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

Oleg Nesterov
2017-10-02T15:21:39Z

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?

Oleg.