From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752578Ab2AQIlX (ORCPT ); Tue, 17 Jan 2012 03:41:23 -0500 Received: from fgwmail6.fujitsu.co.jp ([192.51.44.36]:45176 "EHLO fgwmail6.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752481Ab2AQIlV (ORCPT ); Tue, 17 Jan 2012 03:41:21 -0500 X-SecurityPolicyCheck-FJ: OK by FujitsuOutboundMailChecker v1.4.0 Date: Tue, 17 Jan 2012 17:40:31 +0900 From: Yasunori Goto To: Peter Zijlstra Subject: Re: [BUG] TASK_DEAD task is able to be woken up in special condition Cc: Oleg Nesterov , Ingo Molnar , Hiroyuki KAMEZAWA , Motohiro Kosaki , Linux Kernel ML In-Reply-To: <1326721082.2442.234.camel@twins> References: <20120116205140.6120.E1E9C6FF@jp.fujitsu.com> <1326721082.2442.234.camel@twins> Message-Id: <20120117174031.3118.E1E9C6FF@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Mailer: Becky! ver. 2.56.05 [ja] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Mon, 2012-01-16 at 20:51 +0900, Yasunori Goto wrote: > > > try_to_wake_up() has a problem which may change status from TASK_DEAD to > > TASK_RUNNING in race condition with SMI or guest environment of virtual > > machine. (See: https://lkml.org/lkml/2011/12/21/523) > > As a result, exited task is scheduled() again and panic occurs. > > > > By this patch, do_exit() waits for releasing task->pi_lock which is used > > in try_to_wake_up(). It guarantees the task becomes TASK_DEAD after > > waking up. > > > This Changelog isn't very good. Please spell out the problem instead of > referring to it so that people using git-blame and the like don't then > have to go look up some (possibly non-existent) web-resource. Ahh, Ok. I rewrote the description and comment in patch. Please check them. ----- try_to_wake_up() has a problem which may change status from TASK_DEAD to TASK_RUNNING in race condition with SMI or guest environment of virtual machine. As a result, exited task is scheduled() again and panic occurs. Here is the sequence how it occurs. ----------------------------------+----------------------------- | CPU A | CPU B ----------------------------------+----------------------------- TASK A calls exit().... do_exit() exit_mm() down_read(mm->mmap_sem); rwsem_down_failed_common() set TASK_UNINTERRUPTIBLE set waiter.task <= task A list_add to sem->wait_list : raw_spin_unlock_irq() (I/O interruption occured) __rwsem_do_wake(mmap_sem) list_del(&waiter->list); waiter->task = NULL wake_up_process(task A) try_to_wake_up() (task is still TASK_UNINTERRUPTIBLE) p->on_rq is still 1.) ttwu_do_wakeup() (*A) : (I/O interruption handler finished) if (!waiter.task) schedule() is not called due to waiter.task is NULL. tsk->state = TASK_RUNNING : check_preempt_curr(); : task->state = TASK_DEAD (*B) <--- set TASK_RUNNING (*C) schedule() (exit task is running again) BUG_ON() is called! -------------------------------------------------------- The execution time between (*A) and (*B) is usually very short, because the interruption is disabled, and setting TASK_RUNNING at (*C) must be executed before setting TASK_DEAD. HOWEVER, if SMI is interrupted between (*A) and (*B), (*C) is able to execute AFTER setting TASK_DEAD! Then, exited task is scheduled again, and BUG_ON() is called.... If the system works on guest system of virtual machine, the time between (*A) and (*B) may be also long due to scheduling of hypervisor, and same phenomenon can occur. By this patch, do_exit() waits for releasing task->pi_lock which is used in try_to_wake_up(). It guarantees the task becomes TASK_DEAD after waking up. Signed-off-by: Yasunori Goto --- kernel/exit.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) Index: linux-3.2/kernel/exit.c =================================================================== --- linux-3.2.orig/kernel/exit.c +++ linux-3.2/kernel/exit.c @@ -1038,6 +1038,22 @@ NORET_TYPE void do_exit(long code) preempt_disable(); exit_rcu(); + + /* + * The setting of TASK_RUNNING by try_to_wake_up() may be delayed + * when the following two conditions become true. + * - There is race condition of mmap_sem (It is acquired by + * exit_mm()), and + * - SMI occurs before setting TASK_RUNINNG. + * (or hypervisor of virtual machine switches to other guest) + * As a result, we may become TASK_RUNNING after becoming TASK_DEAD + * + * To avoid it, we have to wait for releasing tsk->pi_lock which + * is held by try_to_wake_up() + */ + smp_mb(); + raw_spin_unlock_wait(&tsk->pi_lock); + /* causes final put_task_struct in finish_task_switch(). */ tsk->state = TASK_DEAD; schedule(); ----- -- Yasunori Goto