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=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 97ECCC433E7 for ; Thu, 15 Oct 2020 15:29:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3167722272 for ; Thu, 15 Oct 2020 15:29:20 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=ffwll.ch header.i=@ffwll.ch header.b="jiwyZuJU" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389713AbgJOP3S (ORCPT ); Thu, 15 Oct 2020 11:29:18 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38322 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388549AbgJOP3R (ORCPT ); Thu, 15 Oct 2020 11:29:17 -0400 Received: from mail-oo1-xc41.google.com (mail-oo1-xc41.google.com [IPv6:2607:f8b0:4864:20::c41]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9FFC8C0613D3 for ; Thu, 15 Oct 2020 08:29:17 -0700 (PDT) Received: by mail-oo1-xc41.google.com with SMTP id o20so816994ook.1 for ; Thu, 15 Oct 2020 08:29:17 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ffwll.ch; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=LJwfZvZzZbL3U3dsefi4j72Jm0TppMPnsxaVcVl6iT0=; b=jiwyZuJU82zqfM3xXQZkNpewO6fCkqA7igabDZ79l9dEml7MlNt9/W6KspOKdTFMBv l0ZZKLNyCyInH/bWkyXoNA3+A+JhbIMgDBR1thG0QOHKB4hsUrxgAlsNauv981ymHMte aG33qVap5atzCP8H/NRFqSDXso79lnD2nYDfc= 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=LJwfZvZzZbL3U3dsefi4j72Jm0TppMPnsxaVcVl6iT0=; b=Mx24w8Bleuldx8g9rw8Lu7TQRpkS1kk3K6uLAiswzDlw4aPvq/q06vauFe7ZybI/U1 IRVC2eF0nMecH6UW/y1NH0OeqB1oE6ut4339KfWJFWy3+N4Y3P6HH/52TrhnF1EteLl/ kBR0vTMS9LJUx/Vlpirfha7xO4GTi2KC+uyaDK04Yx+aFwBw7L+LXlppERPDumd5yaVl SqP/bNt/icmsBqGpiZ73nYB7sfCiL3bU54M2mNar81/F3v5a6YrbOzIND0eFf41/SXu9 B0t9/WSymym9nIyKMuwmA5CZR3+DWNkWNGw+XjAbTzwVjrks6fcDLfZF5DGSDj0FmBL5 kXTA== X-Gm-Message-State: AOAM5322wJO1a0vG1/G1slBq4R4IoJ6OkRVdy0b7jzZLoQHLB/Lm9q9Y 10bOPXrEP1hgP1DGMP2eLPiNZ8oW0JFoSYBF5RNyaQ== X-Google-Smtp-Source: ABdhPJwHGqKFcBCVEj4Z9lD8mpPIRoQJxs6gS7mqbJLVOqKjzGL3o2Yivo5QSuS7JTOQT9EJ/KzY1r55eHOicMya4wY= X-Received: by 2002:a4a:e1d7:: with SMTP id n23mr2788554oot.85.1602775756786; Thu, 15 Oct 2020 08:29:16 -0700 (PDT) MIME-Version: 1.0 References: <20201009075934.3509076-1-daniel.vetter@ffwll.ch> <20201009075934.3509076-15-daniel.vetter@ffwll.ch> <20201009123109.GO5177@ziepe.ca> <20201009143209.GS5177@ziepe.ca> <20201015000939.GD6763@ziepe.ca> In-Reply-To: <20201015000939.GD6763@ziepe.ca> From: Daniel Vetter Date: Thu, 15 Oct 2020 17:29:05 +0200 Message-ID: Subject: Re: [PATCH v2 14/17] resource: Move devmem revoke code to resource framework To: Jason Gunthorpe Cc: Dan Williams , DRI Development , LKML , KVM list , Linux MM , Linux ARM , linux-samsung-soc , "open list:DMA BUFFER SHARING FRAMEWORK" , linux-s390 , Daniel Vetter , Kees Cook , Andrew Morton , John Hubbard , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Jan Kara , Arnd Bergmann , Greg Kroah-Hartman , David Hildenbrand , "Rafael J. Wysocki" Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org On Thu, Oct 15, 2020 at 2:09 AM Jason Gunthorpe wrote: > > On Fri, Oct 09, 2020 at 11:28:54AM -0700, Dan Williams wrote: > > On Fri, Oct 9, 2020 at 7:32 AM Jason Gunthorpe wrote: > > > > > > On Fri, Oct 09, 2020 at 04:24:45PM +0200, Daniel Vetter wrote: > > > > On Fri, Oct 9, 2020 at 2:31 PM Jason Gunthorpe wrote: > > > > > > > > > > On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote: > > > > > > > > > > > +struct address_space *iomem_get_mapping(void) > > > > > > +{ > > > > > > + return iomem_inode->i_mapping; > > > > > > > > > > This should pair an acquire with the release below > > > > > > > > > > > + /* > > > > > > + * Publish /dev/mem initialized. > > > > > > + * Pairs with smp_load_acquire() in revoke_iomem(). > > > > > > + */ > > > > > > + smp_store_release(&iomem_inode, inode); > > > > > > > > > > However, this seems abnormal, initcalls rarely do this kind of stuff > > > > > with global data.. > > > > > > > > > > The kernel crashes if this fs_initcall is raced with > > > > > iomem_get_mapping() due to the unconditional dereference, so I think > > > > > it can be safely switched to a simple assignment. > > > > > > > > Ah yes I checked this all, but forgot to correctly annotate the > > > > iomem_get_mapping access. For reference, see b34e7e298d7a ("/dev/mem: > > > > Add missing memory barriers for devmem_inode"). > > > > > > Oh yikes, so revoke_iomem can run concurrently during early boot, > > > tricky. > > > > It runs early because request_mem_region() can run before fs_initcall. > > Rather than add an unnecessary lock just arrange for the revoke to be > > skipped before the inode is initialized. The expectation is that any > > early resource reservations will block future userspace mapping > > attempts. > > Actually, on this point a simple WRITE_ONCE/READ_ONCE pairing is OK, > Paul once explained that the pointer chase on the READ_ONCE side is > required to be like an acquire - this is why rcu_dereference is just > READ_ONCE Hm so WRITE_ONCE doesn't have any barriers, and we'd need that for updating the pointer. That would leave things rather inconsistent, so I think I'll just leave it as-is for symmetry reasons. None of this code matters for performance anyway, so micro-optimizing barriers seems a bit silly. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch