From nobody Mon Feb 9 08:29:15 2026 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 15053755786221022.6005061699482; Thu, 14 Sep 2017 00:52:58 -0700 (PDT) Received: from localhost ([::1]:46190 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dsOxF-0007oJ-Rc for importer@patchew.org; Thu, 14 Sep 2017 03:52:57 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:51620) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dsOvK-00063v-Md for qemu-devel@nongnu.org; Thu, 14 Sep 2017 03:50:59 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dsOvJ-0006Sf-Mr for qemu-devel@nongnu.org; Thu, 14 Sep 2017 03:50:58 -0400 Received: from mx1.redhat.com ([209.132.183.28]:37266) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dsOvJ-0006Rs-8O for qemu-devel@nongnu.org; Thu, 14 Sep 2017 03:50:57 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 43B6A5F7B0; Thu, 14 Sep 2017 07:50:56 +0000 (UTC) Received: from pxdev.xzpeter.org.com (dhcp-15-224.nay.redhat.com [10.66.15.224]) by smtp.corp.redhat.com (Postfix) with ESMTP id 638FD60F9F; Thu, 14 Sep 2017 07:50:52 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 43B6A5F7B0 Authentication-Results: ext-mx10.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx10.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=peterx@redhat.com From: Peter Xu To: qemu-devel@nongnu.org Date: Thu, 14 Sep 2017 15:50:22 +0800 Message-Id: <1505375436-28439-2-git-send-email-peterx@redhat.com> In-Reply-To: <1505375436-28439-1-git-send-email-peterx@redhat.com> References: <1505375436-28439-1-git-send-email-peterx@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Thu, 14 Sep 2017 07:50:56 +0000 (UTC) Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Laurent Vivier , Fam Zheng , Juan Quintela , Markus Armbruster , peterx@redhat.com, mdroth@linux.vnet.ibm.com, Stefan Hajnoczi , =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= , Paolo Bonzini , "Dr . David Alan Gilbert" Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Content-Type: text/plain; charset="utf-8" This is not a problem if we are only having one single loop thread like before. However, after per-monitor thread is introduced, this is not true any more, and the race can happen. The race can be triggered with "make check -j8" sometimes: qemu-system-x86_64: /root/git/qemu/chardev/char-io.c:91: io_watch_poll_finalize: Assertion `iwp->src =3D=3D NULL' failed. This patch keeps the reference for the watch object when creating in io_add_watch_poll(), so that the object will never be released in the context main loop, especially when the context loop is running in another standalone thread. Meanwhile, when we want to remove the watch object, we always first detach the watch object from its owner context, then we continue with the cleanup. Without this patch, calling io_remove_watch_poll() in main loop thread is not thread-safe, since the other per-monitor thread may be modifying the watch object at the same time. Reviewed-by: Marc-Andr=C3=A9 Lureau Signed-off-by: Peter Xu --- chardev/char-io.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/chardev/char-io.c b/chardev/char-io.c index f810524..3828c20 100644 --- a/chardev/char-io.c +++ b/chardev/char-io.c @@ -122,7 +122,6 @@ GSource *io_add_watch_poll(Chardev *chr, g_free(name); =20 g_source_attach(&iwp->parent, context); - g_source_unref(&iwp->parent); return (GSource *)iwp; } =20 @@ -131,12 +130,24 @@ static void io_remove_watch_poll(GSource *source) IOWatchPoll *iwp; =20 iwp =3D io_watch_poll_from_source(source); + + /* + * Here the order of destruction really matters. We need to first + * detach the IOWatchPoll object from the context (which may still + * be running in another loop thread), only after that could we + * continue to operate on iwp->src, or there may be race condition + * between current thread and the context loop thread. + * + * Let's blame the glib bug mentioned in commit 2b3167 (again) for + * this extra complexity. + */ + g_source_destroy(&iwp->parent); if (iwp->src) { g_source_destroy(iwp->src); g_source_unref(iwp->src); iwp->src =3D NULL; } - g_source_destroy(&iwp->parent); + g_source_unref(&iwp->parent); } =20 void remove_fd_in_watch(Chardev *chr) --=20 2.7.4