From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756151AbZGIVca (ORCPT ); Thu, 9 Jul 2009 17:32:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756295AbZGIVcO (ORCPT ); Thu, 9 Jul 2009 17:32:14 -0400 Received: from mx2.redhat.com ([66.187.237.31]:53519 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756076AbZGIVcN (ORCPT ); Thu, 9 Jul 2009 17:32:13 -0400 Date: Thu, 9 Jul 2009 23:28:49 +0200 From: Oleg Nesterov To: Andrew Morton Cc: David Howells , James Morris , "Paul E. McKenney" , Roland McGrath , Stephen Smalley , linux-kernel@vger.kernel.org Subject: [PATCH -mm 2/2] is_current_single_threaded: don't use ->mmap_sem Message-ID: <20090709212849.GA10245@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 is_current_single_threaded() can safely miss a freshly forked CLONE_VM task, but in this case it must not miss its parent. That is why we take mm->mmap_sem for writing to make sure a thread/task with the same ->mm can't pass exit_mm() and disappear. However we can avoid ->mmap_sem and rely on rcu/barriers: - if we do not see the exiting parent on thread/process list we see the result of list_del_rcu(), in this case we must also see the result of list_add_rcu() which does wmb(). - if we do see the parent but its ->mm == NULL, we need rmb() to make sure we can't miss the child. Signed-off-by: Oleg Nesterov --- lib/is_single_threaded.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) --- WAIT/lib/is_single_threaded.c~ISS_2_RCU 2009-07-09 22:16:29.000000000 +0200 +++ WAIT/lib/is_single_threaded.c 2009-07-09 22:54:41.000000000 +0200 @@ -22,8 +22,6 @@ bool is_current_single_threaded(void) struct task_struct *p, *t; bool ret; - might_sleep(); - if (atomic_read(&task->signal->live) != 1) return false; @@ -31,7 +29,6 @@ bool is_current_single_threaded(void) return true; ret = false; - down_write(&mm->mmap_sem); rcu_read_lock(); for_each_process(p) { if (unlikely(p->flags & PF_KTHREAD)) @@ -45,12 +42,17 @@ bool is_current_single_threaded(void) goto found; if (likely(t->mm)) break; + /* + * t->mm == NULL. Make sure next_thread/next_task + * will see other CLONE_VM tasks which might be + * forked before exiting. + */ + smp_rmb(); } while_each_thread(p, t); } ret = true; found: rcu_read_unlock(); - up_write(&mm->mmap_sem); return ret; }