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=-6.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 C8A63C2D0E4 for ; Tue, 17 Nov 2020 08:30:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 67DF524655 for ; Tue, 17 Nov 2020 08:30:29 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="K9cyBI3X" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726815AbgKQIa2 (ORCPT ); Tue, 17 Nov 2020 03:30:28 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42776 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725355AbgKQIa1 (ORCPT ); Tue, 17 Nov 2020 03:30:27 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1B73BC0613CF for ; Tue, 17 Nov 2020 00:30:27 -0800 (PST) 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=qrM57Vqpe5PlbsXpdoBXSd/3FAipesafHhH3I2g1pD4=; b=K9cyBI3Xd179Jod1LNIGmEB0IV sWLcoxX67RM50qK0e0jSXJ9xpgdY6SHAg4BeHMOaI/IVjMNIZCbKBv6jP9wxBSJaKVhifpV0QCp7H x5cHaTNLhY8lAoZ0uBG6Vk3yQiIxwr0bmI2lYmR8MoloSc01HZPvkshppc/C0tCmHEGOcWEZ/NUm6 bZdZG3UbdPKK111wii4wnar5oUvY5LTNJVKRbKr5Gpva2jjjpxu4t7wrltYQw3+FVzA8Wj/V4CqqL NKpEReuLO+vk3vYPf30buifYdfJqF46i+y6QgkqIYw1fXzW+JLJiX9TBg5geynNTPrafRXwlDFt2/ gn1P1STQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kewNW-00022B-9a; Tue, 17 Nov 2020 08:30:18 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 70B80301959; Tue, 17 Nov 2020 09:30:16 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 5B2642364166A; Tue, 17 Nov 2020 09:30:16 +0100 (CET) Date: Tue, 17 Nov 2020 09:30:16 +0100 From: Peter Zijlstra To: Mel Gorman Cc: Will Deacon , Davidlohr Bueso , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: [PATCH] sched: Fix data-race in wakeup Message-ID: <20201117083016.GK3121392@hirez.programming.kicks-ass.net> References: <20201116091054.GL3371@techsingularity.net> <20201116131102.GA29992@willie-the-truck> <20201116133721.GQ3371@techsingularity.net> <20201116142005.GE3121392@hirez.programming.kicks-ass.net> <20201116193149.GW3371@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201116193149.GW3371@techsingularity.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 16, 2020 at 07:31:49PM +0000, Mel Gorman wrote: > And this works. Yay! > sched_psi_wake_requeue can probably stay with the other three fields > given they are under the rq lock but sched_remote_wakeup needs to move > out. I _think_ we can move the bit into the unserialized section below. It's a bit cheecky, but it should work I think because the only time we actually use this bit, we're guaranteed the task isn't actually running, so current doesn't exist. I suppose the question is wether this is worth saving 31 bits over... How's this? --- Subject: sched: Fix data-race in wakeup From: Peter Zijlstra Date: Tue Nov 17 09:08:41 CET 2020 Mel reported that on some ARM64 platforms loadavg goes bananas and tracked it down to the following data race: CPU0 CPU1 schedule() prev->sched_contributes_to_load = X; deactivate_task(prev); try_to_wake_up() if (p->on_rq &&) // false if (smp_load_acquire(&p->on_cpu) && // true ttwu_queue_wakelist()) p->sched_remote_wakeup = Y; smp_store_release(prev->on_cpu, 0); where both p->sched_contributes_to_load and p->sched_remote_wakeup are in the same word, and thus the stores X and Y race (and can clobber one another's data). Whereas prior to commit c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu") the p->on_cpu handoff serialized access to p->sched_remote_wakeup (just as it still does with p->sched_contributes_to_load) that commit broke that by calling ttwu_queue_wakelist() with p->on_cpu != 0. However, due to p->XXX ttwu() schedule() if (p->on_rq && ...) // false smp_mb__after_spinlock() if (smp_load_acquire(&p->on_cpu) && deactivate_task() ttwu_queue_wakelist()) p->on_rq = 0; p->sched_remote_wakeup = X; We can be sure any 'current' store is complete and 'current' is guaranteed asleep. Therefore we can move p->sched_remote_wakeup into the current flags word. Note: while the observed failure was loadavg accounting gone wrong due to ttwu() cobbering p->sched_contributes_to_load, the reverse problem is also possible where schedule() clobbers p->sched_remote_wakeup, this could result in enqueue_entity() wrecking ->vruntime and causing scheduling artifacts. Fixes: c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu") Reported-by: Mel Gorman Signed-off-by: Peter Zijlstra (Intel) --- include/linux/sched.h | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -775,7 +775,6 @@ struct task_struct { unsigned sched_reset_on_fork:1; unsigned sched_contributes_to_load:1; unsigned sched_migrated:1; - unsigned sched_remote_wakeup:1; #ifdef CONFIG_PSI unsigned sched_psi_wake_requeue:1; #endif @@ -785,6 +784,18 @@ struct task_struct { /* Unserialized, strictly 'current' */ + /* + * p->in_iowait = 1; ttwu() + * schedule() if (p->on_rq && ..) // false + * smp_mb__after_spinlock(); if (smp_load_acquire(&p->on_cpu) && //true + * deactivate_task() ttwu_queue_wakelist()) + * p->on_rq = 0; p->sched_remote_wakeup = X; + * + * Guarantees all stores of 'current' are visible before + * ->sched_remote_wakeup gets used. + */ + unsigned sched_remote_wakeup:1; + /* Bit to tell LSMs we're in execve(): */ unsigned in_execve:1; unsigned in_iowait:1;