linux-kernel - Re: [PATCH v4 1/2] pid: Replace pid bitmap implementation with IDR API

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Date:   Tue, 10 Oct 2017 13:50:35 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Gargi Sharma <gs051095@...il.com>, linux-kernel@...r.kernel.org,
        riel@...riel.com, julia.lawall@...6.fr, mingo@...nel.org,
        pasha.tatashin@...cle.com, ktkhai@...tuozzo.com,
        ebiederm@...ssion.com, hch@...radead.org
Subject: Re: [PATCH v4 1/2] pid: Replace pid bitmap implementation with IDR
 API

On 10/09, Andrew Morton wrote:
>
> > @@ -240,17 +230,11 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
> >  	 *
> >  	 */
> >  	read_lock(&tasklist_lock);
> > -	nr = next_pidmap(pid_ns, 1);
> > -	while (nr > 0) {
> > -		rcu_read_lock();
> > -
> > -		task = pid_task(find_vpid(nr), PIDTYPE_PID);
> > +	nr = 2;
> > +	idr_for_each_entry_continue(&pid_ns->idr, pid, nr) {
> > +		task = pid_task(pid, PIDTYPE_PID);
> >  		if (task && !__fatal_signal_pending(task))
> >  			send_sig_info(SIGKILL, SEND_SIG_FORCED, task);
> > -
> > -		rcu_read_unlock();
> > -
> > -		nr = next_pidmap(pid_ns, nr);
> >  	}
> >  	read_unlock(&tasklist_lock);
> 
> Especially here.  I don't think pidmap_lock is held.  Is that IDR
> iteration safe?

Yes, this doesn't look right, we need rcu_read_lock() or pidmap_lock.

And, we also need rcu_read_lock() for another reason, to protect "struct pid".

Gargi, I suggested to use idr_for_each_entry_continue(), but now I am wondering
if we should use idr_for_each() instead. IIUC this would be a bit faster? Not
that I think this is really important...

Oleg.