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=-2.5 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, USER_AGENT_MUTT 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 B1C53C282C2 for ; Wed, 13 Feb 2019 17:05:55 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 80F9C222D0 for ; Wed, 13 Feb 2019 17:05:55 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="ITfTFXxm" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 80F9C222D0 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-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=bombadil.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=v1riXzPUEllqG18DR/k5EBQJFonMngwBnkAUYSmOG/4=; b=ITfTFXxmI2U7ZI 62gTm/VQVWpMbGh9IwjU7eXvGJ6TUUA6IPEbxGXbEwPjj8wW0wLiRCy4IngBAWWsWDPl49oXvSjHw 8dEiTu3loz9ahnq6hSsDafCZcvzbXqYqrd9tpBQS7el5t+pdHON5RjNG0G/63Oqjwk6TqjtlBeicf 0f58+oTDCfIcVHuW69YwjPfzFv7kz2mg6hH2qwvuPEVvGFpxTGkxozpToRjGe1//8XvfK1SI6b+Ur A9uZSzd5FU+Vyoead38XtetlViIvv6cYn1XZc2491R/R5KT3WLVt0kCFfk4CrbJFSCpxODuIO2/ff 2Pbimn/SRybt1sE1SS/w==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1gtxys-0007Cb-21; Wed, 13 Feb 2019 17:05:54 +0000 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70] helo=foss.arm.com) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1gtxyo-0007C3-Q2 for linux-arm-kernel@lists.infradead.org; Wed, 13 Feb 2019 17:05:52 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 586A5A78; Wed, 13 Feb 2019 09:05:50 -0800 (PST) Received: from brain-police (usa-sjc-mx-foss1.foss.arm.com [217.140.101.70]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6CFDA3F675; Wed, 13 Feb 2019 09:05:47 -0800 (PST) Date: Wed, 13 Feb 2019 17:05:44 +0000 From: Will Deacon To: Thomas Gleixner Subject: Re: [PATCH v2 06/28] kernel: Define gettimeofday vdso common code Message-ID: <20190213170543.GF6346@brain-police> References: <20181129170530.37789-1-vincenzo.frascino@arm.com> <20181129170530.37789-7-vincenzo.frascino@arm.com> <20181207175321.GA11430@edgewater-inn.cambridge.arm.com> <20190208173539.GD24375@fuggles.cambridge.arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190213_090550_856774_97D0597C X-CRM114-Status: GOOD ( 26.22 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, Arnd Bergmann , Catalin Marinas , Daniel Lezcano , Russell King , Ralf Baechle , Mark Salyzyn , Paul Burton , Vincenzo Frascino , Peter Collingbourne , 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+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Thomas, On Fri, Feb 08, 2019 at 08:28:07PM +0100, Thomas Gleixner wrote: > On Fri, 8 Feb 2019, Will Deacon wrote: > > On Fri, Dec 07, 2018 at 05:53:21PM +0000, Will Deacon wrote: > > > Anyway, moving the counter read into the protected region is a little fiddly > > > because the memory barriers we have in there won't give us the ordering we > > > need. We'll instead need to do something nasty, like create a dependency > > > from the counter read to the read of the seqlock: > > > > > > Maybe the untested crufty hack below, although this will be a nightmare to > > > implement in C. > > > > We discussed this in person this week, but you couldn't recall the details > > off the top of your head so I'm replying here. Please could you clarify what > > your concern was with the existing code, and whether or not I've got the > > wrong end of the stick? > > If you just collect the variables under the seqcount protection and have > the readout of the timer outside of it then you are not guaranteeing > consistent state. > > The problem is: > > do { > seq = read_seqcount_begin(d->seq); > last = d->cycle_last; > mult = d->mult; > shift = d->shift; > ns = d->ns_base; > while (read_seqcount_retry(d->seq, seq)); > > ns += ((read_clock() - last) * mult); > ns >>= shift; > > So on the first glance this looks consistent because you collect all data > and then do the read and calc outside the loop. > > But if 'd->mult' gets updated _before_ read_clock() then you can run into a > situation where time goes backwards with the next read. > > Here is the flow you need for that: > > t1 = gettime() > { > collect_data() > > ---> Interrupt, updates mult (mult becomes smaller) > > This can expand over a very long time when the task is scheduled > out here and there are multiple updates in between. The farther > out the read is delayed, the more likely the problem is going to > observable. > > read_clock_and_calc() > } > > t2 = gettime() > { > collect_data() > read_clock_and_calc() > } > > This second read uses updated data, i.e. the smaller mult. So depending on > the distance of the two reads and the difference of the mult, the caller > can observe t2 < t1, which is a NONO. You can observe it on a single > CPU. No virt, SMP, migration needed at all. Thank you for this explanation, I agree that we have a bug here (and have done since day 1 on arm64!). I've replied separately on ktime_get(). Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel