Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751887Ab0BHNV5 (ORCPT ); Mon, 8 Feb 2010 08:21:57 -0500 Received: from mx1.redhat.com ([209.132.183.28]:9155 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751700Ab0BHNVz (ORCPT ); Mon, 8 Feb 2010 08:21:55 -0500 Date: Mon, 8 Feb 2010 14:21:01 +0100 From: Oleg Nesterov To: Tetsuo Handa Cc: tglx@linutronix.de, 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 Message-ID: <20100208132101.GA7129@redhat.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201002082130.JDC57339.OHOVJFQtFSLFMO@I-love.SAKURA.ne.jp> 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: 1423 Lines: 46 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. In any case, I think the patch is nice. 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/