nbd: Defer trace init until after daemonization

Commit Message

Eric Blake Feb. 27, 2025, 10:06 p.m. UTC

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 <[e-mail]>
Signed-off-by: Eric Blake <[e-mail]>
---
 qemu-nbd.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

Patch

diff --git a/qemu-nbd.c b/qemu-nbd.c
index 05b61da51ea..ed5895861bb 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -852,10 +852,6 @@  int main(int argc, char **argv)
         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 @@  int main(int argc, char **argv)
 #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));