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

[Sébastien Barthélémy]

2011/12/20 Sébastien Barthélémy <barthelemy at crans.org>:
> 011/12/19 Mathieu Desnoyers <compudj at krystal.dyndns.org>:
>>
>> Please try to reproduce this problem in a simple application that uses
>> dlopen to load libs.
>
> I omitted to mention it, but I tried that yesterday. Without success: tracing
> worked. I'll keep trying.

There is still a question that bugs me though: the tracing only works if I link
the executable (the one which dlopens the traced .so) against lttng-ust.

So, while the attached example works out of the box, it won't with the following
patch applied:

diff --git a/Makefile b/Makefile
index 4674579..6e25f08 100644
--- a/Makefile
+++ b/Makefile
@@ -3,7 +3,7 @@
 all: run_motion libmotion.so.1.0.0

 run_motion:run_motion.cpp
-       ${CC} -o run_motion -lstdc++ -ldl -llttng-ust run_motion.cpp
+       ${CC} -o run_motion -lstdc++ -ldl run_motion.cpp
 libmotion.so.1.0.0:motion.o ust_motion.o
        ${CC} -shared -Wl,-soname,libmotion.so.1 -o libmotion.so.1.0.0
-ldl -llttng-ust -lstdc++ motion.o ust_motion.o

The program hangs, waiting for ltt-sessiond :

libust[4114/4114]: just registered a tracepoints section from
0x7fab35640aa8 and having 1 tracepoints (in tracepoint_register_lib()
at tracepoint.c:575)
libust[4114/4114]: LTT : ltt ring buffer client init
 (in ltt_ring_buffer_metadata_client_init() at
ltt-ring-buffer-metadata-client.h:318)
libust[4114/4114]: LTT : ltt ring buffer client init
 (in ltt_ring_buffer_client_overwrite_init() at ltt-ring-buffer-client.h:557)
libust[4114/4114]: LTT : ltt ring buffer client init
 (in ltt_ring_buffer_client_discard_init() at ltt-ring-buffer-client.h:557)
libust[4114/4116]: Info: sessiond not accepting connections to global
apps socket (in ust_listener_thread() at lttng-ust-comm.c:633)
libust[4114/4116]: Waiting for global apps sessiond (in
wait_for_sessiond() at lttng-ust-comm.c:556)
libust[4114/4116]: Linux kernels 2.6.33 to 3.0 (with the exception of
stable versions) do not support FUTEX_WAKE on read-only memory
mappings correctly. Please upgrade your kernel (fix is commit
9ea71503a8ed9184d2d0b8ccc4d269d05f7940ae in Linux kernel mainline).
LTTng-UST will use polling mode fallback. (in wait_for_sessiond() at
lttng-ust-comm.c:569)
libust[4114/4116]: Error: futex: Bad address (in wait_for_sessiond()
at lttng-ust-comm.c:571)
libust[4114/4116]: Info: sessiond not accepting connections to global
apps socket (in ust_listener_thread() at lttng-ust-comm.c:633)
libust[4114/4115]: message received
 (in ust_listener_thread() at lttng-ust-comm.c:700)
libust[4114/4114]: Error: Timed out waiting for ltt-sessiond (in
lttng_ust_init() at lttng-ust-comm.c:818)

Any idea why this is so?

> 2011/12/19 Mathieu Desnoyers <compudj at krystal.dyndns.org>:
>> Sébastien Barthélémy (barthelemy at crans.org) wrote:
>>> A better understanding of the way probes are enabled would probably
>>> help me debug it.
>>> Is it documented somewhere? Or could you point me to the right source file?
>>
>> liblttng-ust/tracepoint.c
>> liblttng-ust/ltt-probe.c
>> include/lttng/tracepoint.h include/lttng/ust-tracepoint-event.h
>>
>> would be a good start,

Ok, I eventually understood why I haven't got any linking error on Friday, while
having omitted an object file.
And why the tracepoints were getting registered while I got no output:
the constructor from
include/lttng/ust-tracepoint-event.h was never called.

I have another question though:
in include/lttng/ust-tracepoint-event.h there are the two following declarations

extern struct tracepoint * const __start___tracepoints_ptrs[]
	__attribute__((weak, visibility("hidden")));
extern struct tracepoint * const __stop___tracepoints_ptrs[]
	__attribute__((weak, visibility("hidden")));

I eventually understood that they are defined in liblttng-ust.so

$ nm /usr/local/lib/liblttng-ust.so | grep ___tracepoints_ptrs
0000000000232aa8 a __start___tracepoints_ptrs
0000000000232ab0 a __stop___tracepoints_ptrs

And that they match the boundaries of the tracepoints_ptrs section

$ objdump -h /usr/local/lib/liblttng-ust.so |grep __tracepoints_ptrs
 26 __tracepoints_ptrs 00000008  0000000000232aa8  0000000000232aa8
00032aa8  2**3

But how do you get these symbols defined? Greping through the lttng-ust source
code did not help me much.

Cheers
-- Sebastian
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