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Date: Mon, 2 Oct 2017 17:21:35 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Rik van Riel <riel@surriel.com>
Cc: Gargi Sharma <gs051095@gmail.com>, linux-kernel@vger.kernel.org,
        julia.lawall@lip6.fr, akpm@linux-foundation.org, mingo@kernel.org,
        pasha.tatashin@oracle.com, ktkhai@virtuozzo.com
Subject: Re: [PATCH v2 2/2] pid: Remove pidhash
Message-ID: <20171002152135.GB9481@redhat.com>
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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.