From nobody Thu Apr 3 11:34:00 2025 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=pass(p=none dis=none) header.from=redhat.com ARC-Seal: i=1; a=rsa-sha256; t=1741216012; cv=none; d=zohomail.com; s=zohoarc; b=eaTRlOVYWZ+byE/e+7gZkPOVohfC6j2qLdGCUVZPwsI45AFc6xvDPtCmHTZGHCfAGcCEfMEFoUflrNPS0aPOKClfI9WE/bAyEPtf1TjMn8UV8w9fz7weN6mm5vSHnB0tZJ0JUTl7aDJTIIBfgn9RT3MNb3eSr0cS9r49DK/3CSs= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=zohomail.com; s=zohoarc; t=1741216012; h=Content-Type:Content-Transfer-Encoding:Cc:Cc:Date:Date:From:From:In-Reply-To:List-Subscribe:List-Post:List-Id:List-Archive:List-Help:List-Unsubscribe:MIME-Version:Message-ID:References:Sender:Subject:Subject:To:To:Message-Id:Reply-To; bh=i9l/lI8uB4ISWuy9MK5PFCYsUvJZEkpx1O7spZoc6BU=; b=aBpa8V4bXC2iZ6L518diBcHddPoKKMn2R8ls8kH2NiqQmtq/VQ5AKo3HQze2upItfiddAvO1SVugw9KJSI8xZ7vyKOtuM+8dnLzvyIxMop49cwdzt0M0lBji0oOaKlCHPTeoN3F4CQl0rydx0U2wgz91xL8/T5jo9MmhKKR2BLw= ARC-Authentication-Results: i=1; mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=pass header.from= (p=none dis=none) Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1741216012311305.5838641446975; Wed, 5 Mar 2025 15:06:52 -0800 (PST) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1tpxoS-0007KZ-4q; Wed, 05 Mar 2025 18:06:04 -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 1tpxoQ-0007Hg-9f for qemu-devel@nongnu.org; Wed, 05 Mar 2025 18:06:02 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1tpxoO-0000Cq-90 for qemu-devel@nongnu.org; Wed, 05 Mar 2025 18:06:02 -0500 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-271-yU595C9SOA2_9z-j9OTpWA-1; Wed, 05 Mar 2025 18:05:56 -0500 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (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 A717B1801A03; Wed, 5 Mar 2025 23:05:54 +0000 (UTC) Received: from green.redhat.com (unknown [10.2.16.105]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 726E31955DCE; Wed, 5 Mar 2025 23:05:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1741215959; 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=i9l/lI8uB4ISWuy9MK5PFCYsUvJZEkpx1O7spZoc6BU=; b=fbvozqHw7wzdR2qB1/d4hGdNaGXDvqJWv1GVLVh7v7kKBZnxLfKqZv6Kwxew9Uc1+b4fPq lwn0lqmEnhujV6J5McXcn1R0gfawjgJi7mjcc2KlhN8MeWgCY7Fqzr1IGBp0KgI+W47wP5 gCT5/y7zEYcGrFBBhmUhkCV7QfnUC7A= X-MC-Unique: yU595C9SOA2_9z-j9OTpWA-1 X-Mimecast-MFC-AGG-ID: yU595C9SOA2_9z-j9OTpWA_1741215955 From: Eric Blake To: qemu-devel@nongnu.org Cc: Thomas Huth , Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org (open list:Network Block Dev...) Subject: [PULL 4/4] nbd: Defer trace init until after daemonization Date: Wed, 5 Mar 2025 17:05:24 -0600 Message-ID: <20250305230542.2225013-10-eblake@redhat.com> In-Reply-To: <20250305230542.2225013-6-eblake@redhat.com> References: <20250305230542.2225013-6-eblake@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=eblake@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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 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+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1741216014567019000 Content-Type: text/plain; charset="utf-8" At least the simple trace backend works by spawning a helper thread, and setting up an atexit() handler that coordinates completion with the helper thread. But since atexit registrations survive fork() but helper threads do not, this means that qemu-nbd configured to use the simple trace will deadlock waiting for a thread that no longer exists when it has daemonized. Better is to follow the example of vl.c: don't call any setup functions that might spawn helper threads until we are in the final process that will be doing the work worth tracing. Tested by configuring with --enable-trace-backends=3Dsimple, then running qemu-nbd --fork --trace=3Dnbd_\*,file=3Dqemu-nbd.trace -f raw -r README.r= st followed by `nbdinfo nbd://localhost`, and observing that the trace file is now created without hanging. Reported-by: Thomas Huth Signed-off-by: Eric Blake Message-ID: <20250227220625.870246-2-eblake@redhat.com> Reviewed-by: Thomas Huth --- qemu-nbd.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) diff --git a/qemu-nbd.c b/qemu-nbd.c index 05b61da51ea..ed5895861bb 100644 --- a/qemu-nbd.c +++ b/qemu-nbd.c @@ -852,10 +852,6 @@ int main(int argc, char **argv) export_name =3D ""; } - if (!trace_init_backends()) { - exit(1); - } - trace_init_file(); qemu_set_log(LOG_TRACE, &error_fatal); socket_activation =3D check_socket_activation(); @@ -1045,6 +1041,18 @@ int main(int argc, char **argv) #endif /* WIN32 */ } + /* + * trace_init must be done after daemonization. Why? Because at + * least the simple backend spins up a helper thread as well as an + * atexit() handler that waits on that thread, but the helper + * thread won't survive a fork, leading to deadlock in the child + * if we initialized pre-fork. + */ + if (!trace_init_backends()) { + exit(1); + } + trace_init_file(); + if (opts.device !=3D NULL && sockpath =3D=3D NULL) { sockpath =3D g_malloc(128); snprintf(sockpath, 128, SOCKET_PATH, basename(opts.device)); --=20 2.48.1