Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753949Ab0AYQDu (ORCPT ); Mon, 25 Jan 2010 11:03:50 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753072Ab0AYQDt (ORCPT ); Mon, 25 Jan 2010 11:03:49 -0500 Received: from bombadil.infradead.org ([18.85.46.34]:39181 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753048Ab0AYQDs (ORCPT ); Mon, 25 Jan 2010 11:03:48 -0500 Subject: Re: Bisected rcu hang (kernel/sched.c): was 2.6.33rc4 RCU hang mm spin_lock deadlock(?) after running libvirtd - reproducible. From: Peter Zijlstra To: Michael Breuer Cc: paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org In-Reply-To: <4B5BB535.8040200@majjas.com> References: <4B49015D.9000903@majjas.com> <4B4A341B.6010800@majjas.com> <20100112014909.GB10869@linux.vnet.ibm.com> <4B4E1461.4010806@majjas.com> <4B5BB535.8040200@majjas.com> Content-Type: text/plain; charset="UTF-8" Date: Mon, 25 Jan 2010 17:03:40 +0100 Message-ID: <1264435420.4283.1927.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6449 Lines: 192 On Sat, 2010-01-23 at 21:49 -0500, Michael Breuer wrote: > Libvirtd always triggers the crash; other things that fork and use mmap > sometimes do (vsftpd, for example). I bet its the nscgroup trainwreck, does this fix it? --- commit fabf318e5e4bda0aca2b0d617b191884fda62703 Author: Peter Zijlstra Date: Thu Jan 21 21:04:57 2010 +0100 sched: Fix fork vs hotplug vs cpuset namespaces There are a number of issues: 1) TASK_WAKING vs cgroup_clone (cpusets) copy_process(): sched_fork() child->state = TASK_WAKING; /* waiting for wake_up_new_task() */ if (current->nsproxy != p->nsproxy) ns_cgroup_clone() cgroup_clone() mutex_lock(inode->i_mutex) mutex_lock(cgroup_mutex) cgroup_attach_task() ss->can_attach() ss->attach() [ -> cpuset_attach() ] cpuset_attach_task() set_cpus_allowed_ptr(); while (child->state == TASK_WAKING) cpu_relax(); will deadlock the system. 2) cgroup_clone (cpusets) vs copy_process So even if the above would work we still have: copy_process(): if (current->nsproxy != p->nsproxy) ns_cgroup_clone() cgroup_clone() mutex_lock(inode->i_mutex) mutex_lock(cgroup_mutex) cgroup_attach_task() ss->can_attach() ss->attach() [ -> cpuset_attach() ] cpuset_attach_task() set_cpus_allowed_ptr(); ... p->cpus_allowed = current->cpus_allowed over-writing the modified cpus_allowed. 3) fork() vs hotplug if we unplug the child's cpu after the sanity check when the child gets attached to the task_list but before wake_up_new_task() shit will meet with fan. Solve all these issues by moving fork cpu selection into wake_up_new_task(). Reported-by: Serge E. Hallyn Tested-by: Serge E. Hallyn Signed-off-by: Peter Zijlstra LKML-Reference: <1264106190.4283.1314.camel@laptop> Signed-off-by: Thomas Gleixner diff --git a/kernel/fork.c b/kernel/fork.c index 5b2959b..f88bd98 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1241,21 +1241,6 @@ static struct task_struct *copy_process(unsigned long clone_flags, /* Need tasklist lock for parent etc handling! */ write_lock_irq(&tasklist_lock); - /* - * The task hasn't been attached yet, so its cpus_allowed mask will - * not be changed, nor will its assigned CPU. - * - * The cpus_allowed mask of the parent may have changed after it was - * copied first time - so re-copy it here, then check the child's CPU - * to ensure it is on a valid CPU (and if not, just force it back to - * parent's CPU). This avoids alot of nasty races. - */ - p->cpus_allowed = current->cpus_allowed; - p->rt.nr_cpus_allowed = current->rt.nr_cpus_allowed; - if (unlikely(!cpu_isset(task_cpu(p), p->cpus_allowed) || - !cpu_online(task_cpu(p)))) - set_task_cpu(p, smp_processor_id()); - /* CLONE_PARENT re-uses the old parent */ if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) { p->real_parent = current->real_parent; diff --git a/kernel/sched.c b/kernel/sched.c index 4508fe7..3a8fb30 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -2320,14 +2320,12 @@ static int select_fallback_rq(int cpu, struct task_struct *p) } /* - * Called from: + * Gets called from 3 sites (exec, fork, wakeup), since it is called without + * holding rq->lock we need to ensure ->cpus_allowed is stable, this is done + * by: * - * - fork, @p is stable because it isn't on the tasklist yet - * - * - exec, @p is unstable, retry loop - * - * - wake-up, we serialize ->cpus_allowed against TASK_WAKING so - * we should be good. + * exec: is unstable, retry loop + * fork & wake-up: serialize ->cpus_allowed against TASK_WAKING */ static inline int select_task_rq(struct task_struct *p, int sd_flags, int wake_flags) @@ -2620,9 +2618,6 @@ void sched_fork(struct task_struct *p, int clone_flags) if (p->sched_class->task_fork) p->sched_class->task_fork(p); -#ifdef CONFIG_SMP - cpu = select_task_rq(p, SD_BALANCE_FORK, 0); -#endif set_task_cpu(p, cpu); #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT) @@ -2652,6 +2647,21 @@ void wake_up_new_task(struct task_struct *p, unsigned long clone_flags) { unsigned long flags; struct rq *rq; + int cpu = get_cpu(); + +#ifdef CONFIG_SMP + /* + * Fork balancing, do it here and not earlier because: + * - cpus_allowed can change in the fork path + * - any previously selected cpu might disappear through hotplug + * + * We still have TASK_WAKING but PF_STARTING is gone now, meaning + * ->cpus_allowed is stable, we have preemption disabled, meaning + * cpu_online_mask is stable. + */ + cpu = select_task_rq(p, SD_BALANCE_FORK, 0); + set_task_cpu(p, cpu); +#endif rq = task_rq_lock(p, &flags); BUG_ON(p->state != TASK_WAKING); @@ -2665,6 +2675,7 @@ void wake_up_new_task(struct task_struct *p, unsigned long clone_flags) p->sched_class->task_woken(rq, p); #endif task_rq_unlock(rq, &flags); + put_cpu(); } #ifdef CONFIG_PREEMPT_NOTIFIERS @@ -7139,14 +7150,18 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask) * the ->cpus_allowed mask from under waking tasks, which would be * possible when we change rq->lock in ttwu(), so synchronize against * TASK_WAKING to avoid that. + * + * Make an exception for freshly cloned tasks, since cpuset namespaces + * might move the task about, we have to validate the target in + * wake_up_new_task() anyway since the cpu might have gone away. */ again: - while (p->state == TASK_WAKING) + while (p->state == TASK_WAKING && !(p->flags & PF_STARTING)) cpu_relax(); rq = task_rq_lock(p, &flags); - if (p->state == TASK_WAKING) { + if (p->state == TASK_WAKING && !(p->flags & PF_STARTING)) { task_rq_unlock(rq, &flags); goto again; } -- 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/