On Sat, Feb 28, 2026 at 03:16:11PM +0000, Peter Maydell wrote: > On Mon, 19 May 2025 at 12:00, Daniel P. Berrangé <[email protected]> wrote: > > > > On Thu, May 15, 2025 at 07:20:14PM -0300, Fabiano Rosas wrote: > > > It's possible for the hup_source to have its reference decremented by > > > remove_hup_source() while it's still being added to the context, > > > leading to asserts in glib: > > > > IIUC this must mean that > > > > tcp_chr_free_connection > > > > is being called concurrently with > > > > update_ioc_handlers > > > > I'm wondering if that is really intended, or a sign of a deeper > > bug that we'll just paper over if we add the mutex proposed here. > > > > Are you able to provide stack traces showing the 2 concurrent > > operations that are triggering this problem ? > > (Pulling up this thread from last year since I ran into something > similar running the sanitizers, and Fabiano pointed me at this series > he'd sent that fixes most/all of the problems.) > > I had a look at this, and although I couldn't get a specific > backtrace, I got enough information from asan leak backtraces > and looking at the code that I think I understand where the race > is here: > > vhost-user-test.c:test_server_create_chr() sets up a chardev with: > qemu_chr_new() > qemu_chr_fe_init() > qemu_chr_fe_set_handlers() > > For the "connect-fail" and "flags-mismatch" tests, it includes in > the options to the chardev "reconnect-ms=1000". > > qemu_chr_new() ends up in tcp_chr_open(), which calls > qmp_chardev_open_socket_client(). > > If reconnect_ms is not 0, qmp_chardev_open_socket_client() calls > tcp_chr_connect_client_async() instead of doing the connect synchronously. > That kicks off a qio_task_new() to do the connecting, and immediately > returns, so we return successfully from qemu_chr_new(). So > test_server_create_chr() will continue execution into qemu_chr_fe_init() > and qemu_chr_fe_set_handlers(). > > qemu_chr_fe_set_handlers() calls qemu_chr_be_update_read_handlers(), which > calls the chr_update_read_handler method for the ChardevClass, which here > is tcp_chr_update_read_handler(). That function calls update_ioc_handlers(). > > Meanwhile, the thread that got kicked off to do the connect is > running in parallel. If the connect succeeds then qemu_chr_socket_conneted() > will call tcp_chr_connect(), which will also call > update_ioc_handlers(). (If the connect fails then presumably we end > up in the tcp_char_disconnect or otherwise destroying the chardev, > though I haven't figured out exactly what happens in this case.) > > None of this appears to be taking any kind of lock while it modifies > the SocketChardev fields, so we could have two threads in > update_ioc_handlers() simultaneously.
It is subtle, as the QIOTask object deals with 2 separate callbacks. The callback that is provided to qio_task_new() is what is invoked when the task is marked as completed. This is guaranteed to always be invoked in main event loop context. The callback that is provided to qio_task_run_in_thread() is what runs in the throwaway background thread. When this callback returns, the QIOTask creates a glib idle source in the main event loop to run the completion callback. So wrt char-socket.c, tcp_chr_connect_client_task will run in the background thread, but it only calls QIOChannel APIs so is safe. The qemu_chr_socket_connected method will run in main event loop context, so it does not need locking wrt other chardev I/O callbacks. > How is this intended to work ? Is the test case code incorrect to assume > it can work with the chardev it got back from qemu_chr_new(), or should > the chardev be handling this case? The test is really rather had to understand, but I feel like something in the test ought to be looking at the CHR_EVENT_OPENED event to identify when the connection is actually established ? With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|
