[PATCH 0/1] pidns: fix free_pid() to handle the first fork failure

[Oleg Nesterov <oleg@redhat.com>]

On 09/09, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@redhat.com> writes:
> >
> > Agreed, it also makes sense to clear ->nr_hashed. But I still think
> > that WARN_ON(ns->child_reaper) makes sense too.
>
> I don't know that I like warnings for impossible conditions.

But WARN_ON() should only check for "impossible" conditions ;)

> How could
> we even make a mistake that gets us there?

I do not know! I mean, this should not happen, that is why it adds
a warning.

And note that "ns->nr_hashed = 0" is not really needed, still I agree
it makes sense.

However I won't mind to remove this warning if you really dislike it.

> >> At which point I ask myself what of the pathlogocical case where the
> >> first fork fails but because we created the pid namespace with unshare
> >> there is a concurrent fork from another process into the pid namespace
> >> that succeeds.  Resulting in one pid in the pid namespace that is not
> >> the reaper.
> >
> > But how can setns() work before the first fork() succeeds and makes the
> > ->child_reaper visible in /proc ?
> >
> > Probably I missed something obvious, I didn't sleep today...
>
> Actually that is a very good point.  That is an accidental feature but
> one I very much appreciate today.

OK. Please review v2 then. I also shamelessly stole your comment.

Oleg.