From patchwork Tue Feb 15 13:57:24 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Hanna Czenczek X-Patchwork-Id: 12747109 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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id D1239C433EF for ; Tue, 15 Feb 2022 14:09:59 +0000 (UTC) Received: from localhost ([::1]:52636 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nJyWk-0004tk-QB for qemu-devel@archiver.kernel.org; Tue, 15 Feb 2022 09:09:58 -0500 Received: from eggs.gnu.org ([209.51.188.92]:37542) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nJyKv-0002CR-MV for qemu-devel@nongnu.org; Tue, 15 Feb 2022 08:57:45 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:44967) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nJyKt-0002LL-Gg for qemu-devel@nongnu.org; Tue, 15 Feb 2022 08:57:45 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1644933462; 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: content-transfer-encoding:content-transfer-encoding; bh=0vJaUduL62c1OYe8zy3dfQP7pNPgrIp1nOI9UowtxV8=; b=FEVPICzzmb+ySyP6vKkKtx9jZaSFmACGQhq/f/dIigxq4WEAy6k9ieMCYrQZQ1LFtTwOs6 HZajcm6I1WqU6IbnXhZ7chZfEES2pimeAYCUIJkQwD39K3YeGxwBRE7geK4QrKjHyoD2Gm qUCfrsSVW/vnAW4L0RjgCn4OynV6f8s= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-100-fJS5POwoN4OKvGGXPbCqdA-1; Tue, 15 Feb 2022 08:57:38 -0500 X-MC-Unique: fJS5POwoN4OKvGGXPbCqdA-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id ADACB1006AD3; Tue, 15 Feb 2022 13:57:35 +0000 (UTC) Received: from localhost (unknown [10.39.195.17]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 9ABF47C0F5; Tue, 15 Feb 2022 13:57:29 +0000 (UTC) From: Hanna Reitz To: qemu-block@nongnu.org Subject: [PATCH 0/3] block: Make bdrv_refresh_limits() non-recursive Date: Tue, 15 Feb 2022 14:57:24 +0100 Message-Id: <20220215135727.28521-1-hreitz@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=hreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Received-SPF: pass client-ip=170.10.133.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.083, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Hanna Reitz , qemu-devel@nongnu.org, Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Hi, Most bdrv_refresh_limits() callers do not drain the subtree of the node whose limits are refreshed, so concurrent I/O requests to child nodes can occur (if the node is in an I/O thread). bdrv_refresh_limits() is recursive, so such requests can happen to a node whose limits are being refreshed. bdrv_refresh_limits() is not atomic, and so the I/O requests can encounter invalid limits, like a 0 request_alignment. This will crash qemu (e.g. because of a division by 0, or a failed assertion). On inspection, bdrv_refresh_limits() doesn’t look like it really needs to be recursive. It just has always been. Dropping the recursion fixes those crashes, because all callers of bdrv_refresh_limits() make sure one way or another that concurrent requests to the node whose limits are to be refreshed are at leased paused (by draining, and/or by acquiring the AioContext). I see two other ways to fix it: (A) Have all bdrv_refresh_limits() callers drain the entire subtree, (B) Protect BDS.bl with RCU, which would make concurrent I/O just fine. (A) is kind of ugly, and after starting down that path two times, both times I decided I didn’t want to follow through with it. It was always an AioContext-juggling mess. (E.g. bdrv_set_backing_hd() would need to drain the subtree; but that means having to acquire the `backing_hd` context, too, because `bs` might be moved into that context, and so when `backing_hd` is attached to `bs`, `backing_hd` would be drained in the new context. But we can’t acquire a context twice, so we can only acquire `backing_hd`’s context if the caller hasn’t done so already. But the worst is that we can’t actually acquire that context: If `bs` is moved into `backing_hd`’s context, then `bdrv_set_aio_context_ignore()` requires us not to hold that context. It’s just kind of a mess.) I tried (B), and it worked, and I liked it very much; but it requires quite a bit of refactoring (every BDS.bl reader must then use qatomic_rcu_read() and take the RCU read lock), so it feels really difficult to justify when the fix this series proposes just removes four lines of code. Hanna Reitz (3): block: Make bdrv_refresh_limits() non-recursive iotests: Allow using QMP with the QSD iotests/graph-changes-while-io: New test block/io.c | 4 - tests/qemu-iotests/iotests.py | 29 +++++- .../qemu-iotests/tests/graph-changes-while-io | 91 +++++++++++++++++++ .../tests/graph-changes-while-io.out | 5 + 4 files changed, 124 insertions(+), 5 deletions(-) create mode 100755 tests/qemu-iotests/tests/graph-changes-while-io create mode 100644 tests/qemu-iotests/tests/graph-changes-while-io.out