urcu/lttng (Userspace) and Xenomai

[Lange Norbert]

Hello,

I am trying to figure out if Xenomai would work correctly with Lttng. Currently I haven’t figured out how the system manages buffers,
but I am checking if this would be generally applicable to Xenomai.

I’d like to know if anyone has already used Lttng UST with xenomai threads,
and if there is any need to compile lttng/liburcu for xenomai or using some patches.
(I haven’t seen anything that indicates it would not work).

## urcu flavours
This has a few variants, lttng uses the bulletproof one. Most others should be
faster on average – but all of them might unlock a futex with a raw syscall.

Other flavours like qsbr could likely be faster if the futex sycall would be replaced with a cobalt mutex
(it’s very unlikely this path is executed). Would need some work to get this done (and lttng to use it).

## sys_membarrier
recent kernels and liburcu versions support this syscall, which supposedly
allows removal of reader memory barriers.
The syscall will somehow interrupt the threads (all *running threads* of the process), which implicitly causes a barrier for readers.

Q: I guess this will *not* interrupt xenomai threads, as their shadow linux thread is not *running*?
Q: x86_64 accesses are strictly ordered, do you actually need membarriers at all?

Kind regards, Norbert
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