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

[Ben-melech, Shiran]

On Wed, 2016-06-01 at 15:45 +0200, Lennart Poettering wrote:
> On Wed, 01.06.16 13:33, Ben-melech, Shiran (
> shiran.ben-melech at intel.com) wrote:
> > > Hi,
> > 
> > i'm writing a small C library using sd-bus to communicate with
> > bluetooth low energy devices.
> > 
> > Instead of using a loop for reading from a specific characteristic
> > on
> > the ble device, I want to use eventloop and register for value
> > changes
> > on the object that represent this characteristic.
> > 
> > When i run the eventloop in a different pthread, since on the main
> > i'm
> > writing to the ble service to get the data i want from the
> > characteristic - i get either what seems to be a deadlock or seg
> > faults.
> > 
> > i've used the example on your blog (
> > http://0pointer.net/blog/introducing-sd-event.html) to set up the
> > event.
> > 
> > Can anyone please advise? what might i be doing wrong?
> > sd-event is not thread-safe, but it is threads-aware. That means it
> won't do any locking on its own, but it will work fine in a threaded
> environment as long as you use the same sd_event object (and its
> auxiliary objects, i.e. sd_event_source) only from within one thread.
> > In other words: as long as your don't "reach over" from one thread to
> the other to access sd-event objects you should be safe. If you do,
> then you need to wrap things in your own locking to ensure you never
> end up accessing the same sd-event (or any of its auxiliary
> sd_event_source objects) from multiple threads at the same time.
> > Lennart

Thanks for the quick reply,

I am not doing anything with the event from the main thread unless i'm
missing something.
i even tried just setting and running the eventloop without adding a
match to catch but still get issues.

i have pasted in http://dpaste.com/1CF8QAY
a stripped test code i wrote that the issue can be seen at.
it's just to overview how i used the eventloop since it has some hard
coded stuff so unfortunately you will be able to work with it only if
you happen to have Arduino 101.

Thanks,

Shiran
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