From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750827AbXBRMUJ (ORCPT ); Sun, 18 Feb 2007 07:20:09 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750821AbXBRMUI (ORCPT ); Sun, 18 Feb 2007 07:20:08 -0500 Received: from ogre.sisk.pl ([217.79.144.158]:48510 "EHLO ogre.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750827AbXBRMUF (ORCPT ); Sun, 18 Feb 2007 07:20:05 -0500 From: "Rafael J. Wysocki" To: Oleg Nesterov Subject: Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug Date: Sun, 18 Feb 2007 13:12:21 +0100 User-Agent: KMail/1.9.5 Cc: ego@in.ibm.com, akpm@osdl.org, paulmck@us.ibm.com, mingo@elte.hu, vatsa@in.ibm.com, dipankar@in.ibm.com, venkatesh.pallipadi@intel.com, linux-kernel@vger.kernel.org, Pavel Machek References: <20070214144031.GA15257@in.ibm.com> <200702181132.41264.rjw@sisk.pl> <20070218113259.GB100@tv-sign.ru> In-Reply-To: <20070218113259.GB100@tv-sign.ru> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200702181312.24633.rjw@sisk.pl> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Sunday, 18 February 2007 12:32, Oleg Nesterov wrote: > On 02/18, Rafael J. Wysocki wrote: > > > > On Sunday, 18 February 2007 00:42, Oleg Nesterov wrote: > > > On 02/17, Rafael J. Wysocki wrote: > > > > > > > > On Saturday, 17 February 2007 22:34, Oleg Nesterov wrote: > > > > > > > > > > static inline int is_user_space(struct task_struct *p) > > > > > { > > > > > return p->mm && !(p->flags & PF_BORROWED_MM); > > > > > } > > > > > > > > > > This doesn't look right. First, an exiting task has ->mm == NULL after > > > > > do_exit()->exit_mm(). Probably not a problem. However, PF_BORROWED_MM > > > > > check is racy without task_lock(), so we can have a false positive as > > > > > well. Is it ok? We can freeze aio_wq prematurely. > > > > > > > > Right now aio_wq is not freezeable (PF_NOFREEZE). > > > > > > Right now yes, but we are going to change this? > > > > Well, is there any more reliable (and not racy) method of differentiating > > between kernel threads and user space processes? > > Not that I know of. At least, we can take task_lock() to really rule out > kernel threads at FREEZER_USER_SPACE stage. Something like this? --- kernel/power/process.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) Index: linux-2.6.20-mm2/kernel/power/process.c =================================================================== --- linux-2.6.20-mm2.orig/kernel/power/process.c +++ linux-2.6.20-mm2/kernel/power/process.c @@ -8,6 +8,7 @@ #undef DEBUG +#include #include #include #include @@ -92,7 +93,12 @@ static void cancel_freezing(struct task_ static inline int is_user_space(struct task_struct *p) { - return p->mm && !(p->flags & PF_BORROWED_MM); + int ret; + + task_lock(p); + ret = p->mm && !(p->flags & PF_BORROWED_MM); + task_unlock(p); + return ret; } static unsigned int try_to_freeze_tasks(int freeze_user_space)