nbd: Defer trace init until after daemonization
At least the simple trace backend works by spawning a helper thread,
and setting up an atexit() handler that coordinates completion with
the helper thread. But since atexit registrations survive fork() but
helper threads do not, this means that qemu-nbd configured to use the
simple trace will deadlock waiting for a thread that no longer exists
when it has daemonized.
Better is to follow the example of vl.c: don't call any setup
functions that might spawn helper threads until we are in the final
process that will be doing the work worth tracing.
Tested by configuring with --enable-trace-backends=simple, then running
qemu-nbd --fork --trace=nbd_\*,file=qemu-nbd.trace -f raw -r README.rst
followed by `nbdinfo nbd://localhost`, and observing that the trace
file is now created without hanging.
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-ID: <20250227220625.870246-2-eblake@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
diff --git a/qemu-nbd.c b/qemu-nbd.c
index 05b61da..ed58958 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -852,10 +852,6 @@
export_name = "";
}
- if (!trace_init_backends()) {
- exit(1);
- }
- trace_init_file();
qemu_set_log(LOG_TRACE, &error_fatal);
socket_activation = check_socket_activation();
@@ -1045,6 +1041,18 @@
#endif /* WIN32 */
}
+ /*
+ * trace_init must be done after daemonization. Why? Because at
+ * least the simple backend spins up a helper thread as well as an
+ * atexit() handler that waits on that thread, but the helper
+ * thread won't survive a fork, leading to deadlock in the child
+ * if we initialized pre-fork.
+ */
+ if (!trace_init_backends()) {
+ exit(1);
+ }
+ trace_init_file();
+
if (opts.device != NULL && sockpath == NULL) {
sockpath = g_malloc(128);
snprintf(sockpath, 128, SOCKET_PATH, basename(opts.device));