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=-10.0 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham 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 70093C433B4 for ; Thu, 29 Apr 2021 17:27:44 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 09318608FE for ; Thu, 29 Apr 2021 17:27:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 09318608FE Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:37602 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lcARz-0003De-0V for qemu-devel@archiver.kernel.org; Thu, 29 Apr 2021 13:27:43 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60902) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lcAIS-0003o4-8T for qemu-devel@nongnu.org; Thu, 29 Apr 2021 13:17:52 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:37294) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lcAIP-0003WR-DB for qemu-devel@nongnu.org; Thu, 29 Apr 2021 13:17:51 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1619716666; 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: in-reply-to:in-reply-to:references:references; bh=9f0rZVQf1diPevuokX4ABEJ2HpC9lZbwiTJDjh6jHao=; b=h4tK/836y9jrQE7HmyUzf4pElDB7Kg1IDk2VtDkVzt6nuqn0DUo5dQnY5VlVtV8LZW4jMQ bymtsnzdvDeZY3EoE5W7HkuXheLyniu6ypx5X/iEnaPFbhFQNSUCTagk+oC+I36ws5QD3u NM67g7rKPuYcEEBgM8dzG4aSjztip78= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-294-J8uFrdAcOMqi0IiJ3tjV9g-1; Thu, 29 Apr 2021 13:17:42 -0400 X-MC-Unique: J8uFrdAcOMqi0IiJ3tjV9g-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 53BB519253C5; Thu, 29 Apr 2021 17:17:41 +0000 (UTC) Received: from localhost (ovpn-114-114.ams2.redhat.com [10.36.114.114]) by smtp.corp.redhat.com (Postfix) with ESMTP id C35AA4EF63; Thu, 29 Apr 2021 17:17:33 +0000 (UTC) Date: Thu, 29 Apr 2021 18:17:32 +0100 From: "Richard W.M. Jones" To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Subject: libnbd thread-local errors and dlclose (was: Re: [ANNOUNCE] libblkio v0.1.0 preview release) Message-ID: <20210429171732.GX26415@redhat.com> References: <20210429142259.GR26415@redhat.com> <20210429150038.GT26415@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=rjones@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=rjones@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.22, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , pkrempa@redhat.com, Alberto Garcia , slp@redhat.com, qemu-block@nongnu.org, Markus Armbruster , qemu-devel@nongnu.org, mreitz@redhat.com, Vladimir Sementsov-Ogievskiy , Stefan Hajnoczi , Klaus Jensen , philmd@redhat.com, sgarzare@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Apr 29, 2021 at 04:08:22PM +0100, Daniel P. Berrangé wrote: > On Thu, Apr 29, 2021 at 04:00:38PM +0100, Richard W.M. Jones wrote: > > On Thu, Apr 29, 2021 at 03:41:29PM +0100, Stefan Hajnoczi wrote: > > > On Thu, Apr 29, 2021 at 03:22:59PM +0100, Richard W.M. Jones wrote: > > > > libvirt originally, and now libnbd, keep a per-thread error message > > > > (stored in thread-local storage). It's a lot nicer than having to > > > > pass &errmsg to every function. You can just write: > > > > > > > > if (nbd_connect_tcp (nbd, "remote", "nbd") == -1) { > > > > fprintf (stderr, > > > > "failed to connect to remote server: %s (errno = %d)\n", > > > > nbd_get_error (), nbd_get_errno ()); > > > > exit (EXIT_FAILURE); > > > > } > > > > > > > > (https://libguestfs.org/libnbd.3.html#ERROR-HANDLING) > > > > > > > > It means you can extend the range of error information available in > > > > future. Also you can return a 'const char *' and the application > > > > doesn't have to worry about lifetimes, at least in the common case. > > > > > > Thanks for sharing the idea, I think it would work well for libblkio > > > too. > > > > > > Do you ignore the dlclose(3) memory leak? > > > > I believe this mechanism in libnbd ensures there is no leak in the > > ordinary shared library (not dlopen/dlclose) case: > > > > https://gitlab.com/nbdkit/libnbd/-/blob/f9257a9fdc68706a4079deb4ced61e1d468f28d6/lib/errors.c#L35 > > > > However I'm not sure what happens in the dlopen case, so I'm going to > > test that out now ... > > pthread_key destructors are a disaster waiting to happen with > dlclose, if the dlclose happens while non-main threads are > still running. When those threads exit, they'll run the > destructor which points to a function that is no longer > resident in memory. While libnbd had a memory leak there, now fixed by: https://gitlab.com/nbdkit/libnbd/-/commit/026d281c57dd95485cc9bf829918b5efd9e32ddb I don't actually think we have the bug you're describing. We have a destructor (errors_free()) which runs on dlclose, which deletes the thread-local storage key, so the destructor will not run after the library has been unloaded. I added a test for this which works fine for me: https://gitlab.com/nbdkit/libnbd/-/commit/831d142787aba4f5b638418e02cf7e0f2a051565 Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org