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

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Date:   Wed, 27 Sep 2017 18:28:08 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Gargi Sharma <gs051095@...il.com>
Cc:     linux-kernel@...r.kernel.org, riel@...riel.com,
        julia.lawall@...6.fr, akpm@...ux-foundation.org, mingo@...nel.org,
        pasha.tatashin@...cle.com, ktkhai@...tuozzo.com,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [PATCH v2 2/2] pid: Remove pidhash

If I was not clear...

in short, after this patch the very first idr_alloc_cyclic() is already
wrong. Because, once again, the new not-fully-initialized pid can be found
by find_pid_ns().

perhaps you should chane the previous patch to do
idr_alloc_cyclic(ptr = NULL) and use idr_replace() in this patch after
the PIDNS_HASH_ADDING check.

And I just noticed you didn't cc Eric Biederman <ebiederm@...ssion.com>,
please do next time.


On 09/27, 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.
> 
> Oleg.