[lttng-dev] [PATCH lttng-ust] Improve tracelog handling, reduce exported functions

[Mathieu Desnoyers]

----- On May 20, 2021, at 11:54 AM, Norbert Lange nolange79 at gmail.com wrote:
[...]

>> What prevents you from linking against lttng-ust.so again ?
> 
> I did not poke around enough with Lttng to be confident it wont have
> side effects,
> I really don't want it active in production. It doesn't seem there is
> much public knowledge with Xenomai either.
> lttng-ust.so will spawn threads, lttng-ust-tracepoint.so is mostly passive,

There is indeed a split between instrumentation and runtime threads done
with lttng-ust-tracepoint.so vs lttng-ust.so.

I understand that this split is missing for tracelog and tracef, and
would be a good thing to have.

I would be interested to move the tracelog and tracef implementation
from liblttng-ust.so to liblttng-ust-tracepoint.so, even this late
in the -rc cycle, because all users of tracelog/tracef need to link
against liblttng-ust-tracepoint.so anyway. So moving these symbols
should not affect anyone.

Can you give it a try and let me know if it works for you ?

> So Id want a dynamic tracepoint-provider than i can dlopen (so that
> the signal masks are inherited,
> I hope you dont touch them).

The signals are all blocked for lttng-ust listener threads. We don't
modify the signal masks in the tracepoint probes. Not sure which is
the target of your question though.

> 
> Of course I could just remove all lttng libraries on the production
> system aswell. Still doesnt change that
> tracelog and tracef doesnt work that way.

Would moving the tracelog/tracef implementation to liblttng-ust-tracepoint.so
solve your issues ?

> 
> I implemented my own tracelog/tracef using the normal lttng
> tracepoints for now, they totally break on source level with 2.13
> aswell ;)
> is it ok if I do this to access them:
> 
> #define TRACEPOINT_DEFINE
> #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
> // 2.12
> // #include <lttng/lttng-ust-tracelog.h>
> // #include <lttng/lttng-ust-tracef.h>
> // 2.13
> #include <lttng/tp/lttng-ust-tracelog.h>
> #include <lttng/tp/lttng-ust-tracef.h>
> 
> ie. I would load lttng-ust.so later and can then use those tracepoints.

Reimplementing the tracelog/tracef providers is not an intended use-case.
I'd very much prefer if we move their implementation to
liblttng-ust-tracepoint.so.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com