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 7B3D2C021BC for ; Wed, 26 Feb 2025 09:30:40 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1tnDkE-0000Vf-SZ; Wed, 26 Feb 2025 04:30:23 -0500 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 1tnDk7-0000VI-SC for qemu-devel@nongnu.org; Wed, 26 Feb 2025 04:30:16 -0500 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 1tnDk2-0004U7-Qq for qemu-devel@nongnu.org; Wed, 26 Feb 2025 04:30:15 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1740562207; 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=upC73uqGDSad4dgmkmYpSCQglOQkWO24vlQuufBvuZY=; b=dnxxyfV4K9B7RD/RMCoC2ZvoHJqMw1iOCJ7dvLuK7SKBB6RKgGYo1irSxzVIIdmnFcmgJ2 NPHqJdq6L9SGy6SY8DUzJJQmLZyhfz5++1Mb69JQk3UP+zAXxM968DzXdB9vQAt/B+DykO 6kUaV76FjqWRNpfJEXJKE8/ViOQUzQU= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-654-8sIVtFc2O7WQ6aGHImzEvA-1; Wed, 26 Feb 2025 04:30:05 -0500 X-MC-Unique: 8sIVtFc2O7WQ6aGHImzEvA-1 X-Mimecast-MFC-AGG-ID: 8sIVtFc2O7WQ6aGHImzEvA_1740562204 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (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 mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 1FACD1801A15; Wed, 26 Feb 2025 09:30:04 +0000 (UTC) Received: from redhat.com (dhcp-192-195.str.redhat.com [10.33.192.195]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id AD9A81800358; Wed, 26 Feb 2025 09:30:01 +0000 (UTC) Date: Wed, 26 Feb 2025 10:29:59 +0100 From: Kevin Wolf To: Thomas Huth Cc: qemu-devel@nongnu.org, Stefan Hajnoczi , Hanna Reitz , qemu-block@nongnu.org, Eric Blake , Mads Ynddal Subject: Re: [PATCH] trace/simple: Fix hang when using simpletrace with fork() Message-ID: References: <20250226085015.1143991-1-thuth@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250226085015.1143991-1-thuth@redhat.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 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: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.443, 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, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=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 Am 26.02.2025 um 09:50 hat Thomas Huth geschrieben: > When compiling QEMU with --enable-trace-backends=simple , the > iotest 233 is currently hanging. This happens because qemu-nbd > calls trace_init_backends() first - which causes simpletrace to > install its writer thread and the atexit() handler - before > calling fork(). But the simpletrace writer thread is then only > available in the parent process, not in the child process anymore. > Thus when the child process exits, its atexit handler waits forever > on the trace_empty_cond condition to be set by the non-existing > writer thread, so the process never finishes. > > Fix it by installing a pthread_atfork() handler, too, which > makes sure that the trace_writeout_enabled variable gets set > to false again in the child process, so we can use it in the > atexit() handler to check whether we still need to wait on the > writer thread or not. > > Signed-off-by: Thomas Huth I can see how this would fix the hang, but do we actually get the trace events written out somewhere then? Or do we need to make sure that the child process has a writer thread, too? Of course, the question would then be how the two processes writing into the same trace file interact. Kevin