On Tue, Nov 11, 2025 at 11:01:44PM +0800, Jie Song wrote: > From: Jie Song <[email protected]> > > When starting a dummy QEMU process with virsh, monitor_init_qmp() enables > IOThread monitoring of the QMP fd by default. However, a race condition > exists during the initialization phase: the IOThread only removes the > main thread's fd watch when it reaches > qio_net_listener_set_client_func_full(), > which may be delayed under high system load. > > This creates a window between monitor_qmp_setup_handlers_bh() and > qio_net_listener_set_client_func_full() where both the main thread and > IOThread are simultaneously monitoring the same fd and processing events. > This race can cause either the main thread or the IOThread to hang and > become unresponsive. > > Fix this by proactively cleaning up the listener's IO sources in > monitor_init_qmp() before the IOThread initializes QMP monitoring, > ensuring exclusive fd ownership and eliminating the race condition. > > The fix introduces socket_chr_listener_cleanup() to destroy and unref > all existing IO sources on the socket chardev listener, guaranteeing > that no concurrent fd monitoring occurs during the transition to > IOThread handling. > > Signed-off-by: Jie Song <[email protected]> > --- > chardev/char-socket.c | 18 ++++++++++++++++++ > include/chardev/char-socket.h | 2 ++ > monitor/qmp.c | 6 ++++++ > 3 files changed, 26 insertions(+) > > diff --git a/chardev/char-socket.c b/chardev/char-socket.c > index 62852e3caf..073a9da855 100644 > --- a/chardev/char-socket.c > +++ b/chardev/char-socket.c > @@ -656,6 +656,24 @@ static void tcp_chr_telnet_destroy(SocketChardev *s) > } > } > > +void socket_chr_listener_cleanup(Chardev *chr) > +{ > + SocketChardev *s = SOCKET_CHARDEV(chr); > + > + if (s->listener) { > + QIONetListener *listener = s->listener; > + size_t i; > + > + for (i = 0; i < listener->nsioc; i++) {
This directly accesses listener->nsioc outside of net-listener.c. I've got a pending patch that frowns on this type of usage (here's the link to v2; v3 is coming soon): https://lore.kernel.org/qemu-devel/[email protected]/T/#m69a13da54c24ad55351b6a004ec1c0cba7a7b49c But it might be possible to do what you want without peeking inside the listener; have you tested calling qio_net_listener_set_client_func_full() to change the callback to NULL prior to doing the handover to iothread, and then reregistering tcp_chr_accept after that point? -- Eric Blake, Principal Software Engineer Red Hat, Inc. Virtualization: qemu.org | libguestfs.org
