[systemd-devel] EXT: sdbus_event loop state mark as volatile?

[Stephen Hemminger]

On Fri, 6 Sep 2019 16:04:33 +0100
Simon McVittie <smcv at collabora.com> wrote:

> On Fri, 06 Sep 2019 at 06:57:22 +0000, Ray, Ian (GE Healthcare) wrote:
> > If thread-safety is a design goal (and I don’t believe that it is [1])
> > then atomic or thread-safe primitives should be used.
> > 
> > [1] https://lists.freedesktop.org/archives/systemd-devel/2017-March/038519.html  
> 
> [1] is about sd-bus, not sd-event, and doesn't say anything about whether
> sd-event is designed to be thread-safe or not.
> 
> However, I think you're correct to say that struct sd_event is also only
> designed to be used from the single thread that "owns" it.
> 
> If you need a thread-safe event loop, then you need something like
> GLib's GMainContext, with mutexes to protect its data structures against
> concurrent access, and a well-defined mechanism for one main-context to
> "post" events to other main-contexts (which might be running concurrently
> in a different thread). Many other event loops are available; GMainContext
> happens to be the one I'm most familiar with, and I know that it is
> designed to be thread-safe.
> 
> The price that things like GMainContext pay for being thread-safe is
> that they are more complex and less efficient than sd-event: in general,
> all operations on a thread-aware event loop have to pay the complexity
> and performance cost of being thread-aware, even if the current program
> only has one thread.
> 
>     smcv

Excuse me for reviving an old thread. But I see similar problem today
(especially on Arm). The sd-event model uses signals so it is inherently
subject to thread issues.

It looks like a stronger memory model is needed here (not volatile).
Other projects use __atomic builtins for this.