[lttng-dev] some troubles having lttgng-ust actually log something on a shared library

[Sébastien Barthélémy]

2011/12/22 Mathieu Desnoyers <compudj at krystal.dyndns.org>:
> I've now splitted liblttng-ust into two libs: liblttng-ust-tracepoint
> (dlopen'd by the application to register the tracepoints), and
> liblttng-ust (which is used by the tracepoint probes, and talks to
> lttng-sessiond).
>
> I've updated the README file in lttng-ust to specify that the tracepoint
> probes should not be dlopen()'d. dlopen has just too many nasty
> side-effects with TLS (grabbing a lock also taken in constructors is
> a major one). Please use LD_PRELOAD or link the application directly
> with the tracepoint probe .so. Even if I change the locking scheme
> within liblttng-ust to play nicely with the dynamic linker mutex, the
> whole thing would still be fragile due to the possibility to hit that
> mutex from an active tracepoint (which could nest in various other
> mutexes).

In my application, the probe activation problem came from the fact that the
constructor was not carried on from the static library to the (dlopened) shared
library.

I changed the the static library to a shared one, and it worked.

I reproduced this behavior on a minimalistic example. On this example,
I also tried
to activate the probes through LD_PRELOAD, it worked flawlessly.

> Note that you can still dlopen() the file that is instrumented with
> tracepoints (the file where you define e.g. TRACEPOINT_DEFINE and add
> your tracepoint() statements). It's only the probe (with
> TRACEPOINT_CREATE_PROBES defined) that cannot be dlopen'd.

ok, I'll then try to use LD_PRELOAD and the new splitted libs on the main app,
probably next week.

Cheers
-- Sébastien