USDT is missing documentation

[krzysiek4321]

As of now the only mention of usdt outside of libbpf function definitions is in program types, which comes from libbpf.

We could start by including this comment from libbpf source code.

Linux kernel selftests is a comprehensive example that I think should be at least linked in documentation.

I'd love to see a section about setting up and using semaphores with USDT as I'm having problems with that.
I couldn't yet figure out how when checking with readelf -n executable to make semaphore non zero.

My understanding is that for usdt with provider "customProvider" and name "probe_name" one should create a global variable like this
unsigned short customProvider_probe_name_semaphore SEC(".probes");
But this doesn't work for me.

I hope you find this issue worth pursuing.
Best regards,
Krzysztof

[krzysiek4321]

Never mind about semaphore issue.

I've checked in sys/sdt.h and learned that you need to define _SDT_HAS_SEMAPHORES
to have them working.
Here is the snipped of code from sys/sdt.h

#if defined _SDT_HAS_SEMAPHORES
#define _SDT_SEMAPHORE(p,n) \
        _SDT_ASM_1(             _SDT_ASM_ADDR p##_##n##_semaphore)
#else
#define _SDT_SEMAPHORE(p,n) _SDT_ASM_1(         _SDT_ASM_ADDR 0)
#endif