Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754886AbaKDQLG (ORCPT ); Tue, 4 Nov 2014 11:11:06 -0500 Received: from terminus.zytor.com ([198.137.202.10]:59475 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754034AbaKDQLD (ORCPT ); Tue, 4 Nov 2014 11:11:03 -0500 Date: Tue, 4 Nov 2014 08:09:54 -0800 From: tip-bot for Kirill Tkhai Message-ID: Cc: ktkhai@parallels.com, hpa@zytor.com, peterz@infradead.org, tglx@linutronix.de, mingo@kernel.org, linux-kernel@vger.kernel.org, juri.lelli@arm.com, torvalds@linux-foundation.org Reply-To: linux-kernel@vger.kernel.org, mingo@kernel.org, torvalds@linux-foundation.org, juri.lelli@arm.com, peterz@infradead.org, tglx@linutronix.de, hpa@zytor.com, ktkhai@parallels.com In-Reply-To: <1414420852.19914.186.camel@tkhai> References: <1414420852.19914.186.camel@tkhai> To: linux-tip-commits@vger.kernel.org Subject: [tip:sched/core] sched/deadline: Implement cancel_dl_timer() to use in switched_from_dl() Git-Commit-ID: 67dfa1b756f250972bde31d65e3f8fde6aeddc5b X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit-ID: 67dfa1b756f250972bde31d65e3f8fde6aeddc5b Gitweb: http://git.kernel.org/tip/67dfa1b756f250972bde31d65e3f8fde6aeddc5b Author: Kirill Tkhai AuthorDate: Mon, 27 Oct 2014 17:40:52 +0300 Committer: Ingo Molnar CommitDate: Tue, 4 Nov 2014 07:17:50 +0100 sched/deadline: Implement cancel_dl_timer() to use in switched_from_dl() Currently used hrtimer_try_to_cancel() is racy: raw_spin_lock(&rq->lock) ... dl_task_timer raw_spin_lock(&rq->lock) ... raw_spin_lock(&rq->lock) ... switched_from_dl() ... ... hrtimer_try_to_cancel() ... ... switched_to_fair() ... ... ... ... ... ... ... ... raw_spin_unlock(&rq->lock) ... (asquired) ... ... ... ... ... ... do_exit() ... ... schedule() ... ... raw_spin_lock(&rq->lock) ... raw_spin_unlock(&rq->lock) ... ... ... raw_spin_unlock(&rq->lock) ... raw_spin_lock(&rq->lock) ... ... (asquired) put_task_struct() ... ... free_task_struct() ... ... ... ... raw_spin_unlock(&rq->lock) ... (asquired) ... ... ... ... ... (use after free) ... So, let's implement 100% guaranteed way to cancel the timer and let's be sure we are safe even in very unlikely situations. rq unlocking does not limit the area of switched_from_dl() use, because this has already been possible in pull_dl_task() below. Let's consider the safety of of this unlocking. New code in the patch is working when hrtimer_try_to_cancel() fails. This means the callback is running. In this case hrtimer_cancel() is just waiting till the callback is finished. Two 1) Since we are in switched_from_dl(), new class is not dl_sched_class and new prio is not less MAX_DL_PRIO. So, the callback returns early; it's right after !dl_task() check. After that hrtimer_cancel() returns back too. The above is: raw_spin_lock(rq->lock); ... ... dl_task_timer() ... raw_spin_lock(rq->lock); switched_from_dl() ... hrtimer_try_to_cancel() ... raw_spin_unlock(rq->lock); ... hrtimer_cancel() ... ... raw_spin_unlock(rq->lock); ... return HRTIMER_NORESTART; ... ... raw_spin_lock(rq->lock); ... 2) But the below is also possible: dl_task_timer() raw_spin_lock(rq->lock); ... raw_spin_unlock(rq->lock); raw_spin_lock(rq->lock); ... switched_from_dl() ... hrtimer_try_to_cancel() ... ... return HRTIMER_NORESTART; raw_spin_unlock(rq->lock); ... hrtimer_cancel(); ... raw_spin_lock(rq->lock); ... In this case hrtimer_cancel() returns immediately. Very unlikely case, just to mention. Nobody can manipulate the task, because check_class_changed() is always called with pi_lock locked. Nobody can force the task to participate in (concurrent) priority inheritance schemes (the same reason). All concurrent task operations require pi_lock, which is held by us. No deadlocks with dl_task_timer() are possible, because it returns right after !dl_task() check (it does nothing). If we receive a new dl_task during the time of unlocked rq, we just don't have to do pull_dl_task() in switched_from_dl() further. Signed-off-by: Kirill Tkhai [ Added comments] Signed-off-by: Peter Zijlstra (Intel) Acked-by: Juri Lelli Cc: Linus Torvalds Link: http://lkml.kernel.org/r/1414420852.19914.186.camel@tkhai Signed-off-by: Ingo Molnar --- kernel/sched/core.c | 4 ++++ kernel/sched/deadline.c | 34 +++++++++++++++++++++++++++------- kernel/sched/sched.h | 5 +++++ 3 files changed, 36 insertions(+), 7 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 0cd34e6..379cb87 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1008,6 +1008,9 @@ inline int task_curr(const struct task_struct *p) return cpu_curr(task_cpu(p)) == p; } +/* + * Can drop rq->lock because from sched_class::switched_from() methods drop it. + */ static inline void check_class_changed(struct rq *rq, struct task_struct *p, const struct sched_class *prev_class, int oldprio) @@ -1015,6 +1018,7 @@ static inline void check_class_changed(struct rq *rq, struct task_struct *p, if (prev_class != p->sched_class) { if (prev_class->switched_from) prev_class->switched_from(rq, p); + /* Possble rq->lock 'hole'. */ p->sched_class->switched_to(rq, p); } else if (oldprio != p->prio || dl_task(p)) p->sched_class->prio_changed(rq, p, oldprio); diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index 2e31a30..9d483e8 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -563,11 +563,6 @@ void init_dl_task_timer(struct sched_dl_entity *dl_se) { struct hrtimer *timer = &dl_se->dl_timer; - if (hrtimer_active(timer)) { - hrtimer_try_to_cancel(timer); - return; - } - hrtimer_init(timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL); timer->function = dl_task_timer; } @@ -1610,10 +1605,35 @@ void init_sched_dl_class(void) #endif /* CONFIG_SMP */ +/* + * Ensure p's dl_timer is cancelled. May drop rq->lock for a while. + */ +static void cancel_dl_timer(struct rq *rq, struct task_struct *p) +{ + struct hrtimer *dl_timer = &p->dl.dl_timer; + + /* Nobody will change task's class if pi_lock is held */ + lockdep_assert_held(&p->pi_lock); + + if (hrtimer_active(dl_timer)) { + int ret = hrtimer_try_to_cancel(dl_timer); + + if (unlikely(ret == -1)) { + /* + * Note, p may migrate OR new deadline tasks + * may appear in rq when we are unlocking it. + * A caller of us must be fine with that. + */ + raw_spin_unlock(&rq->lock); + hrtimer_cancel(dl_timer); + raw_spin_lock(&rq->lock); + } + } +} + static void switched_from_dl(struct rq *rq, struct task_struct *p) { - if (hrtimer_active(&p->dl.dl_timer) && !dl_policy(p->policy)) - hrtimer_try_to_cancel(&p->dl.dl_timer); + cancel_dl_timer(rq, p); __dl_clear_params(p); diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index ec3917c..49b941f 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1157,6 +1157,11 @@ struct sched_class { void (*task_fork) (struct task_struct *p); void (*task_dead) (struct task_struct *p); + /* + * The switched_from() call is allowed to drop rq->lock, therefore we + * cannot assume the switched_from/switched_to pair is serliazed by + * rq->lock. They are however serialized by p->pi_lock. + */ void (*switched_from) (struct rq *this_rq, struct task_struct *task); void (*switched_to) (struct rq *this_rq, struct task_struct *task); void (*prio_changed) (struct rq *this_rq, struct task_struct *task, -- 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/