Re: [systemd-devel] EXT: sdbus_event loop state mark as volatile?
On Fri, 06 Sep 2019 at 06:57:22 +, 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 ___ systemd-devel mailing list systemd-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/systemd-devel
Re: [systemd-devel] EXT: sdbus_event loop state mark as volatile?
> On 5 Sep 2019, at 20.46, Stephen Hemminger wrote:
>
> The libsystemd bus event loop is:
>
>
>while (e->state != SD_EVENT_FINISHED) {
>r = sd_event_run(e, (uint64_t) -1);
>
> But since e->state is changed by another thread it
What other thread?
> should be marked volatile to avoid compiler thinking
> the state doesn't get changed.
>
The “volatile” keyword does not equate to thread safety.
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
While “volatile” _is_ a useful hint to the compiler, it provides *no*
atomic or thread-safe guarantees: for example about the ordering and
visibility of operations in a multi-core system.
It is true that for _certain_ { chipset, compiler, compiler-option }
combinations we can effectively reason about atomicity [for example
of word-size integers], however this does not generalise, and is
certainly not portable.
Ian
>
> diff --git a/src/libsystemd/sd-event/sd-event.c
> b/src/libsystemd/sd-event/sd-event.c
> index 5adbceeb0247..b7be2472a398 100644
> --- a/src/libsystemd/sd-event/sd-event.c
> +++ b/src/libsystemd/sd-event/sd-event.c
> @@ -90,7 +90,7 @@ struct sd_event {
>
> uint64_t iteration;
> triple_timestamp timestamp;
> -int state;
> +volatile int state;
>
> bool exit_requested:1;
> bool need_process_child:1;
> ___
> systemd-devel mailing list
> systemd-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/systemd-devel
___
systemd-devel mailing list
systemd-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/systemd-devel
Re: [systemd-devel] Bad accelerometer values cause incorrect screen rotation
On Thu, Sep 5, 2019 at 9:00 PM Bastien Nocera wrote: > Daniel, if you run into many more problems, there's also the > possibility of adding a boot argument to disable the accelerometer (or > maybe its effects?), either in iio-sensor-proxy or gnome-shell. Thanks for the suggestion, manually adding something through the bootloader menu may indeed be a bit more practical than the laptop acrobatics workaround. For cases where we know which driver is used this can probably already be done, by adding a modprobe.blacklist= boot arg. I appreciate the quick action on the HP laptop case. Let's see how much that reduces the problem occurance rate. Thanks! Daniel ___ systemd-devel mailing list systemd-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/systemd-devel
