Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758547AbbBHSN3 (ORCPT ); Sun, 8 Feb 2015 13:13:29 -0500 Received: from mx1.redhat.com ([209.132.183.28]:35781 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756680AbbBHSN2 (ORCPT ); Sun, 8 Feb 2015 13:13:28 -0500 Date: Sun, 8 Feb 2015 19:12:08 +0100 From: Oleg Nesterov To: Ian Kent Cc: Jeff Layton , Kernel Mailing List , David Howells , Trond Myklebust , "J. Bruce Fields" , Benjamin Coddington , Al Viro , "Eric W. Biederman" Subject: Re: [RFC PATCH 3/8] kmod - teach call_usermodehelper() to use a namespace Message-ID: <20150208181208.GA19886@redhat.com> References: <20150205021553.8382.16297.stgit@pluto.fritz.box> <20150205023410.8382.13695.stgit@pluto.fritz.box> <20150206070859.7eb499b0@tlielax.poochiereds.net> <1423364852.2641.2.camel@pluto.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1423364852.2641.2.camel@pluto.fritz.box> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1617 Lines: 45 On 02/08, Ian Kent wrote: > > On Fri, 2015-02-06 at 07:08 -0500, Jeff Layton wrote: > > On Thu, 05 Feb 2015 10:34:11 +0800 > > Ian Kent wrote: > > > > > +{ > > > + struct task_struct *tsk; > > > + > > > + rcu_read_lock(); > > > + tsk = find_task_by_vpid(1); > > > + if (tsk) > > > + get_task_struct(tsk); > > > + rcu_read_unlock(); > > > > I'm not terribly familiar with the task_struct lifetime rules... > > > > I assume that you can be assured that tsk won't go away while you hold > > the rcu_read_lock, but is doing a get_task_struct while holding it > > sufficient to pin it after you drop the lock? > > > > IOW, could the refcount on the task_struct do a 0->1 transition here and > > end up being freed anyway after you've grabbed a reference? > > Good point, I thought getting a reference under he read lock would be > enough but maybe I need more checks as I do with dentrys. I'll check > that. This is fine. If find_task_by_vpid() succeeds then delayed_put_task_struct() can't be called until rcu_read_unlock() at least, so this task_struct has a reference. But I can't understand why do you need this helper... I guess I need to read the whole series first. find_task_by_vpid(1) can never fail, but it can be zombie... At seems we only need this task_struct for task_pid_nr(tsk) in umh_enter_ns(tsk) ? Confused, but please ignore. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/