > +                               page = NULL;
> > +                       }
> >                        spin_unlock_irq(&pidmap_lock);
> > +                       kfree(page);

And this change pessimizes (a little bit) the likely case, when
the race doesn't happen. And imho this change doesn't make the
code more readable.

But this is subjective, and technically the patch is correct
afaics.

> >                        if (unlikely(!map->page))
> >                         �

Hmm. Off-topic, but why alloc_pidmap() does not do this right
after kzalloc() ?

Oleg.