On 27/02/2025 23.06, Eric Blake wrote:
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 <th...@redhat.com> Signed-off-by: Eric Blake <ebl...@redhat.com> --- qemu-nbd.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) 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();
This also sounds like the best option to me! Reviewed-by: Thomas Huth <th...@redhat.com>
