Shared semaphore between user and kernel spaces

Question

Short version

Is it possible to share a semaphore (or any other synchronization lock) between user space and kernel space? Named POSIX semaphores have kernel persi

Answer 1

I have read your project's README and I have the following observations. Apologies in advance:

Firstly there already is a universal interface to real time systems. It is called POSIX; certainly VxWorks, Integrity and QNX are POSIX compliant and in my experience there are very few problems with portability if you develop within the POSIX API. Whether POSIX is sane or not is another matter, but it's the one we all use.

[The reason most RTOSes are POSIX compliant is because one of the big markets for them is defence equipment. And the US DoD won't let you use an OS for their non-IT equipment (eg Radars) unless it is POSIX compliant... This has pretty much made it commercially impossible to do an RTOS without giving it POSIX]

Secondly Linux itself can be made into a pretty good real time OS by applying the PREMPT_RT patch set. Of all the RTOSes out there this is probably the best one at the moment from the point of view of making efficient use of all these multi core CPUs. However it's not quite such a hard-realtime OS as the others, so its quid pro quo.

RTAI takes a different approach of in effect placing their own RTOS underneath Linux and making Linux nothing more than one task running in their OS. This approach is ok up to a point, but the big penalty of RTAI is that the real time bit is now (as far as I can tell) not POSIX compliant (though the API looks like they've just stuck rt_ on the front of some POSIX function names) and interaction with other things is now, as you're discovering, quite complicated.

PREEMPT_RT is a much more intrusive patch set than RTAI, but the payback is that everything else (like POSIX and valgrind) stays completely normal. Plus nice things like FTrace are available. Book keeping is then a case of merely using existing tools, not having to write new ones. Also it looks like PREEMPT_RT is gradually worming its way into the mainstream Linux kernel anyway. That would render other patch sets like RTAI pretty much pointless.

So Linux + PREEMPT_RT gives us realtime POSIX plus a bunch of tools, just like all the other RTOSes out there; commonality across the board. Which kinda sounds like the goal of your project.

I apologise for not helping with the with the "how" of your project, and it is highly ungentlemanly of me to query the "why?" of it too. But I feel it is important to know that there are established things out there that seem to heavily overlap with what you're trying to do. Unseating King POSIX is going to be difficult.