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=-12.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,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 AF501C7113B for ; Mon, 21 Jan 2019 11:25:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 87BE020663 for ; Mon, 21 Jan 2019 11:25:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728071AbfAULZ5 (ORCPT ); Mon, 21 Jan 2019 06:25:57 -0500 Received: from terminus.zytor.com ([198.137.202.136]:51897 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727665AbfAULZ5 (ORCPT ); Mon, 21 Jan 2019 06:25:57 -0500 Received: from terminus.zytor.com (localhost [127.0.0.1]) by terminus.zytor.com (8.15.2/8.15.2) with ESMTPS id x0LBPiHY2495707 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO); Mon, 21 Jan 2019 03:25:44 -0800 Received: (from tipbot@localhost) by terminus.zytor.com (8.15.2/8.15.2/Submit) id x0LBPilc2495704; Mon, 21 Jan 2019 03:25:44 -0800 Date: Mon, 21 Jan 2019 03:25:44 -0800 X-Authentication-Warning: terminus.zytor.com: tipbot set sender to tipbot@zytor.com using -f From: tip-bot for Prateek Sood Message-ID: Cc: prsood@codeaurora.org, mingo@kernel.org, linux-kernel@vger.kernel.org, dbueso@suse.de, torvalds@linux-foundation.org, tglx@linutronix.de, andrea.parri@amarulasolutions.com, hpa@zytor.com, peterz@infradead.org Reply-To: prsood@codeaurora.org, mingo@kernel.org, linux-kernel@vger.kernel.org, andrea.parri@amarulasolutions.com, peterz@infradead.org, hpa@zytor.com, tglx@linutronix.de, dbueso@suse.de, torvalds@linux-foundation.org In-Reply-To: <1543590656-7157-1-git-send-email-prsood@codeaurora.org> References: <1543590656-7157-1-git-send-email-prsood@codeaurora.org> To: linux-tip-commits@vger.kernel.org Subject: [tip:locking/core] sched/wait: Fix rcuwait_wake_up() ordering Git-Commit-ID: 6dc080eeb2ba01973bfff0d79844d7a59e12542e 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 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit-ID: 6dc080eeb2ba01973bfff0d79844d7a59e12542e Gitweb: https://git.kernel.org/tip/6dc080eeb2ba01973bfff0d79844d7a59e12542e Author: Prateek Sood AuthorDate: Fri, 30 Nov 2018 20:40:56 +0530 Committer: Ingo Molnar CommitDate: Mon, 21 Jan 2019 11:15:36 +0100 sched/wait: Fix rcuwait_wake_up() ordering For some peculiar reason rcuwait_wake_up() has the right barrier in the comment, but not in the code. This mistake has been observed to cause a deadlock in the following situation: P1 P2 percpu_up_read() percpu_down_write() rcu_sync_is_idle() // false rcu_sync_enter() ... __percpu_up_read() [S] ,- __this_cpu_dec(*sem->read_count) | smp_rmb(); [L] | task = rcu_dereference(w->task) // NULL | | [S] w->task = current | smp_mb(); | [L] readers_active_check() // fail `-> Where the smp_rmb() (obviously) fails to constrain the store. [ peterz: Added changelog. ] Signed-off-by: Prateek Sood Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Andrea Parri Acked-by: Davidlohr Bueso Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Fixes: 8f95c90ceb54 ("sched/wait, RCU: Introduce rcuwait machinery") Link: https://lkml.kernel.org/r/1543590656-7157-1-git-send-email-prsood@codeaurora.org Signed-off-by: Ingo Molnar --- kernel/exit.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/exit.c b/kernel/exit.c index 284f2fe9a293..3fb7be001964 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -307,7 +307,7 @@ void rcuwait_wake_up(struct rcuwait *w) * MB (A) MB (B) * [L] cond [L] tsk */ - smp_rmb(); /* (B) */ + smp_mb(); /* (B) */ /* * Avoid using task_rcu_dereference() magic as long as we are careful,