From patchwork Mon Feb 11 05:50:32 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Stefan Hajnoczi X-Patchwork-Id: 1039649 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=nongnu.org (client-ip=209.51.188.17; helo=lists.gnu.org; envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org; receiver=) Authentication-Results: ozlabs.org; dmarc=fail (p=none dis=none) header.from=redhat.com Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 43yZps305vz9s5c for ; Mon, 11 Feb 2019 16:58:45 +1100 (AEDT) Received: from localhost ([127.0.0.1]:45014 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gt4c7-0007bK-AX for incoming@patchwork.ozlabs.org; Mon, 11 Feb 2019 00:58:43 -0500 Received: from eggs.gnu.org ([209.51.188.92]:51333) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gt4aP-0006ku-JG for qemu-devel@nongnu.org; Mon, 11 Feb 2019 00:56:59 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gt4aO-0005RU-IM for qemu-devel@nongnu.org; Mon, 11 Feb 2019 00:56:57 -0500 Received: from mx1.redhat.com ([209.132.183.28]:50618) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gt4aM-0005Ot-9g; Mon, 11 Feb 2019 00:56:54 -0500 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 514B0C04959F; Mon, 11 Feb 2019 05:50:53 +0000 (UTC) Received: from localhost (unknown [10.64.242.44]) by smtp.corp.redhat.com (Postfix) with ESMTP id 5D9CF600C3; Mon, 11 Feb 2019 05:50:48 +0000 (UTC) From: Stefan Hajnoczi To: qemu-devel@nongnu.org Date: Mon, 11 Feb 2019 13:50:32 +0800 Message-Id: <20190211055040.13528-2-stefanha@redhat.com> In-Reply-To: <20190211055040.13528-1-stefanha@redhat.com> References: <20190211055040.13528-1-stefanha@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Mon, 11 Feb 2019 05:50:53 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [PULL 1/9] iothread: fix iothread hang when stop too soon 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 , Peter Maydell , Thomas Huth , qemu-block@nongnu.org, "Michael S. Tsirkin" , Markus Armbruster , "Dr . David Alan Gilbert" , Peter Xu , Max Reitz , Stefan Hajnoczi , =?utf-8?b?THVrw6HFoSBEb2t0b3I=?= , Paolo Bonzini , Kevin Wolf , Eduardo Habkost Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" From: Peter Xu Lukas reported an hard to reproduce QMP iothread hang on s390 that QEMU might hang at pthread_join() of the QMP monitor iothread before quitting: Thread 1 #0 0x000003ffad10932c in pthread_join #1 0x0000000109e95750 in qemu_thread_join at /home/thuth/devel/qemu/util/qemu-thread-posix.c:570 #2 0x0000000109c95a1c in iothread_stop #3 0x0000000109bb0874 in monitor_cleanup #4 0x0000000109b55042 in main While the iothread is still in the main loop: Thread 4 #0 0x000003ffad0010e4 in ?? #1 0x000003ffad553958 in g_main_context_iterate.isra.19 #2 0x000003ffad553d90 in g_main_loop_run #3 0x0000000109c9585a in iothread_run at /home/thuth/devel/qemu/iothread.c:74 #4 0x0000000109e94752 in qemu_thread_start at /home/thuth/devel/qemu/util/qemu-thread-posix.c:502 #5 0x000003ffad10825a in start_thread #6 0x000003ffad00dcf2 in thread_start IMHO it's because there's a race between the main thread and iothread when stopping the thread in following sequence: main thread iothread =========== ============== aio_poll() iothread_get_g_main_context set iothread->worker_context iothread_stop schedule iothread_stop_bh execute iothread_stop_bh [1] set iothread->running=false (since main_loop==NULL so skip to quit main loop. Note: although main_loop is NULL but worker_context is not!) atomic_read(&iothread->worker_context) [2] create main_loop object g_main_loop_run() [3] pthread_join() [4] We can see that when execute iothread_stop_bh() at [1] it's possible that main_loop is still NULL because it's only created until the first check of the worker_context later at [2]. Then the iothread will hang in the main loop [3] and it'll starve the main thread too [4]. Here the simple solution should be that we check again the "running" variable before check against worker_context. CC: Thomas Huth CC: Dr. David Alan Gilbert CC: Stefan Hajnoczi CC: Lukáš Doktor CC: Markus Armbruster CC: Eric Blake CC: Paolo Bonzini Reported-by: Lukáš Doktor Signed-off-by: Peter Xu Tested-by: Thomas Huth Message-id: 20190129051432.22023-1-peterx@redhat.com Signed-off-by: Stefan Hajnoczi --- iothread.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/iothread.c b/iothread.c index 2fb1cdf55d..e615b7ae52 100644 --- a/iothread.c +++ b/iothread.c @@ -63,7 +63,11 @@ static void *iothread_run(void *opaque) while (iothread->running) { aio_poll(iothread->ctx, true); - if (atomic_read(&iothread->worker_context)) { + /* + * We must check the running state again in case it was + * changed in previous aio_poll() + */ + if (iothread->running && atomic_read(&iothread->worker_context)) { GMainLoop *loop; g_main_context_push_thread_default(iothread->worker_context);