Hi Martin,
I have been struggling for a while with this issue (see the whole thread):
http://lists.lttng.org/pipermail/lttng-dev/2014-May/023035.html
and landed on the same conclusions as yours (found your message by
searching for __start___tracepoints_ptr!).
So at least you're not alone!
So, did you ever manage to get any of your questions answered:
>> 1) Have you run into a problem like this? Is there a known
fix/workaround?
>> 2) __start____tracepoints_ptrs is declared as extern in
tracepoint.h, but it
>> is not defined. This appears to be some sort of undocumented linker
magic.
>> http://gcc.gnu.org/ml/gcc-help/2010-04/msg00120.html is the only
reference I
>> could find. Do you know where this behavior is documented or
specified (if
>> at all)?
>> 3) Do you know why the symbol visibility for __start___tracepoints_ptrs
>> changed between 4.6.4 to 4.7.2?
Thank you so much!
Gerlando
BTW, I'm also running GCC 4.7.2 (lttng-ust is cross-compiled, the test
application is natively compiled).
On an x86_64 host running either GCC 4.4.6 or 4.4.7, the issue is not
observed.
On 04/30/2014 11:57 PM, Martin Ünsal wrote:
Incidentally I also asked for help on the GNU linker-specific part
(question 2) here:
http://gcc.gnu.org/ml/gcc-help/2014-04/msg00164.html
Martin
On Wed, Apr 30, 2014 at 2:21 PM, Martin Ünsal <[email protected]> wrote:
Hi LTTng folks
I have a strange problem using LTTng-UST on an ARM based platform. I have
done some diagnosis but I am running low on ideas and was hoping for help
from the experts. I am using lttng-tools 2.2.0, lttng-ust 2.2.0, liburcu
0.8.1. I know these are old but upgrading is easier said than done
unfortunately. I didn't see anything related to this problem in relnotes,
mailing list traffic, or master branch, but I could have missed something.
The problem showed up when I switched from GCC 4.6.4 to 4.7.2. Conceptually,
the situation is that I have a single executable, call it MyProgram, with
two plugins loaded at runtime with dlopen(), lets call them libPlugin1.so
and libPlugin2.so. There are three different LTTng-UST tracepoint providers,
one each for the executable and the two plugins. With GCC 4.7.2, tracepoints
in libPlugin1 stopped working. The tracepoints in MyProgram and in
libPlugin2 continue to work correctly.
I have established without a doubt that the toolchain upgrade is the cause
of the regression.
In the debugger, I confirmed that the tracepoint for libPlugin1.so is being
executed, but __tracepoint_##provider##___##name.state is always 0 even when
I enable the tracepoint in lttng-tools. As a result the tracepoint callback
is not being invoked when it should be. In MyProgram and libPlugin2.so, the
.state variable correctly reflects whether the tracepoint is enabled, and if
the tracepoint is enabled, the tracepoint callback is invoked.
Next I set a breakpoint in tracepoint_register_lib() and looked at
tracepoints_start parameter.
1) With GCC 4.6.4 everything is as expected:
a) tracepoint_register_lib() for MyProgram called with
MyProgramProvider's __start___tracepoints_ptrs.
b) tracepoint_register_lib() after libPlugin1 dlopen() called with
libPlugin1Provider's __start___tracepoints_ptrs
c) tracepoint_register_lib() after libPlugin2 dlopen() called with
libPlugin2Provider's __start___tracepoint_ptrs
2) With GCC 4.7.2 there is a problem:
a) tracepoint_register_lib() for MyProgram called with
MyProgramProvider's __start___tracepoints_ptrs.
b) tracepoint_register_lib() after libPlugin1 dlopen() called with
MyProgramProvider's __start___tracepoints_ptrs (!!!! THIS IS WRONG !!!!)
c) tracepoint_register_lib() after libPlugin2 dlopen() called with
libPlugin2Provider's __start___tracepoint_ptrs
I looked at the symbol table for libPlugin1.so to see if it would shed some
light on the problem.
1) With GCC 4.6.4:
# objdump -t /usr/lib/.debug/libPlugin1.so | grep __start___tracepoints_ptrs
00025bb0 l *ABS* 00000000 __start___tracepoints_ptrs
# objdump -t /usr/lib/.debug/libPlugin2.so | grep __start___tracepoints_ptrs
00041eb4 l *ABS* 00000000 __start___tracepoints_ptrs
2) With GCC 4.7.2:
# objdump -t /usr/lib/.debug/libPlugin1.so | grep __start___tracepoints_ptrs
00025a90 g __tracepoints_ptrs 00000000 __start___tracepoints_ptrs
# objdump -t /usr/lib/.debug/libPlugin2.so | grep __start___tracepoints_ptrs
00041eb4 g __tracepoints_ptrs 00000000 __start___tracepoints_ptrs
My hypothesis at this point is that since __start___tracepoints_ptrs changed
from a local to a global symbol, the dynamic loader no longer knows how to
select the correct weak symbol. I cannot explain why libPlugin2 still loads
its provider correctly, perhaps it is just getting lucky.
A few questions come to mind...
1) Have you run into a problem like this? Is there a known fix/workaround?
2) __start____tracepoints_ptrs is declared as extern in tracepoint.h, but it
is not defined. This appears to be some sort of undocumented linker magic.
http://gcc.gnu.org/ml/gcc-help/2010-04/msg00120.html is the only reference I
could find. Do you know where this behavior is documented or specified (if
at all)?
3) Do you know why the symbol visibility for __start___tracepoints_ptrs
changed between 4.6.4 to 4.7.2?
Thanks for any help. This is a real puzzler for me.
Martin
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