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=-11.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,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 EC466C3A5A1 for ; Fri, 23 Aug 2019 02:12:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C9F3223405 for ; Fri, 23 Aug 2019 02:12:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389743AbfHWCMf (ORCPT ); Thu, 22 Aug 2019 22:12:35 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:33728 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389361AbfHWCMV (ORCPT ); Thu, 22 Aug 2019 22:12:21 -0400 Received: from [5.158.153.53] (helo=tip-bot2.lab.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1i0z3p-0000xy-Px; Fri, 23 Aug 2019 04:12:17 +0200 Received: from [127.0.1.1] (localhost [IPv6:::1]) by tip-bot2.lab.linutronix.de (Postfix) with ESMTP id 791A11C0883; Fri, 23 Aug 2019 04:12:17 +0200 (CEST) Date: Fri, 23 Aug 2019 02:12:17 -0000 From: tip-bot2 for Frederic Weisbecker Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: timers/core] hrtimer: Improve comments on handling priority inversion against softirq kthread Cc: linux-kernel@vger.kernel.org, Thomas Gleixner , Frederic Weisbecker In-Reply-To: <20190820132656.GC2093@lenoir> References: <20190820132656.GC2093@lenoir> MIME-Version: 1.0 Message-ID: <156652633741.11664.5398841645445804148.tip-bot2@tip-bot2> X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following commit has been merged into the timers/core branch of tip: Commit-ID: 0bee3b601b77dbe7981b5474ae8758d6bf60177a Gitweb: https://git.kernel.org/tip/0bee3b601b77dbe7981b5474ae8758d6bf60177a Author: Frederic Weisbecker AuthorDate: Tue, 20 Aug 2019 15:12:23 +02:00 Committer: Thomas Gleixner CommitterDate: Tue, 20 Aug 2019 22:05:46 +02:00 hrtimer: Improve comments on handling priority inversion against softirq kthread The handling of a priority inversion between timer cancelling and a a not well defined possible preemption of softirq kthread is not very clear. Especially in the posix timers side it's unclear why there is a specific RT wait callback. All the nice explanations can be found in the initial changelog of f61eff83cec9 (hrtimer: Prepare support for PREEMPT_RT"). Extract the detailed informations from there and put it into comments. Signed-off-by: Frederic Weisbecker Signed-off-by: Thomas Gleixner Link: https://lkml.kernel.org/r/20190820132656.GC2093@lenoir --- kernel/time/hrtimer.c | 14 ++++++++++---- kernel/time/posix-timers.c | 6 ++++++ 2 files changed, 16 insertions(+), 4 deletions(-) diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c index 4991227..8333537 100644 --- a/kernel/time/hrtimer.c +++ b/kernel/time/hrtimer.c @@ -1201,10 +1201,16 @@ static void hrtimer_sync_wait_running(struct hrtimer_cpu_base *cpu_base, * deletion of a timer failed because the timer callback function was * running. * - * This prevents priority inversion, if the softirq thread on a remote CPU - * got preempted, and it prevents a life lock when the task which tries to - * delete a timer preempted the softirq thread running the timer callback - * function. + * This prevents priority inversion: if the soft irq thread is preempted + * in the middle of a timer callback, then calling del_timer_sync() can + * lead to two issues: + * + * - If the caller is on a remote CPU then it has to spin wait for the timer + * handler to complete. This can result in unbound priority inversion. + * + * - If the caller originates from the task which preempted the timer + * handler on the same CPU, then spin waiting for the timer handler to + * complete is never going to end. */ void hrtimer_cancel_wait_running(const struct hrtimer *timer) { diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c index 9e37783..0ec5b7a 100644 --- a/kernel/time/posix-timers.c +++ b/kernel/time/posix-timers.c @@ -810,6 +810,12 @@ static void common_timer_wait_running(struct k_itimer *timer) hrtimer_cancel_wait_running(&timer->it.real.timer); } +/* + * On PREEMPT_RT this prevent priority inversion against softirq kthread in + * case it gets preempted while executing a timer callback. See comments in + * hrtimer_cancel_wait_running. For PREEMPT_RT=n this just results in a + * cpu_relax(). + */ static struct k_itimer *timer_wait_running(struct k_itimer *timer, unsigned long *flags) {