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=-7.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 BB5E0C3A59E for ; Mon, 2 Sep 2019 12:31:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 939D62168B for ; Mon, 2 Sep 2019 12:31:33 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="qyt5M2sH" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731621AbfIBMbc (ORCPT ); Mon, 2 Sep 2019 08:31:32 -0400 Received: from merlin.infradead.org ([205.233.59.134]:44178 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729999AbfIBMbc (ORCPT ); Mon, 2 Sep 2019 08:31:32 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=lBLD1ncc2hG4ju0AyE3DjcGoR5OZ27h6EU0DPKLq93w=; b=qyt5M2sHlu4gjAY1O6NgGLy91 EoYi9ltTA+5ZM4d+xmzo3toUP8JrlqXyvIg7/gY7maGaaa0oXQkxWjAYSPgAvwuDtgy5v01S8EqVE EkMXrFGlwX4KfojVcf/s6F7p/lQ63rvMXTZ6qmwZMq8YTc9LntWlJcEO8lLd6+qLotRcLGqwhYBYD aO95Xgw7qL6pegat+9HipWcBnb35mwdNl74YPSaOV76g6CwI1cMmT5wnltcbWGEBXnBSKe1R405iw 5Fg2MdPWYenEncekJj0F5WIVCNU5jwzhbBl55sZmK1l8N3TJeKriw1p0FOKz15Cx5aikYFVr/K4X1 Xe/0z1C4g==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1i4lUE-00080K-Ua; Mon, 02 Sep 2019 12:31:11 +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 A0E5430116F; Mon, 2 Sep 2019 14:30:30 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 539C029B2A005; Mon, 2 Sep 2019 14:31:06 +0200 (CEST) Date: Mon, 2 Sep 2019 14:31:06 +0200 From: Peter Zijlstra To: Alessio Balsini Cc: mingo@kernel.org, juri.lelli@redhat.com, linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com, luca.abeni@santannapisa.it, bristot@redhat.com, dvyukov@google.com, tglx@linutronix.de, vpillai@digitalocean.com, rostedt@goodmis.org, kernel-team@android.com Subject: Re: [RFC][PATCH 01/13] sched/deadline: Impose global limits on sched_attr::sched_period Message-ID: <20190902123106.GS2386@hirez.programming.kicks-ass.net> References: <20190726145409.947503076@infradead.org> <20190726161357.397880775@infradead.org> <20190802172104.GA134279@google.com> <20190805115309.GJ2349@hirez.programming.kicks-ass.net> <20190822122949.GA245353@google.com> <20190822165125.GW2369@hirez.programming.kicks-ass.net> <20190831144117.GA133727@google.com> <20190902091623.GQ2349@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190902091623.GQ2349@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 02, 2019 at 11:16:23AM +0200, Peter Zijlstra wrote: > On Sat, Aug 31, 2019 at 03:41:17PM +0100, Alessio Balsini wrote: > > Right! > > > > Verified that sysctl_sched_dl_period_max and sysctl_sched_dl_period_min values > > are now always consistent. > > > > I spent some time in trying to figure out if not having any mutex in > > __checkparam_dl() is safe. There can surely happen that "max < min", e.g.: > > Sharing my thoughts, a "BUG_ON(max < min)" in __checkparam_dl() is then a > > guaranteed source of explosions, but the good news is that "if (period < min || > > period > max" in __checkparam_dl() surely fails if "max < min". Also the fact > > that, when we are writing the new sysctl_sched_dl_* values, only one is > > actually changed at a time, that surely helps to preserve the consistency. > > > > But is that enough? > > Strictly speaking, no, I suppose it is not. We can have two changes in > between the two READ_ONCE()s and then we'd be able to observe a > violation. > > The easy way to fix that is do something like: > > + synchronize_rcu(); > mutex_unlock(&mutex); > > in sched_dl_period_handler(). And do: > > + preempt_disable(); > max = (u64)READ_ONCE(sysctl_sched_dl_period_max) * NSEC_PER_USEC; > min = (u64)READ_ONCE(sysctl_sched_dl_period_min) * NSEC_PER_USEC; > + preempt_enable(); > > in __checkparam_dl(). > > That would prohibit we see two changes, and seeing only the single > change is safe. I pushed out a new version; and added patch to sched_rt_handler() on top. Please have a look at: git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git sched/wip-deadline I'll move these two patches to sched/core if everything looks good.