From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751005AbXBTSfE (ORCPT ); Tue, 20 Feb 2007 13:35:04 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932103AbXBTSfE (ORCPT ); Tue, 20 Feb 2007 13:35:04 -0500 Received: from ogre.sisk.pl ([217.79.144.158]:33451 "EHLO ogre.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751005AbXBTSfB (ORCPT ); Tue, 20 Feb 2007 13:35:01 -0500 From: "Rafael J. Wysocki" To: Oleg Nesterov Subject: Re: freezer problems Date: Tue, 20 Feb 2007 19:29:01 +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> <20070220001209.GA15991@tv-sign.ru> <200702200132.12847.rjw@sisk.pl> In-Reply-To: <200702200132.12847.rjw@sisk.pl> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200702201929.03776.rjw@sisk.pl> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tuesday, 20 February 2007 01:32, Rafael J. Wysocki wrote: > On Tuesday, 20 February 2007 01:12, Oleg Nesterov wrote: > > On 02/20, Rafael J. Wysocki wrote: > > > > > > On Monday, 19 February 2007 23:41, Oleg Nesterov wrote: > > > > On 02/19, Rafael J. Wysocki wrote: > > > > > > > > > > On Monday, 19 February 2007 21:23, Oleg Nesterov wrote: > > > > > > > > > > > > @@ -199,6 +189,10 @@ static void thaw_tasks(int thaw_user_spa > > > > > > > > > > > > > > do_each_thread(g, p) { > > > > > > > + if (freezer_should_skip(p)) > > > > > > > + cancel_freezing(p); > > > > > > > + } while_each_thread(g, p); > > > > > > > + do_each_thread(g, p) { > > > > > > > if (!freezeable(p)) > > > > > > > continue; > > > > > > > > > > > > Any reason for 2 separate do_each_thread() loops ? > > > > > > > > > > Yes. If there is a "freeze" request pending for the vfork parent (TIF_FREEZE > > > > > set), we have to cancel it before the child is unfrozen, since otherwise the > > > > > parent may go freezing after we try to reset PF_FROZEN for it. > > > > > > > > I see, thanks... thaw_process() doesn't take TIF_FREEZE into account. > > > > > > > > But doesn't this mean we have a race? > > > > > > > > Suppose that try_to_freeze_tasks() failed. It does cancel_freezing() for each > > > > process before return, but what if some thread already checked TIF_FREEZE and > > > > (for simplicity) it is preempted before frozen_process() in refrigerator(). > > > > > > > > thaw_tasks() runs, ignores this task (P), returns. P gets CPU, and becomes > > > > frozen, but nobody will thaw it. > > > > > > > > No? > > > > > > Well, I think this is highly theoretical. Namely, try_to_freeze_tasks() only > > > fails after the timeout that's currently set to 20 sec., and it yields the CPU > > > in each iteration of the main loop. The task in question would have to refuse > > > being frozen for 20 sec. and then suddenly decide to freeze itself right before > > > try_to_freeze_tasks() checks the timeout for the very last time. Then, it > > > would have to get preempted at this very moment and stay unfrozen at least > > > until thaw_tasks() starts running and in fact even longer. > > > > Yes, yes, it is pure theroretical, > > > > > I think we may avoid this by making try_to_freeze_tasks() sleep for some time > > > after it has reset TIF_FREEZE for all tasks in the error path, if anyone is > > > ever able to trigger it. > > > > This makes this race (pure theroretical) ** 2 :) > > > > Still. May be it make sense to introduce cancel_freezing_and_thaw() function > > (not right now) which stops the task from sleeping in refrigirator reliably. > > Hm. In the case discussed above we have a task that's right before calling > frozen_process(), so we can't thaw it, because it's not frozen. It will be > frozen just in a while, but try_to_freeze_tasks() and thaw_tasks() have no > way to check this. > > I think to close this race the refrigerator should check TIF_FREEZE and set > PF_FROZEN _and_ reset TIF_FREEZE under a lock that would also have to be > taken by try_to_freeze_tasks() in the beginning of the error path. This will > ensure that all tasks either freeze themselves before the error path in > try_to_freeze_tasks() is executed, or remain unfrozen. > > I'll try to prepare a patch to illustrate this, but right now I'm too tired to > do it. :-) Something like this, perhaps: --- include/linux/freezer.h | 10 +++------- kernel/power/process.c | 18 ++++++++++++++++-- 2 files changed, 19 insertions(+), 9 deletions(-) Index: linux-2.6.20-mm2/include/linux/freezer.h =================================================================== --- linux-2.6.20-mm2.orig/include/linux/freezer.h +++ linux-2.6.20-mm2/include/linux/freezer.h @@ -58,17 +58,13 @@ static inline void frozen_process(struct clear_tsk_thread_flag(p, TIF_FREEZE); } -extern void refrigerator(void); +extern int refrigerator(void); extern int freeze_processes(void); extern void thaw_processes(void); static inline int try_to_freeze(void) { - if (freezing(current)) { - refrigerator(); - return 1; - } else - return 0; + return refrigerator(); } /* @@ -104,7 +100,7 @@ static inline void freeze(struct task_st static inline int thaw_process(struct task_struct *p) { return 1; } static inline void frozen_process(struct task_struct *p) { BUG(); } -static inline void refrigerator(void) {} +static inline int refrigerator(void) { return 0; } static inline int freeze_processes(void) { BUG(); return 0; } static inline void thaw_processes(void) {} 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 @@ -24,6 +24,8 @@ #define FREEZER_KERNEL_THREADS 0 #define FREEZER_USER_SPACE 1 +spinlock_t refrigerator_lock; + static inline int freezeable(struct task_struct * p) { if ((p == current) || @@ -34,15 +36,23 @@ static inline int freezeable(struct task } /* Refrigerator is place where frozen processes are stored :-). */ -void refrigerator(void) +int refrigerator(void) { /* Hmm, should we be allowed to suspend when there are realtime processes around? */ long save; + + spin_lock(&refrigerator_lock); + if (freezing(current)) { + frozen_process(current); + spin_unlock(&refrigerator_lock); + } else { + spin_unlock(&refrigerator_lock); + return 0; + } save = current->state; pr_debug("%s entered refrigerator\n", current->comm); - frozen_process(current); spin_lock_irq(¤t->sighand->siglock); recalc_sigpending(); /* We sent fake signal, clean it up */ spin_unlock_irq(¤t->sighand->siglock); @@ -53,6 +63,7 @@ void refrigerator(void) } pr_debug("%s left refrigerator\n", current->comm); current->state = save; + return 1; } static inline void freeze_process(struct task_struct *p) @@ -143,6 +154,7 @@ static unsigned int try_to_freeze_tasks( "kernel threads", TIMEOUT / HZ, todo); read_lock(&tasklist_lock); + spin_lock(&refrigerator_lock); do_each_thread(g, p) { if (is_user_space(p) == !freeze_user_space) continue; @@ -152,6 +164,7 @@ static unsigned int try_to_freeze_tasks( cancel_freezing(p); } while_each_thread(g, p); + spin_unlock(&refrigerator_lock); read_unlock(&tasklist_lock); } @@ -169,6 +182,7 @@ int freeze_processes(void) unsigned int nr_unfrozen; printk("Stopping tasks ... "); + spin_lock_init(&refrigerator_lock); nr_unfrozen = try_to_freeze_tasks(FREEZER_USER_SPACE); if (nr_unfrozen) return nr_unfrozen;