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authorPeter Xu <peterx@redhat.com>2019-01-29 13:14:32 +0800
committerStefan Hajnoczi <stefanha@redhat.com>2019-02-12 11:49:17 +0800
commit6c95363d97087e665748cf4d42fafdeb6f714e53 (patch)
treeb952ae95c258446160a0fc05b2296f14a2a062e8
parent22c5f446514a2a4bb0dbe1fea26713da92fc85fa (diff)
iothread: fix iothread hang when stop too soon
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 <thuth@redhat.com> CC: Dr. David Alan Gilbert <dgilbert@redhat.com> CC: Stefan Hajnoczi <stefanha@redhat.com> CC: Lukáš Doktor <ldoktor@redhat.com> CC: Markus Armbruster <armbru@redhat.com> CC: Eric Blake <eblake@redhat.com> CC: Paolo Bonzini <pbonzini@redhat.com> Reported-by: Lukáš Doktor <ldoktor@redhat.com> Signed-off-by: Peter Xu <peterx@redhat.com> Tested-by: Thomas Huth <thuth@redhat.com> Message-id: 20190129051432.22023-1-peterx@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
-rw-r--r--iothread.c6
1 files changed, 5 insertions, 1 deletions
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);