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 8895FC19F2E for ; Thu, 27 Feb 2025 07:06:10 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1tnXxs-0006Uy-3O; Thu, 27 Feb 2025 02:05:48 -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 1tnXxp-0006J8-7c for qemu-devel@nongnu.org; Thu, 27 Feb 2025 02:05:45 -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 1tnXxn-0006nb-GX for qemu-devel@nongnu.org; Thu, 27 Feb 2025 02:05:44 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1740639941; 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=vbK1WhZmOcf3gcVh2+VrhIk2EyIIOIP/wZG8mYOXTE4=; b=EKdzZOWnWlQpVmxKXvT10KqwGvWAqAETIdr+JfA8JsNbQDkYeuoBwoVUvw6hn0daYk5083 3DOrZ+nTii8SQ+miFwPjCZnEcE+TtYFb/eirrJfndOgFKz4Q0Y+O6JFxCWY4vBVynb/Bcl P0pVutiUmAWC66M/XiNg44tPNtwXdzk= 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-206-NZhNlWmGPFWoDMPpnSlUAw-1; Thu, 27 Feb 2025 02:05:35 -0500 X-MC-Unique: NZhNlWmGPFWoDMPpnSlUAw-1 X-Mimecast-MFC-AGG-ID: NZhNlWmGPFWoDMPpnSlUAw_1740639932 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 F13CE1801A13; Thu, 27 Feb 2025 07:05:30 +0000 (UTC) Received: from localhost (unknown [10.2.16.46]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id A5CAA1800945; Thu, 27 Feb 2025 07:05:26 +0000 (UTC) Date: Thu, 27 Feb 2025 15:05:23 +0800 From: Stefan Hajnoczi To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Cc: Thomas Huth , qemu-devel@nongnu.org, Kevin Wolf , Hanna Reitz , qemu-block@nongnu.org, Eric Blake , Mads Ynddal , danielhb413@gmail.com Subject: Re: [PATCH] trace/simple: Fix hang when using simpletrace with fork() Message-ID: <20250227070523.GC85709@fedora> References: <20250226085015.1143991-1-thuth@redhat.com> <2a7c4f21-ee27-4407-8191-dd1f0547990c@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="Z5Rz4Crf7PowmNb2" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 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: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.44, 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=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 --Z5Rz4Crf7PowmNb2 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Feb 26, 2025 at 09:53:53AM +0000, Daniel P. Berrang=E9 wrote: > On Wed, Feb 26, 2025 at 10:38:56AM +0100, Thomas Huth wrote: > > On 26/02/2025 10.15, Daniel P. Berrang=E9 wrote: > > > On Wed, Feb 26, 2025 at 09:50:15AM +0100, Thomas Huth wrote: > > > > When compiling QEMU with --enable-trace-backends=3Dsimple , 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. > > > >=20 > > > > 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. > > > >=20 > > > > Signed-off-by: Thomas Huth > > > > --- > > > > trace/simple.c | 17 ++++++++++++++++- > > > > 1 file changed, 16 insertions(+), 1 deletion(-) > > > >=20 > > > > diff --git a/trace/simple.c b/trace/simple.c > > > > index c0aba00cb7f..269bbda69f1 100644 > > > > --- a/trace/simple.c > > > > +++ b/trace/simple.c > > > > @@ -380,8 +380,22 @@ void st_print_trace_file_status(void) > > > > void st_flush_trace_buffer(void) > > > > { > > > > - flush_trace_file(true); > > > > + flush_trace_file(trace_writeout_enabled); > > > > +} > > > > + > > > > +#ifndef _WIN32 > > > > +static void trace_thread_atfork(void) > > > > +{ > > > > + /* > > > > + * If we fork, the writer thread does not exist in the child, = so > > > > + * make sure to allow st_flush_trace_buffer() to clean up corr= ectly. > > > > + */ > > > > + g_mutex_lock(&trace_lock); > > > > + trace_writeout_enabled =3D false; > > > > + g_cond_signal(&trace_empty_cond); > > > > + g_mutex_unlock(&trace_lock); > > > > } > > > > +#endif > > >=20 > > > This doesn't seem right to me. This is being run in the child and whi= le > > > it may avoid the hang when the child exits, surely it still leaves tr= acing > > > non-functional in the child as we're lacking the thread to write out = the > > > trace data. > >=20 > > Well, you cannot write to the same file from the parent and child at the > > same time, so one of both needs to be shut up AFAIU. And the simpletrace > > code cannot now which one of the two processes should be allowed to con= tinue > > with the logging, so we either have to disable tracing in one of the two > > processes, or think of something completely different, e.g. using > > pthread_atfork(abort, NULL, NULL) to make people aware that they are not > > allowed to start tracing before calling fork()...? But in that case we = still > > need a qemu-nbd expert to fix qemu-nbd, so that it does not initialize = the > > trace backend before calling fork(). >=20 > As precedent, in system/vl.c we delay trace_init() until after daemonizing > which is the simple way to avoid the worst of the danger. That sounds good to me. Adding Daniel Henrique Barboza because he fixed a similar issue in commit 10b6ee1616f9 ("vl.c: do not execute trace_init_backends() before daemonizing"). Stefan --Z5Rz4Crf7PowmNb2 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmfADrMACgkQnKSrs4Gr c8ggAAgAww5j9KlaOt6ulTSjVzJUgETqogbybtmFOGE9ryiOlNohRGEsxsETKwvB 4HmTWkvfgBU/vYvVrCB/K9UM/Lz1QAaU8W3U/+vEhcfc3/J/kNfHeVjlvVL0JT6+ jYH80v4JL3ETXwS3r1cPPsniOapeTHHF0hBPrHn+qbgJS2/rbfEhae2n6u6bhCIb tFNL/OZZ7m60nMQ1b79q8hw0hH+ytdubJxK04Ckn2uwSClpZDBH0ied65x0dbdu9 pl7kyzOLLmn+yWXL1hWw4ZTYt2w5MLi9meEwvrE4vAq6uUGCmKH2Evej39yo1fgy PnUIo+PwXPZfDonNkfqxatwSVEsonA== =wbeE -----END PGP SIGNATURE----- --Z5Rz4Crf7PowmNb2--