Am 11.11.2025 um 15:43 hat Daniel P. Berrangé geschrieben: > On Wed, Nov 05, 2025 at 03:57:29PM -0600, Eric Blake wrote: > > On Tue, Nov 04, 2025 at 11:13:48AM +0000, Daniel P. Berrangé wrote: > > > On Mon, Nov 03, 2025 at 02:10:57PM -0600, Eric Blake wrote: > > > > The point of QIONetListener is to allow a server to listen to more > > > > than one socket address at a time, and respond to clients in a > > > > first-come-first-serve order across any of those addresses. While > > > > some servers (like NBD) really do want to serve multiple simultaneous > > > > clients, many other servers only care about the first client to > > > > connect, and will immediately deregister the callback, possibly by > > > > dropping their reference to the QIONetListener. The existing code > > > > ensures that all other pending callbacks remain safe once the first > > > > callback drops the listener, by adding an extra reference to the > > > > listener for each GSource created, where those references pair to the > > > > eventual teardown of each GSource after a given callbacks has been > > > > serviced or aborted. But it is equally acceptable to hoist the > > > > reference to the listener outside the loop - as long as there is a > > > > callback function registered, it is sufficient to have a single > > > > reference live for the entire array of sioc, rather than one reference > > > > per sioc in the array. > > > > > > > > Hoisting the reference like this will make it easier for an upcoming > > > > patch to still ensure the listener cannot be prematurely garbage > > > > collected during the user's callback, even when the callback no longer > > > > uses a per-sioc GSource. > > > > > > It isn't quite this simple. Glib reference counts the callback > > > func / data, holding a reference when dispatching the callback. > > > > > > IOW, even if the GSource is unrefed, the callback 'notify' > > > function won't be called if the main loop is in the process > > > of dispatching. > > > > I'm not sure I follow your argument. Glib holds a reference on the > > GSource object, not on the opaque data that is handed to the GSource. > > It is possible to use g_source_set_callback_indirect() where GSource > > can learn how to use the same reference counting on data as external > > code, by the use of function pointers for ref and unref, but QIO uses > > merely g_source_set_callback(). > > https://gitlab.gnome.org/GNOME/glib/-/blob/main/glib/gmain.c#L1844 > > shows that glib then wraps that opaque pointer into an internal > > GSourceCallback object which itself is reference counted, so that the > > notify function is not called until the GSource is finalized, but that > > is reference counting on the container, not on the opaque object > > itself (which in this patch is the QIONetListener). > > > > > > > > With this change, the reference on 'listener' can now be > > > released even if the callback is currently dispatching. > > > > So if I'm understanding your concern, you're worried that the unwatch > > code can finish looping through the g_source_destroy and then reach > > the point where it unrefs listener, but that a late-breaking client > > connection can trigger a callback that can still be executing in > > another thread/coroutine after the listener is unref'ed but before the > > GSource has been finalized? If so, would squashing this in fix the > > problem you are seeing? > > Consider the following scenario, where we have two threads, one > is calling QIONetListener APIs, and one is the event thread. > > In the current code, when we unref() the GSource for the socket > watch, the destroy-notify does not get called, because the > event thread is in the middle of a dispatch callback for the > I/O event. When the dispatch callback returns control to the > event loop, the GSourceCallback is unrefed, and this triggers > the destroy-notify call, which unrefs the listener. > > The flow looks like this: > > Thread 1: > qio_net_listener_set_client_func(lstnr, f, ...); > => foreach sock: socket > => object_ref(lstnr) > => sock_src = qio_channel_socket_add_watch_source(sock, ...., > lstnr, object_unref); > > Thread 2: > poll() > => event POLLIN on socket > => ref(GSourceCallback) > => call dispatch(sock) > ...do stuff.. > > Thread 1: > qio_net_listener_set_client_func(lstnr, NULL, ...); > => foreach sock: socket > => g_source_unref(sock_src) > unref(lstnr) (the final reference) > => finalize(lstnr) > > Thread 2: > => return dispatch(sock) > => unref(GSourceCallback) > => destroy-notify > => object_unref > > > > > diff --git i/io/net-listener.c w/io/net-listener.c > > index 9f4e3c0be0c..1fcbbeb7a76 100644 > > --- i/io/net-listener.c > > +++ w/io/net-listener.c > > @@ -67,8 +67,10 @@ static void qio_net_listener_aio_func(void *opaque) > > { > > QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(opaque); > > > > + object_ref(OBJECT(sioc->listener)); > > qio_net_listener_channel_func(QIO_CHANNEL(sioc), G_IO_IN, > > sioc->listener); > > + object_unref(OBJECT(sioc->listener)); > > } > > Now consider this patch, plus this extra hunk... > > Thread 1: > qio_net_listener_set_client_func(lstnr, f, ...); > => object_ref(listener) > => foreach sock: socket > => sock_src = qio_channel_socket_add_watch_source(sock, ...., > lstnr, NULL); > > Thread 2: > poll() > => event POLLIN on socket > => call dispatch(sock) > => ref(lstnr) > ...do stuff.. > > Thread 1: > qio_net_listener_set_client_func(lstnr, NULL, ...); > => foreach sock: socket > => g_source_unref(sock_src) > => object_unref(listener) > unref(lstnr) (still 1 reference left) > > Thread 2: > => unref(lstnr) (the final reference) > => finalize(lstnr) > => return dispatch(sock) > => unref(GSourceCallback) > > > That appears to work ok, however, there's still a race window that is > not solved. Between the time thread 2 sees POLLIN, and when it calls > the dispatch(sock) function, it is possible that thread 1 will drop > the last reference: > > > > Thread 1: > qio_net_listener_set_client_func(lstnr, f, ...); > => object_ref(listener) > => foreach sock: socket > => sock_src = qio_channel_socket_add_watch_source(sock, ...., > lstnr, NULL); > > Thread 2: > poll() > => event POLLIN on socket > > Thread 1: > qio_net_listener_set_client_func(lstnr, NULL, ...); > => foreach sock: socket > => g_source_unref(sock_src) > => object_unref(listener) > unref(lstnr) (still 1 reference left)
Is what you're worried about that there is still a reference left in the opaque pointer of an fd handler, but it's not in the refcount and therefore this already frees the listener while thread 2 will still access it? > > Thread 2: > => call dispatch(sock) > => ref(lstnr) > ...do stuff.. > => unref(lstnr) (the final reference) > => finalize(lstnr) > => return dispatch(sock) > => unref(GSourceCallback) > > > I don't see a way to solve this without synchronization with the event > loop for releasing the reference on the opaque data for the dispatcher > callback. That's what the current code does, but I'm seeing no way for > the AioContext event loop callbacks to have anything equivalent. This > feels like a gap in the AioContext design. I think the way you would normally do this is schedule a BH in thread 2 to do the critical work. If you delete the fd handler and unref the listener in thread 2, then there is no race. But maybe adding a callback for node deletion in AioHandler wouldn't hurt because the opaque pointer pretty much always references something and doing an unref when deleting the AioHandler should be a pretty common pattern. > This is admittedly an incredibly hard to trigger race condition. It would > need a client to be calling a QMP command that tears down the NBD server, > at the exact same time as a new NBD client was incoming. Or the same kind > of scenario for other pieces of QEMU code using QIONetListener. This still > makes me worried though, as rare races have a habit of hitting QEMU > eventually. Aren't both QMP and incoming NBD connections always handled in the main thread? I'm not sure if I know a case where we would actually get this pattern with two different threads today. Of course, that doesn't mean that we couldn't get it in the future. Kevin
