Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754040Ab0BHRI2 (ORCPT ); Mon, 8 Feb 2010 12:08:28 -0500 Received: from www.tglx.de ([62.245.132.106]:32795 "EHLO www.tglx.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753948Ab0BHRIZ (ORCPT ); Mon, 8 Feb 2010 12:08:25 -0500 Date: Mon, 8 Feb 2010 18:07:47 +0100 (CET) From: Thomas Gleixner To: Oleg Nesterov cc: Tetsuo Handa , linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com, linux-security-module@vger.kernel.org Subject: Re: [PATCH] Update comment on find_task_by_pid_ns In-Reply-To: <20100208132101.GA7129@redhat.com> Message-ID: References: <20091210001308.247025548@linutronix.de> <20091210004703.029784964@linutronix.de> <20091210142036.GA8226@redhat.com> <200912110008.BEJ00511.FVOJLtSMOFHOQF@I-love.SAKURA.ne.jp> <201002082130.JDC57339.OHOVJFQtFSLFMO@I-love.SAKURA.ne.jp> <20100208132101.GA7129@redhat.com> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1612 Lines: 50 On Mon, 8 Feb 2010, Oleg Nesterov wrote: > On 02/08, Tetsuo Handa wrote: > > > > [PATCH] Update comment on find_task_by_pid_ns > > > > Caller of find_task_by_vpid() and find_task_by_pid_ns() needs to call > > rcu_read_lock() rather than read_lock(&tasklist_lock) because find_pid_ns() > > uses RCU primitives but spinlock does not prevent RCU callback if preemptive > > RCU ( CONFIG_TREE_PREEMPT_RCU ) is enabled. > > I agree with the patch, but the changelog looks a bit confusing to me. > Perhaps this is just me, though. > > tasklist does protect the task and its pid, it can't go away. The problem > is that find_pid_ns() itself is unsafe without rcu lock, it can race with > copy_process()->free_pid(any_pid). > > IOW, if we change copy_process() > > --- kernel/fork.c > +++ kernel/fork.c > @@ -1304,8 +1304,11 @@ static struct task_struct *copy_process( > return p; > > bad_fork_free_pid: > - if (pid != &init_struct_pid) > + if (pid != &init_struct_pid) { > + read_lock(&tasklist_lock); > free_pid(pid); > + read_unlock(&tasklist_lock); > + } > bad_fork_cleanup_io: > if (p->io_context) > exit_io_context(p); > > then find_task_by_pid_ns/etc could be used under tasklist safely even > with PREEMPT_RCU. We try to get rid of the read_lock sites of tasklist_lock, so please let's not think about adding more :) Thanks, tglx -- 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/