[systemd-devel] systemd+dbus: system boot stops at terminal login screen sometimes

[Michal Schmidt]

Chen Jie wrote:
> Let me explain it further.
> First syslogv() in dbus may block, while rsyslog.service is starting
> and meanwhile the kernel socket buffer was full.
> Attachment syslog-test.c was a program simulates the situation.
> 
> So while rsyslog.service is starting, on the systemd side:
> 1. [ExecStartPre] stops systemd-kmsg-syslogd.service
> 2. It now running bus_init() to publish its API to dbus
> 3. Step 2 timeout, because dbus-daemon didn't reply in time.
> 
> Why dbus-daemon didn't response quickly? Because it blocked on
> syslogv(), which was waiting for someone consumes the message(the
> kernel socket buffer was full), but sadly the consumer -- rsyslog
> didn't be started because systemd blocked.
> 
> So systemd waits for dbus, and dbus waits for startup of rsyslog,
> rsyslog waits for systemd to start it.

Thanks for debugging this.
You are right. This kind of a deadlock is possible.

Does the deadlock go away if you just modify rsyslog.service so that
instead of stopping systemd-kmsg-syslogd in ExecStartPre it would
do it in ExecStartPost?

> IMHO, We need to put functions that may block in separate threads,
> for
> example bus_init(), shutdown_connection(), log_meta().

Oh, I'm sure this works, but I'm scared of the additional
complexity of threads. 

Michal