diff options
authorPeter Maydell <peter.maydell@linaro.org>2018-11-02 18:47:47 +0000
committerPeter Maydell <peter.maydell@linaro.org>2018-11-05 13:15:05 +0000
commit0ee664061743eaf02845077757ad9490acdc489f (patch)
parent18df5062286ba245e86596953938338511a3f79c (diff)
util/qemu-thread-posix: Fix qemu_thread_atexit* for OSXfix-thread-atexit
Our current implementation of qemu_thread_atexit* is broken on OSX. This is because it works by cerating a piece of thread-specific data with pthread_key_create() and using the destructor function for that data to run the notifier function passed to it by the caller of qemu_thread_atexit_add(). The expected use case is that the caller uses a __thread variable as the notifier, and uses the callback to clean up information that it is keeping per-thread in __thread variables. Unfortunately, on OSX this does not work, because on OSX a __thread variable may be destroyed (freed) before the pthread_key_create() destructor runs. (POSIX imposes no ordering constraint here; the OSX implementation happens to implement __thread variables in terms of pthread_key_create((), whereas Linux uses different mechanisms that mean the __thread variables will still be present when the pthread_key_create() destructor is run.) Fix this by switching to a scheme similar to the one qemu-thread-win32 uses for qemu_thread_atexit: keep the thread's notifiers on a __thread variable, and run the notifiers on calls to qemu_thread_exit() and on return from the start routine passed to qemu_thread_start(). We do this with the pthread_cleanup_push() API. We take advantage of the qemu_thread_atexit_add() API permission not to run thread notifiers on process exit to avoid having to special case the main thread. Suggested-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Peter Maydell <peter.maydell@linaro.org> --- qemu-thread-win32.c tries to handle the "main thread" case by using an atexit() handler to run its notifiers. I don't think this will work because the atexit handler may not run on the main thread, in which case notify callback functions which refer to __thread variables will get the wrong ones.
1 files changed, 20 insertions, 24 deletions
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index dfa66ff2fb..865e476df5 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -443,42 +443,34 @@ void qemu_event_wait(QemuEvent *ev)
-static pthread_key_t exit_key;
-union NotifierThreadData {
- void *ptr;
- NotifierList list;
-QEMU_BUILD_BUG_ON(sizeof(union NotifierThreadData) != sizeof(void *));
+static __thread NotifierList thread_exit;
+ * Note that in this implementation you can register a thread-exit
+ * notifier for the main thread, but it will never be called.
+ * This is OK because main thread exit can only happen when the
+ * entire process is exiting, and the API allows notifiers to not
+ * be called on process exit.
+ */
void qemu_thread_atexit_add(Notifier *notifier)
- union NotifierThreadData ntd;
- ntd.ptr = pthread_getspecific(exit_key);
- notifier_list_add(&ntd.list, notifier);
- pthread_setspecific(exit_key, ntd.ptr);
+ notifier_list_add(&thread_exit, notifier);
void qemu_thread_atexit_remove(Notifier *notifier)
- union NotifierThreadData ntd;
- ntd.ptr = pthread_getspecific(exit_key);
- pthread_setspecific(exit_key, ntd.ptr);
-static void qemu_thread_atexit_run(void *arg)
- union NotifierThreadData ntd = { .ptr = arg };
- notifier_list_notify(&ntd.list, NULL);
-static void __attribute__((constructor)) qemu_thread_atexit_init(void)
+static void qemu_thread_atexit_notify(void *arg)
- pthread_key_create(&exit_key, qemu_thread_atexit_run);
+ /*
+ * Called when non-main thread exits (via qemu_thread_exit()
+ * or by returning from its start routine.)
+ */
+ notifier_list_notify(&thread_exit, NULL);
typedef struct {
void *(*start_routine)(void *);
void *arg;
@@ -490,6 +482,7 @@ static void *qemu_thread_start(void *args)
QemuThreadArgs *qemu_thread_args = args;
void *(*start_routine)(void *) = qemu_thread_args->start_routine;
void *arg = qemu_thread_args->arg;
+ void *r;
/* Attempt to set the threads name; note that this is for debug, so
@@ -501,7 +494,10 @@ static void *qemu_thread_start(void *args)
- return start_routine(arg);
+ pthread_cleanup_push(qemu_thread_atexit_notify, NULL);
+ r = start_routine(arg);
+ pthread_cleanup_pop(1);
+ return r;
void qemu_thread_create(QemuThread *thread, const char *name,