[systemd-devel] Using eventloop from pthread while using sd-bus functions in main threads causes deadlock or corruption

[Ben-melech, Shiran]

On Tue, 2016-06-07 at 15:47 +0200, Lennart Poettering wrote:
> On Sun, 05.06.16 13:03, Ben-melech, Shiran (
> shiran.ben-melech at intel.com) wrote:
> 
> > I've added mutex locks over all the sd functions using the bus as
> > well
> > as around the event loop.
> > 
> > This is how i run the event loop inside the pthread:
> >         
> > while (sd_event_get_state(event) != SD_EVENT_FINISHED) {
> >        
> > pthread_mutex_lock (&lock);
> >         sd_event_run(event, 1000);
> >        
> > pthread_mutex_unlock (&lock);
> >         if (r < 0) {
> >         	sd_eve
> > nt_get_exit_code(event, &r);
> >                 syslog(LOG_ERR, "%s: Error
> > sd_event_run with 	code %d", __FUNCTION__, r);
> >                
> > return NULL;
> >         }
> > 	sched_yield();
> > }
> > 
> > This did solve the seg faults and deadlocks but the callbacks are
> > missed and never called. In addition the execution is extremely
> > slow since the event loop is catching the lock most of the time.
> 
> You have to lock around all functions of sd-event you call.  That
> includes sd_event_get_state() and sd_event_get_exit_code().
> 
> If you want to unlock the mutex while sd-event is waiting for events,
> I'd recommend using sd_event_get_fd() and poll()ing on the returned
> fd
> instead of ever calling sd_event_run() or sd_event_wait().
> 
> But the rule is really: if you want to access the same sd_event
> object
> from multiple threads, you have to comprehensively lock around it.
> 
> Lennart
> 

Thank you for the help, I've found that my mistake was initializing the
sd_event from the main thread while running the loop from the pthread.
now everything seems to be in order.

Is there a way to cancel the event loop from the main thread without
killing the pthread?

Best Regards,

Shiran
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