linux-kernel - Re: [PATCH v2 2/2] pid: Remove pidhash

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Date:   Mon, 02 Oct 2017 11:22:44 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Gargi Sharma <gs051095@...il.com>, linux-kernel@...r.kernel.org,
        julia.lawall@...6.fr, akpm@...ux-foundation.org, mingo@...nel.org,
        pasha.tatashin@...cle.com, ktkhai@...tuozzo.com
Subject: Re: [PATCH v2 2/2] pid: Remove pidhash

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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