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=-0.8 required=3.0 tests=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 3021AC433E1 for ; Thu, 21 May 2020 04:38:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DF218206D4 for ; Thu, 21 May 2020 04:38:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=intel-com.20150623.gappssmtp.com header.i=@intel-com.20150623.gappssmtp.com header.b="ySiq0WKu" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727091AbgEUEiK (ORCPT ); Thu, 21 May 2020 00:38:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47742 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726394AbgEUEiJ (ORCPT ); Thu, 21 May 2020 00:38:09 -0400 Received: from mail-ej1-x643.google.com (mail-ej1-x643.google.com [IPv6:2a00:1450:4864:20::643]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 71ABFC061A0E for ; Wed, 20 May 2020 21:38:09 -0700 (PDT) Received: by mail-ej1-x643.google.com with SMTP id z5so7280060ejb.3 for ; Wed, 20 May 2020 21:38:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=intel-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=5KCOIIzeGC6Wr485lQFHvKtNsuBP0Oc0+69jKUw3by0=; b=ySiq0WKuoG1C+5Z4Vg6tTZipO3Me/HdFVu3eL1WLrsp9IuYT5SUXZ8gV3gm9kR2mu+ x2gdrYsEYD44ygiBAP717RAXIj/S1kUDoPWQexjPUXer+qInFOUqPIsNllw1zEG4Ybl8 K9QwsJ3lYpL0jUPVKiPf+l9+EjGbIgSDnF5zjKaMML2Di5HIbtT56HdSPiC3wbLfsERa jVOT0IQsVeeRIvZddIVE7IssnaCp98tK1TIGPVtnq+rz5d0Pee2ZSwxM7XzBLlEQ2Z1t THVfPkkYP+kWuG7EBseJ/ZFKi6G8B1OQYspLqlUycTR2pLShmLeA72kSRf1hQsCDsMhn lKLg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=5KCOIIzeGC6Wr485lQFHvKtNsuBP0Oc0+69jKUw3by0=; b=bFtapPl+j9HrhAcPDvXZRNaP7EKfoQPmMhQZcBhWJosja7sp6KwAv0aHOkfsDUi0mG 2bUe2Y3Zdmn6YLT3Qz7blF6AacjI8hJt/c5ACinQY+zHt8eIJLaDtAcVvoAqLxbhWvEE x+gbjQgnai36gkO1SBM2mo8ZhfxW82aI6tRDVVOsA3+jmseaS0brMyZPXUNkBybzhwDF NUL3FuarTQpE9HH7GeU90WpJVGP3mMfVGS79zNFE6Cn3OV/uBGjo6WkatutbeQVE9EcM lsHYO5pQ0FgZls5LgZ4CNhlQOvjZZTUT/yNnqB977AO79xhR0PZd6xV17lme0UkAdk+N HTCQ== X-Gm-Message-State: AOAM533rKnMzsPgf8M6CoZ9A24FWYlS4cU4zvVbASDXZc1f78HjqfKUx IOMATAqLvveCBOyYgUNcVwI2CLRrBCsnAopl+KGltg== X-Google-Smtp-Source: ABdhPJw3DDYskRhdkTyiCNA2LABXPQAcEKzN/xK3GpvX8gosenDGbFcGGo2iovgmqIIkIOGIZ98Xa6vCprEsXzr4xao= X-Received: by 2002:a17:906:a8d:: with SMTP id y13mr1867800ejf.455.1590035888160; Wed, 20 May 2020 21:38:08 -0700 (PDT) MIME-Version: 1.0 References: <159002475918.686697.11844615159862491335.stgit@dwillia2-desk3.amr.corp.intel.com> <20200521022628.GE16070@bombadil.infradead.org> In-Reply-To: <20200521022628.GE16070@bombadil.infradead.org> From: Dan Williams Date: Wed, 20 May 2020 21:37:57 -0700 Message-ID: Subject: Re: [PATCH v3] /dev/mem: Revoke mappings when a driver claims the region To: Matthew Wilcox Cc: Greg KH , Arnd Bergmann , Ingo Molnar , Kees Cook , Russell King , Andrew Morton , Linux Kernel Mailing List , Linux MM Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 20, 2020 at 7:26 PM Matthew Wilcox wrote: > > On Wed, May 20, 2020 at 06:35:25PM -0700, Dan Williams wrote: > > +static struct inode *devmem_inode; > > + > > +#ifdef CONFIG_IO_STRICT_DEVMEM > > +void revoke_devmem(struct resource *res) > > +{ > > + struct inode *inode = READ_ONCE(devmem_inode); > > + > > + /* > > + * Check that the initialization has completed. Losing the race > > + * is ok because it means drivers are claiming resources before > > + * the fs_initcall level of init and prevent /dev/mem from > > + * establishing mappings. > > + */ > > + smp_rmb(); > > + if (!inode) > > + return; > > But we don't need the smp_rmb() here, right? READ_ONCE and WRITE_ONCE > are a DATA DEPENDENCY barrier (in Documentation/memory-barriers.txt parlance) > so the smp_rmb() is superfluous ... Is it? I did not grok that from Documentation/memory-barriers.txt. READ_ONCE and WRITE_ONCE are certainly ordered with respect to each other in the same function, but I thought they still depend on barriers for smp ordering? > > > + /* > > + * Use a unified address space to have a single point to manage > > + * revocations when drivers want to take over a /dev/mem mapped > > + * range. > > + */ > > + inode->i_mapping = devmem_inode->i_mapping; > > + inode->i_mapping->host = devmem_inode; > > umm ... devmem_inode->i_mapping->host doesn't already point to devmem_inode? Not if inode is coming from: mknod ./newmem c 1 1 ...that's the problem that a unified inode solves. You can mknod all you want, but mapping and mapping->host will point to a common instance. > > > + > > + /* publish /dev/mem initialized */ > > + smp_wmb(); > > + WRITE_ONCE(devmem_inode, inode); > > As above, unnecessary barrier, I think. Well, if you're not sure, how sure should I be?