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

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Date:   Mon, 02 Oct 2017 09:35:13 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Oleg Nesterov <oleg@...hat.com>, Gargi Sharma <gs051095@...il.com>
Cc:     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 Wed, 2017-09-27 at 17:45 +0200, Oleg Nesterov wrote:
> On 09/27, Gargi Sharma wrote:
> > 
> > -#define find_next_offset(map, off)					
> > \
> > -		find_next_zero_bit((map)->page, BITS_PER_PAGE,
> > off)
> > -
> 
> this should go into the previous patch, but this is minor...
> 
> > @@ -208,12 +200,10 @@ struct pid *alloc_pid(struct pid_namespace
> > *ns)
> >  
> >  	upid = pid->numbers + ns->level;
> >  	spin_lock_irq(&pidmap_lock);
> > -	if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
> > +	if (!(ns->pid_allocated & PIDNS_ADDING))
> >  		goto out_unlock;
> >  	for ( ; upid >= pid->numbers; --upid) {
> > -		hlist_add_head_rcu(&upid->pid_chain,
> > -				&pid_hash[pid_hashfn(upid->nr,
> > upid->ns)]);
> > -		upid->ns->nr_hashed++;
> > +		upid->ns->pid_allocated++;
> 
> No, this is wrong.
> 
> It is too late to check PIDNS_HASH_ADDING/PIDNS_ADDING and increment
> pid_allocated,
> once we call idr_alloc_cyclic() this pid is already "hashed" in that
> it can be found
> by find_pid_ns() with this patch applied.
> 
> And of course, it is too late to do atomic_set(&pid->count, 1) and
> initialize
> pid->tasks[type] lists by the same reason.

Hi Oleg,

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.

We are thinking something along these lines:

1) First, check if this is a new namespace (PIDNS_ADDING), and
   do the call to pid_ns_prepare_proc, before we even call idr_alloc.
   Maybe something like:

   if (unlikely(ns->nr_allocated == PIDNS_ADDING)) {
      if (pid_ns_prepare_proc(ns)) {
         disable_pid_allocations(ns);
         goto out_free_ns;
   }

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++;
       ...
  }

Would that resolve your objection, or are we barking up the wrong tree?

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