From patchwork Wed Sep 6 19:01:38 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Stefan Hajnoczi X-Patchwork-Id: 13375896 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 2C10BEE14D0 for ; Wed, 6 Sep 2023 19:02:33 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qdxmj-0006Af-4h; Wed, 06 Sep 2023 15:01:53 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qdxmi-0006AQ-0c for qemu-devel@nongnu.org; Wed, 06 Sep 2023 15:01:52 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qdxmf-00087O-L2 for qemu-devel@nongnu.org; Wed, 06 Sep 2023 15:01:51 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1694026908; 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=5H8pelRVsxJc9RClUG2Va6wOhXzoURG4z+1hJARkfMQ=; b=PlCjIh1bEl9FQFlmEfQ77JGDzE0KMPioIVprBU3x51yALOIeMPwYTBtLZCoSnJ8NLyQRnV 7jrDvaf+LphdbP7e6Ki18i/0LIetz2vQ58eOOogSowswi4bn3az0A41t9JlzVNjlao6/c+ CJHPshOfXST7FQVjcgPXSIHmY+g1HTo= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-196-qwPGnw9UOxyb2nkJPL1MWQ-1; Wed, 06 Sep 2023 15:01:44 -0400 X-MC-Unique: qwPGnw9UOxyb2nkJPL1MWQ-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 8A90D38157A7; Wed, 6 Sep 2023 19:01:43 +0000 (UTC) Received: from localhost (unknown [10.39.193.221]) by smtp.corp.redhat.com (Postfix) with ESMTP id E2A1D10EA0; Wed, 6 Sep 2023 19:01:42 +0000 (UTC) From: Stefan Hajnoczi To: qemu-devel@nongnu.org Cc: "Dr. David Alan Gilbert" , Eduardo Habkost , pbonzini@redhat.com, Markus Armbruster , Eric Blake , kwolf@redhat.com, Maxim Levitsky , =?utf-8?q?Daniel_P?= =?utf-8?q?=2E_Berrang=C3=A9?= , Stefan Hajnoczi Subject: [RFC 0/3] qmp: make qmp_device_add() a coroutine Date: Wed, 6 Sep 2023 15:01:38 -0400 Message-ID: <20230906190141.1286893-1-stefanha@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org It is not safe to call drain_call_rcu() from qmp_device_add() because some call stacks are not prepared for drain_call_rcu() to drop the Big QEMU Lock (BQL). For example, device emulation code is protected by the BQL but when it calls aio_poll() -> ... -> qmp_device_add() -> drain_call_rcu() then the BQL is dropped. See https://bugzilla.redhat.com/show_bug.cgi?id=2215192 for a concrete bug of this type. Another limitation of drain_call_rcu() is that it cannot be invoked within an RCU read-side critical section since the reclamation phase cannot complete until the end of the critical section. Unfortunately, call stacks have been seen where this happens (see https://bugzilla.redhat.com/show_bug.cgi?id=2214985). This patch series introduces drain_call_rcu_co(), which does the same thing as drain_call_rcu() but asynchronously. By yielding back to the event loop we can wait until the caller drops the BQL and leaves its RCU read-side critical section. Patch 1 changes HMP so that coroutine monitor commands yield back to the event loop instead of running inside a nested event loop. Patch 2 introduces the new drain_call_rcu_co() API. Patch 3 converts qmp_device_add() into a coroutine monitor command and uses drain_call_rcu_co(). I'm sending this as an RFC because I don't have confirmation yet that the bugs mentioned above are fixed by this patch series. Stefan Hajnoczi (3): hmp: avoid the nested event loop in handle_hmp_command() rcu: add drain_call_rcu_co() API qmp: make qmp_device_add() a coroutine MAINTAINERS | 2 ++ docs/devel/rcu.txt | 21 ++++++++++++++++ qapi/qdev.json | 1 + include/monitor/qdev.h | 3 ++- include/qemu/rcu.h | 1 + util/rcu-internal.h | 8 ++++++ monitor/hmp.c | 28 +++++++++++---------- monitor/qmp-cmds.c | 2 +- softmmu/qdev-monitor.c | 34 +++++++++++++++++++++++--- util/rcu-co.c | 55 ++++++++++++++++++++++++++++++++++++++++++ util/rcu.c | 3 ++- hmp-commands.hx | 1 + util/meson.build | 2 +- 13 files changed, 140 insertions(+), 21 deletions(-) create mode 100644 util/rcu-internal.h create mode 100644 util/rcu-co.c