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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 BB4CBC433F5 for ; Fri, 10 Sep 2021 14:03:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 983A06108B for ; Fri, 10 Sep 2021 14:03:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233741AbhIJOFF (ORCPT ); Fri, 10 Sep 2021 10:05:05 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51510 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233658AbhIJOFA (ORCPT ); Fri, 10 Sep 2021 10:05:00 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C62BDC061574 for ; Fri, 10 Sep 2021 07:03:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=wOqrrvDZiOq5cww66uejzparN3JwZPm6+E/WejBRpDQ=; b=H13Bubyutnr2EXQUUbC5DxLhRv fWrgsTuFg9+FtSCekMRuJdXAwrAzq5fn+rfGYXt0KiJVcxmIGUiyz28NDBGHL4rfIyPN9eQFqN4p1 D2Wkb2oYlqnYTTRRNuwl4YcLilqB1CTHtRLFPYDAZLWOVXeHE3opqzn3BiSDGuYeZLThAvjvfA8i4 bKbQ6Xy5E1VzmE5EwmGhfL9r5YZ8jP/6BlkzZxabNAtvjopb8gTOnEXSKeMnf/wQ5NJ85FMQQg7Qb 9FmMhlDPD+e7Vv5JvUExV9zkmGJAA3w6Hg88cfkn3GX14Ihk3WeKXfIDAi4X597LWOkVSLXhfAKs6 rjlsRoQA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mOh5u-00B3r6-4P; Fri, 10 Sep 2021 14:01:43 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 2CA7B300047; Fri, 10 Sep 2021 16:01:29 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 167EE23CBCBF6; Fri, 10 Sep 2021 16:01:29 +0200 (CEST) Date: Fri, 10 Sep 2021 16:01:29 +0200 From: Peter Zijlstra To: Will Deacon Cc: tglx@linutronix.de, boqun.feng@gmail.com, linux-kernel@vger.kernel.org, Ingo Molnar , Juri Lelli , Steven Rostedt , Davidlohr Bueso , Waiman Long , Sebastian Andrzej Siewior , Mike Galbraith , Daniel Bristot de Oliveira Subject: Re: [PATCH 1/4] sched/wakeup: Strengthen current_save_and_set_rtlock_wait_state() Message-ID: References: <20210909105915.757320973@infradead.org> <20210909110203.767330253@infradead.org> <20210909134524.GB9722@willie-the-truck> <20210910125658.GA1454@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 10, 2021 at 03:17:04PM +0200, Peter Zijlstra wrote: > On Fri, Sep 10, 2021 at 01:57:26PM +0100, Will Deacon wrote: > > On Thu, Sep 09, 2021 at 04:27:46PM +0200, Peter Zijlstra wrote: > > > Moo yes, so the earlier changelog I wrote was something like: > > > > > > current_save_and_set_rtlock_wait_state(); > > > for (;;) { > > > if (try_lock()) > > > break; > > > > > > raw_spin_unlock_irq(&lock->wait_lock); > > > if (!cond) > > > schedule(); > > > raw_spin_lock_irq(&lock->wait_lock); > > > > > > set_current_state(TASK_RTLOCK_WAIT); > > > } > > > current_restore_rtlock_saved_state(); > > > > > > which is more what the code looks like before these patches, and in that > > > case the @cond load can be lifted before __state. > > > > Ah, so that makes more sense, thanks. I can't see how the try_lock() could > > be reordered though, as it's going to have to do an atomic rmw. > > OK, lemme go update the Changelog and make it __flags for bigeasy :-) The patch now reads: --- Subject: sched/wakeup: Strengthen current_save_and_set_rtlock_wait_state() From: Peter Zijlstra Date: Thu, 09 Sep 2021 12:59:16 +0200 While looking at current_save_and_set_rtlock_wait_state() I'm thinking it really ought to use smp_store_mb(), because using it for a more traditional wait loop like: current_save_and_set_rtlock_wait_state(); for (;;) { if (cond) schedule(); set_current_state(TASK_RTLOCK_WAIT); } current_restore_rtlock_saved_state(); is actually broken, since the cond load could be re-ordered against the state store, which could lead to a missed wakeup -> BAD (tm). While there, make them consistent with the IRQ usage in set_special_state(). Fixes: 5f220be21418 ("sched/wakeup: Prepare for RT sleeping spin/rwlocks") Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20210909110203.767330253@infradead.org --- include/linux/sched.h | 21 ++++++++++++--------- 1 file changed, 12 insertions(+), 9 deletions(-) --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -218,7 +218,7 @@ struct task_group; */ #define set_special_state(state_value) \ do { \ - unsigned long flags; /* may shadow */ \ + unsigned long __flags; /* may shadow */ \ \ raw_spin_lock_irqsave(¤t->pi_lock, flags); \ debug_special_state_change((state_value)); \ @@ -245,7 +245,8 @@ struct task_group; * if (try_lock()) * break; * raw_spin_unlock_irq(&lock->wait_lock); - * schedule_rtlock(); + * if (!cond) + * schedule_rtlock(); * raw_spin_lock_irq(&lock->wait_lock); * set_current_state(TASK_RTLOCK_WAIT); * } @@ -253,22 +254,24 @@ struct task_group; */ #define current_save_and_set_rtlock_wait_state() \ do { \ - lockdep_assert_irqs_disabled(); \ - raw_spin_lock(¤t->pi_lock); \ + unsigned long __flags; /* may shadow */ \ + \ + raw_spin_lock_irqsave(¤t->pi_lock, flags); \ current->saved_state = current->__state; \ debug_rtlock_wait_set_state(); \ - WRITE_ONCE(current->__state, TASK_RTLOCK_WAIT); \ - raw_spin_unlock(¤t->pi_lock); \ + smp_store_mb(current->__state, TASK_RTLOCK_WAIT); \ + raw_spin_unlock_irqrestore(¤t->pi_lock, flags); \ } while (0); #define current_restore_rtlock_saved_state() \ do { \ - lockdep_assert_irqs_disabled(); \ - raw_spin_lock(¤t->pi_lock); \ + unsigned long __flags; /* may shadow */ \ + \ + raw_spin_lock_irqsave(¤t->pi_lock, flags); \ debug_rtlock_wait_restore_state(); \ WRITE_ONCE(current->__state, current->saved_state); \ current->saved_state = TASK_RUNNING; \ - raw_spin_unlock(¤t->pi_lock); \ + raw_spin_unlock_irqrestore(¤t->pi_lock, flags); \ } while (0); #define get_current_state() READ_ONCE(current->__state)