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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0B2D8C27C40 for ; Thu, 23 Nov 2023 02:19:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235073AbjKWCTn (ORCPT ); Wed, 22 Nov 2023 21:19:43 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51938 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234719AbjKWCTi (ORCPT ); Wed, 22 Nov 2023 21:19:38 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A5CCA1A8 for ; Wed, 22 Nov 2023 18:19:44 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1700705984; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=k2aZzryTXfwVm4bHgxcYkBHTBkYi7Y8xjXsoz4m2WKU=; b=cCmbqFd41WsUIqjG2zmPm8mJDn4Y1vRRzG5OVZaGBONVGWlPsHXPPlYajPhfVcM/QLTJio bg5O3YwM76hR2oqNA61HSJ/OGr3QMxpuGsRmYW/gRedv8Xjj3pAJD1bVm8poumB3AtF1jg 56egW/5Ln8q6KLIjJBxgGtz9rRysE9A= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-678-_OvJFeQrOSu_YCCI76BTjg-1; Wed, 22 Nov 2023 21:19:40 -0500 X-MC-Unique: _OvJFeQrOSu_YCCI76BTjg-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 47A3E80D722; Thu, 23 Nov 2023 02:19:40 +0000 (UTC) Received: from fedora (unknown [10.72.120.3]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 4D79A492BFA; Thu, 23 Nov 2023 02:19:34 +0000 (UTC) Date: Thu, 23 Nov 2023 10:19:30 +0800 From: Ming Lei To: Christoph Hellwig Cc: Yu Kuai , axboe@kernel.dk, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, yukuai3@huawei.com, yi.zhang@huawei.com, yangerkun@huawei.com Subject: Re: [PATCH v3 2/3] block: introduce new field bd_flags in block_device Message-ID: References: <20231122103103.1104589-1-yukuai1@huaweicloud.com> <20231122103103.1104589-3-yukuai1@huaweicloud.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.10 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 22, 2023 at 04:47:51AM -0800, Christoph Hellwig wrote: > On Wed, Nov 22, 2023 at 04:19:40PM +0800, Ming Lei wrote: > > On Tue, Nov 21, 2023 at 11:53:17PM -0800, Christoph Hellwig wrote: > > > On Wed, Nov 22, 2023 at 03:45:24PM +0800, Ming Lei wrote: > > > > All the existed 'bool' flags are not atomic RW, so I think it isn't > > > > necessary to define 'bd_flags' as 'unsigned long' for replacing them. > > > > > > So because the old code wasn't correct we'll never bother? The new > > > flag and the new placement certainly make this more critical as well. > > > > Can you explain why the old code was wrong? > > > > 1) ->bd_read_only and ->bd_make_it_fail > > > > - set from userspace interface(ioctl or sysfs) > > - check in IO code path > > > > so changing it into atomic bit doesn't make difference from user > > viewpoint. > > > > > 2) ->bd_write_holder > > > > disk->open_mutex is held for read & write this flag > > > > 3) ->bd_has_submit_bio > > > > This flag is setup as oneshot before adding disk, and check in FS io code > > path. > > On architectures that can't do byte-level atomics all three can corrupt > each other Yeah, C/C++ doesn't provide such guarantee, but many modern ARCHs [1] guarantees that RW on naturally aligned type is atomic. I verified the point on x86/arm64/ppc64le by the following code, and all three STOREs are done in single instruction. struct data { int b; char a; char a2; char a3; char a4; } __attribute__((aligned(8))); void atomic_test() { struct data d; d.b = 1; d.a = 2; d.a3 = 3; printf("%d %d %d\n", d.b, d.a, d.a3); } [1] https://preshing.com/20130618/atomic-vs-non-atomic-operations/ > and even worse bd_partno. Granted that is only alpha these > days IIRC, but it's still buggy. bd_has_submit_bio and bd_partno can be thought as read only, and the two can be corrupted? bd_dev may have similar trouble with bd_partno for ARCHs which don't provide atomic RW on naturally aligned int. Thanks, Ming