From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 513BEC43331 for ; Fri, 27 Mar 2020 23:30:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 27B00206E6 for ; Fri, 27 Mar 2020 23:30:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727192AbgC0XaI convert rfc822-to-8bit (ORCPT ); Fri, 27 Mar 2020 19:30:08 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:55088 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726065AbgC0XaI (ORCPT ); Fri, 27 Mar 2020 19:30:08 -0400 Received: from bigeasy by Galois.linutronix.de with local (Exim 4.80) (envelope-from ) id 1jHyQJ-0007UJ-Mc; Sat, 28 Mar 2020 00:30:00 +0100 Date: Sat, 28 Mar 2020 00:29:59 +0100 From: Sebastian Andrzej Siewior To: kernel test robot Cc: Thomas Gleixner , Ingo Molnar , "Peter Zijlstra (Intel)" , linux-kernel@vger.kernel.org, LKP , Tejun Heo , Lai Jiangshan Subject: [PATCH v2] workqueue: Remove the warning in wq_worker_sleeping() Message-ID: <20200327232959.rpylymw2edhtxuwr@linutronix.de> References: <20200327074308.GY11705@shao2-debian> <20200327175350.rw5gex6cwum3ohnu@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8BIT In-Reply-To: <20200327175350.rw5gex6cwum3ohnu@linutronix.de> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The kernel test robot triggered a warning with the following race: task-ctx A interrupt-ctx B worker -> process_one_work() -> work_item() -> schedule(); -> sched_submit_work() -> wq_worker_sleeping() -> ->sleeping = 1 atomic_dec_and_test(nr_running) __schedule(); *interrupt* async_page_fault() -> local_irq_enable(); -> schedule(); -> sched_submit_work() -> wq_worker_sleeping() -> if (WARN_ON(->sleeping)) return -> __schedule() -> sched_update_worker() -> wq_worker_running() -> atomic_inc(nr_running); -> ->sleeping = 0; -> sched_update_worker() -> wq_worker_running() if (!->sleeping) return In this context the warning is pointless everything is fine. An interrupt before wq_worker_sleeping() will perform the ->sleeping assignment (0 -> 1 > 0) twice. An interrupt after wq_worker_sleeping() will trigger the warning and nr_running will be decremented (by A) and incremented once (only by B, A will skip it). This is the case until the ->sleeping is zeroed again in wq_worker_running(). Remove the WARN statement because this condition may happen. Document that preemption around wq_worker_sleeping() needs to be disabled to protect ->sleeping and not just as an optimisation. Fixes: 6d25be5782e48 ("sched/core, workqueues: Distangle worker accounting from rq lock") Link: https://lkml.kernel.org/r/20200327074308.GY11705@shao2-debian Reported-by: kernel test robot Signed-off-by: Sebastian Andrzej Siewior --- v1…v2: - Drop the warning instead of using cmpxchg_local(). Tglx pointed out that wq_worker_sleeping() is already invoked with disabled preemption so the race described can not happen. - Document that it is required to have preemption disabled within wq_worker_sleeping() to protect ->sleeping (against the race described in v1). kernel/sched/core.c | 3 ++- kernel/workqueue.c | 6 ++++-- 2 files changed, 6 insertions(+), 3 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 1a9983da4408d..7181ea73e5566 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4112,7 +4112,8 @@ static inline void sched_submit_work(struct task_struct *tsk) * it wants to wake up a task to maintain concurrency. * As this function is called inside the schedule() context, * we disable preemption to avoid it calling schedule() again - * in the possible wakeup of a kworker. + * in the possible wakeup of a kworker and because wq_worker_sleeping() + * requires it. */ if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) { preempt_disable(); diff --git a/kernel/workqueue.c b/kernel/workqueue.c index 4e01c448b4b48..39b968146f5f7 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -858,7 +858,8 @@ void wq_worker_running(struct task_struct *task) * @task: task going to sleep * * This function is called from schedule() when a busy worker is - * going to sleep. + * going to sleep. Preemption needs to be disabled to protect ->sleeping + * assignment. */ void wq_worker_sleeping(struct task_struct *task) { @@ -875,7 +876,8 @@ void wq_worker_sleeping(struct task_struct *task) pool = worker->pool; - if (WARN_ON_ONCE(worker->sleeping)) + /* Return if preempted before wq_worker_running() was reached */ + if (worker->sleeping) return; worker->sleeping = 1; -- 2.26.0