From 6c95363d97087e665748cf4d42fafdeb6f714e53 Mon Sep 17 00:00:00 2001 From: Peter Xu Date: Tue, 29 Jan 2019 13:14:32 +0800 Subject: [PATCH] iothread: fix iothread hang when stop too soon MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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);