When using --daemonize, the initial lead process will fork a child and then wait to be notified that setup is complete via a pipe, before it exits. When using --preconfig there is an extra call to main_loop() before the notification is done from os_setup_post(). Thus the parent process won't exit until the mgmt application connects to the monitor and tells QEMU to leave the RUN_STATE_PRECONFIG. The mgmt application won't connect to the monitor until daemonizing has completed though.
This is a chicken and egg problem, leading to deadlock at startup. The only viable way to fix this is to call os_setup_post() before the early main_loop_wait() call when in RUN_STATE_PRECONFIG. This has the downside that any errors from this point onwards won't be handled well by the mgmt application, because it will think QEMU has started successfully, so not be expecting an abrupt exit. The only way to deal with that is to move as much user input validation as possible to before the main_loop() call. This is left as an exercise for future interested developers. Based on: From: Daniel P. Berrangé <berra...@redhat.com> Subject: [PATCH v2 2/2] vl: fix use of --daemonize with --preconfig Message-Id: <20180604120345.12955-3-berra...@redhat.com> Signed-off-by: Igor Mammedov <imamm...@redhat.com> --- v3: - rewrite to apply on top of 1/2 v4: - do not init static boolean to false as it's aready false (Eric Blake <ebl...@redhat.com>) CC: berra...@redhat.com CC: mre...@redhat.com CC: pbonz...@redhat.com CC: ehabk...@redhat.com CC: ldok...@redhat.com CC: ebl...@redhat.com --- os-posix.c | 6 ++++++ vl.c | 2 +- 2 files changed, 7 insertions(+), 1 deletion(-) diff --git a/os-posix.c b/os-posix.c index 9ce6f74..0246195 100644 --- a/os-posix.c +++ b/os-posix.c @@ -309,8 +309,14 @@ void os_daemonize(void) void os_setup_post(void) { + static bool os_setup_post_done; int fd = 0; + if (os_setup_post_done) { + return; + } + os_setup_post_done = true; + if (daemonize) { if (chdir("/")) { error_report("not able to chdir to /: %s", strerror(errno)); diff --git a/vl.c b/vl.c index fa44138..d6fa67f 100644 --- a/vl.c +++ b/vl.c @@ -1960,6 +1960,7 @@ static void main_loop(void) #ifdef CONFIG_PROFILER ti = profile_getclock(); #endif + os_setup_post(); main_loop_wait(false); #ifdef CONFIG_PROFILER dev_time += profile_getclock() - ti; @@ -4707,7 +4708,6 @@ int main(int argc, char **argv, char **envp) } accel_setup_post(current_machine); - os_setup_post(); main_loop(); -- 2.7.4