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

[Lennart Poettering]

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

-- 
Lennart Poettering, Red Hat