Re: [PATCH v2 2/2] pid: Remove pidhash

Rik van Riel
2017-10-02T15:22:53Z

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On Mon, 2017-10-02 at 17:21 +0200, Oleg Nesterov wrote:
> Hi Rik,
>
> On 10/02, Rik van Riel wrote:
> >
> > Gargi and I are looking at that code, and trying to figure out
> > exactly what needs to be done to make all of this correct.
>
> see another email I sent to Gargi a minute ago,
>
> > 2) With pid_ns_prepare_proc out of the way, we can put all the code
> >    from below where the call to pid_ns_prepare_proc is now (except
> >    error handing) into the main loop of pid allocation, so we can
> >    do all that stuff under the pidmap_lock:
> >
> >    for (i = ns->level; i >= 0; i--) {
> >        ...
> >        idr_alloc_cyclic(...)
> >        get_pid_ns(ns);
> >        atomic_set(&pid->count, 1);
> >        for (...)
> >             INIT_HLIST_HEAD(...)
> >        ns->nr_allocated++;
> >        ...
> >   }
>
> I do not see how this can fix the problem with not-fully-initialized
> pid returned by find_pid_ns().
>
> As for PIDNS_ADDING/PIDNS_HASH_ADDING, _perhaps_ we can cleanup this
> logic
> a bit and do the check earlier, but imo this needs another/separate
> change.
>
> I'd suggest to keep the current logic and the order of initialization
> and
> just do
>
> for (i = ns->level; i >= 0; i--) {
> ...
>
> // do not expose the new pid to find_pid_ns() until it
> // is fully initialized
> nr = idr_alloc_cyclic(&tmp->idr, /*pid*/ NULL, ...);
> ...
> }
>
> ...
>
> spin_lock_irq(&pidmap_lock);
> if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
> goto out_unlock;
> for ( ; upid >= pid->numbers; --upid) {
> - hlist_add_head_rcu(&upid->pid_chain,
> - &pid_hash[pid_hashfn(upid->nr, upid-
> >ns)]);
> + // finally make it visible to find_pid_ns()
> + idr_replace(upid->ns-idr, pid, upid->nr);
> upid->ns->nr_hashed++;
> }
> spin_unlock_irq(&pidmap_lock);
>
> Or I missed something?

You are right, that would both fix the problem, and keep the error
paths relatively simple.

Gargi, what do you think?

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