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=-3.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, 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 BE956C56202 for ; Wed, 18 Nov 2020 08:06:40 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 2CDD2223C7 for ; Wed, 18 Nov 2020 08:06:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="g0FSgu3L" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2CDD2223C7 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=5MMSXceXMTXAJxdW2he4drobsIoPWiOkcdRdScJQHgo=; b=g0FSgu3L9+s/uLLbaVGKXpR/r IyrS7AtIxNvh1fhywRFLl0vUbcQd+ZNu0Neug1OgCD1K2uAFjLkjL1NMpENkJt7n0gsMyh95lc0vn IciXE8yV3Z3WGsporhzXxVhXOuwrhHW5otjZC3K14KRBRAH363SKhfdRgztjbluxNgpMY6HkZmtc0 hwOhm1o3NqFbcG8uoroufOov2LzzIvC9HFjKeCjuFQPetKCi+efVashffjFZWs3/HBCwVECG2oNoq gPavwg19nFAuzwme6EhTfOqsxAVb2hgTddfOQFifzNsRfFSFy1svfp0rcOoQvdeFakm449NwDooPQ MLroC4EXQ==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kfISw-0004Tx-Dw; Wed, 18 Nov 2020 08:05:22 +0000 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 #3 (Red Hat Linux)) id 1kfISt-0004Te-2Z; Wed, 18 Nov 2020 08:05:19 +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 3CA2B301959; Wed, 18 Nov 2020 09:05:15 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 178C8200E0A33; Wed, 18 Nov 2020 09:05:15 +0100 (CET) Date: Wed, 18 Nov 2020 09:05:15 +0100 From: Peter Zijlstra To: Valentin Schneider Subject: Re: [PATCH] sched: Fix data-race in wakeup Message-ID: <20201118080515.GR3121392@hirez.programming.kicks-ass.net> References: <20201116142005.GE3121392@hirez.programming.kicks-ass.net> <20201116193149.GW3371@techsingularity.net> <20201117083016.GK3121392@hirez.programming.kicks-ass.net> <20201117091545.GA31837@willie-the-truck> <20201117092936.GA3121406@hirez.programming.kicks-ass.net> <20201117094621.GE3121429@hirez.programming.kicks-ass.net> <20201117161318.GP3121392@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mel Gorman , Davidlohr Bueso , Will Deacon , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Nov 17, 2020 at 07:32:16PM +0000, Valentin Schneider wrote: > > On 17/11/20 16:13, Peter Zijlstra wrote: > > On Tue, Nov 17, 2020 at 03:37:24PM +0000, Valentin Schneider wrote: > > > >> >> + /* > >> >> + * This field must not be in the scheduler word above due to wakelist > >> >> + * queueing no longer being serialized by p->on_cpu. However: > >> >> + * > >> >> + * p->XXX = X; 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 = Y; > >> >> + * > >> >> + * guarantees all stores of 'current' are visible before > >> >> + * ->sched_remote_wakeup gets used, so it can be in this word. > >> >> + */ > >> > > >> > Isn't the control dep between that ttwu() p->on_rq read and > >> > p->sched_remote_wakeup write "sufficient"? > >> > >> smp_acquire__after_ctrl_dep() that is, since we need > >> ->on_rq load => 'current' bits load + store > > > > I don't think we need that extra barrier; after all, there will be a > > complete schedule() between waking the task and it actually becoming > > current. > > Apologies for the messy train of thought; what I was trying to say is that > we have already the following, which AIUI is sufficient: > > * p->XXX = X; ttwu() > * schedule() if (p->on_rq && ..) // false > * smp_mb__after_spinlock(); smp_acquire__after_ctrl_dep(); > * deactivate_task() ttwu_queue_wakelist() > * p->on_rq = 0; p->sched_remote_wakeup = Y; > Ah, you meant the existing smp_acquire__after_ctrl_dep(). Yeah, that's not required here either ;-) The reason I had the ->on_cpu thing in there is because it shows we violate the regular ->on_cpu handoff rules, not for the acquire. The only ordering that matters on the RHS of that thing is the ->on_rq load to p->sched_remote_wakeup store ctrl dep. That, combined with the LHS, guarantees there is a strict order on the stores. Makes sense? _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel