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nbd: Defer trace init until after daemonization

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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>
This commit is contained in:
Eric Blake 2025-02-27 16:06:15 -06:00
parent 57f3962bf1
commit 3e16834856
1 changed files with 12 additions and 4 deletions
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16
qemu-nbd.c
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@ -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));