Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757124Ab2FZNss (ORCPT ); Tue, 26 Jun 2012 09:48:48 -0400 Received: from merlin.infradead.org ([205.233.59.134]:41844 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756781Ab2FZNsr convert rfc822-to-8bit (ORCPT ); Tue, 26 Jun 2012 09:48:47 -0400 Message-ID: <1340718515.21991.83.camel@twins> Subject: Re: [RFC][PATCH] sched: Fix race in task_group() From: Peter Zijlstra To: Stefan Bader Cc: mingo@kernel.org, Oleg Nesterov , Paul Turner , Mike Galbraith , Andrew Vagin , linux-kernel , Tejun Heo Date: Tue, 26 Jun 2012 15:48:35 +0200 In-Reply-To: <4FE48A09.7050305@canonical.com> References: <1340364965.18025.71.camel@twins> <4FE48A09.7050305@canonical.com> Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-Mailer: Evolution 3.2.2- Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5443 Lines: 152 Here's one that's actually compile tested (with the right CONFIG_foo enabled) and I fixed the autogroup lockdep splat. --- Subject: sched: Fix race in task_group() From: Peter Zijlstra Date: Fri, 22 Jun 2012 13:36:05 +0200 Stefan reported a crash on a kernel before a3e5d1091c1 ("sched: Don't call task_group() too many times in set_task_rq()"), he found the reason to be that the multiple task_group() invocations in set_task_rq() returned different values. Looking at all that I found a lack of serialization and plain wrong comments. The below tries to fix it using an extra pointer which is updated under the appropriate scheduler locks. Its not pretty, but I can't really see another way given how all the cgroup stuff works. Reported-by: Stefan Bader Signed-off-by: Peter Zijlstra --- include/linux/init_task.h | 12 +++++++++++- include/linux/sched.h | 5 ++++- kernel/sched/core.c | 9 ++++++++- kernel/sched/sched.h | 23 ++++++++++------------- 4 files changed, 33 insertions(+), 16 deletions(-) --- a/include/linux/init_task.h +++ b/include/linux/init_task.h @@ -123,8 +123,17 @@ extern struct group_info init_groups; extern struct cred init_cred; +extern struct task_group root_task_group; + +#ifdef CONFIG_CGROUP_SCHED +# define INIT_CGROUP_SCHED(tsk) \ + .sched_task_group = &root_task_group, +#else +# define INIT_CGROUP_SCHED(tsk) +#endif + #ifdef CONFIG_PERF_EVENTS -# define INIT_PERF_EVENTS(tsk) \ +# define INIT_PERF_EVENTS(tsk) \ .perf_event_mutex = \ __MUTEX_INITIALIZER(tsk.perf_event_mutex), \ .perf_event_list = LIST_HEAD_INIT(tsk.perf_event_list), @@ -168,6 +177,7 @@ extern struct cred init_cred; }, \ .tasks = LIST_HEAD_INIT(tsk.tasks), \ INIT_PUSHABLE_TASKS(tsk) \ + INIT_CGROUP_SCHED(tsk) \ .ptraced = LIST_HEAD_INIT(tsk.ptraced), \ .ptrace_entry = LIST_HEAD_INIT(tsk.ptrace_entry), \ .real_parent = &tsk, \ --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1246,6 +1246,9 @@ struct task_struct { const struct sched_class *sched_class; struct sched_entity se; struct sched_rt_entity rt; +#ifdef CONFIG_CGROUP_SCHED + struct task_group *sched_task_group; +#endif #ifdef CONFIG_NUMA unsigned long numa_contrib; @@ -2749,7 +2752,7 @@ extern int sched_group_set_rt_period(str extern long sched_group_rt_period(struct task_group *tg); extern int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk); #endif -#endif +#endif /* CONFIG_CGROUP_SCHED */ extern int task_can_switch_user(struct user_struct *up, struct task_struct *tsk); --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1096,7 +1096,7 @@ void set_task_cpu(struct task_struct *p, * a task's CPU. ->pi_lock for waking tasks, rq->lock for runnable tasks. * * sched_move_task() holds both and thus holding either pins the cgroup, - * see set_task_rq(). + * see task_group(). * * Furthermore, all task_rq users should acquire both locks, see * task_rq_lock(). @@ -7712,6 +7712,7 @@ void sched_destroy_group(struct task_gro */ void sched_move_task(struct task_struct *tsk) { + struct task_group *tg; int on_rq, running; unsigned long flags; struct rq *rq; @@ -7726,6 +7727,12 @@ void sched_move_task(struct task_struct if (unlikely(running)) tsk->sched_class->put_prev_task(rq, tsk); + tg = container_of(task_subsys_state_check(tsk, cpu_cgroup_subsys_id, + lockdep_is_held(&tsk->sighand->siglock)), + struct task_group, css); + tg = autogroup_task_group(tsk, tg); + tsk->sched_task_group = tg; + #ifdef CONFIG_FAIR_GROUP_SCHED if (tsk->sched_class->task_move_group) tsk->sched_class->task_move_group(tsk, on_rq); --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -554,22 +554,19 @@ extern int group_balance_cpu(struct sche /* * Return the group to which this tasks belongs. * - * We use task_subsys_state_check() and extend the RCU verification with - * pi->lock and rq->lock because cpu_cgroup_attach() holds those locks for each - * task it moves into the cgroup. Therefore by holding either of those locks, - * we pin the task to the current cgroup. + * We cannot use task_subsys_state() and friends because the cgroup + * subsystem changes that value before the cgroup_subsys::attach() method + * is called, therefore we cannot pin it and might observe the wrong value. + * + * The same is true for autogroup's p->signal->autogroup->tg, the autogroup + * core changes this before calling sched_move_task(). + * + * Instead we use a 'copy' which is updated from sched_move_task() while + * holding both task_struct::pi_lock and rq::lock. */ static inline struct task_group *task_group(struct task_struct *p) { - struct task_group *tg; - struct cgroup_subsys_state *css; - - css = task_subsys_state_check(p, cpu_cgroup_subsys_id, - lockdep_is_held(&p->pi_lock) || - lockdep_is_held(&task_rq(p)->lock)); - tg = container_of(css, struct task_group, css); - - return autogroup_task_group(p, tg); + return p->sched_task_group; } /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */ -- 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/