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 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 B58E8C77B7D for ; Wed, 17 May 2023 16:08:24 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pzJgs-0007uJ-Kr; Wed, 17 May 2023 12:07:50 -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 1pzJgq-0007tv-LS for qemu-devel@nongnu.org; Wed, 17 May 2023 12:07:48 -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 1pzJgp-00013F-4S for qemu-devel@nongnu.org; Wed, 17 May 2023 12:07:48 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1684339666; 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=09SQ9bOlirhw4bPHOjDqwW0d3DjSvHx0yW6Tc+IoEZU=; b=OdJnHDExz7J+KnXKn5JRG4yGZtRoH+Z3Gn83Gov/Gwtdl97ZDWfL4wO8V8jGLZ9Tp0HEGG hCct6e/7atiR5iNV4zDzCfneZQU8Fa4iJQ0px117cADYS9/BzSCMcgpsuVabHZgMjlJY/D whdgrsZyZP/ybb0ZrQ26zd8I2G4y3ek= Received: from mimecast-mx02.redhat.com (mx3-rdu2.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-396-v4atq87mORWxTDQKi3aviw-1; Wed, 17 May 2023 12:07:43 -0400 X-MC-Unique: v4atq87mORWxTDQKi3aviw-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 6160A2A5955C; Wed, 17 May 2023 16:07:42 +0000 (UTC) Received: from redhat.com (unknown [10.39.194.19]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8FE19483EC2; Wed, 17 May 2023 16:07:41 +0000 (UTC) Date: Wed, 17 May 2023 18:07:40 +0200 From: Kevin Wolf To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, Fam Zheng , Paolo Bonzini , qemu-block@nongnu.org Subject: Re: [PATCH 0/2] aio-posix: do not nest poll handlers Message-ID: References: <20230502184134.534703-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230502184134.534703-1-stefanha@redhat.com> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@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_H2=-0.001, 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 02.05.2023 um 20:41 hat Stefan Hajnoczi geschrieben: > The following stack exhaustion was reported in > https://bugzilla.redhat.com/show_bug.cgi?id=2186181: > > ... > #51 0x000055884fca7451 aio_poll (qemu-kvm + 0x9d6451) > #52 0x000055884fab9cbd bdrv_poll_co (qemu-kvm + 0x7e8cbd) > #53 0x000055884fab654b blk_io_plug (qemu-kvm + 0x7e554b) > #54 0x000055884f927fef virtio_blk_handle_vq (qemu-kvm + 0x656fef) > #55 0x000055884f96d384 virtio_queue_host_notifier_aio_poll_ready (qemu-kvm + 0x69c384) > #56 0x000055884fca671b aio_dispatch_handler (qemu-kvm + 0x9d571b) > #57 0x000055884fca7451 aio_poll (qemu-kvm + 0x9d6451) > #58 0x000055884fab9cbd bdrv_poll_co (qemu-kvm + 0x7e8cbd) > #59 0x000055884fab654b blk_io_plug (qemu-kvm + 0x7e554b) > #60 0x000055884f927fef virtio_blk_handle_vq (qemu-kvm + 0x656fef) > #61 0x000055884f96d384 virtio_queue_host_notifier_aio_poll_ready (qemu-kvm + 0x69c384) > #62 0x000055884fca671b aio_dispatch_handler (qemu-kvm + 0x9d571b) > #63 0x000055884fca7451 aio_poll (qemu-kvm + 0x9d6451) > ... > > This happens because some block layer APIs in QEMU 8.0 run in coroutines in > order to take the graph rdlock. Existing virtqueue handler functions weren't > written with this in mind. > > A simplified example of the problem is: > > void my_fd_handler(void *opaque) > { > do_something(); > event_notifier_test_and_clear(opaque); > do_something_else(); > } > > When do_something() calls aio_poll(), my_fd_handler() will be entered again > immediately because the fd is still readable and stack exhaustion will occur. > > When do_something_else() calls aio_poll(), there is no stack exhaustion since > the event notifier has been cleared and the fd is not readable. > > The actual bug is more involved. The handler in question is a poll handler, not > an fd handler, but the principle is the same. > > I haven't been able to reproduce the bug, but I have included a test case that > demonstrates the problem. Thanks, applied to the block branch. Kevin