Hi Paul,
thank you very much for sharing this.
I had in the meantime run into the same suggestion by
Henrik Wallin on a thread opened by Martin
(https://gcc.gnu.org/ml/gcc-help/2014-05/msg00028.html).
Further updates from Martin also suggest the issue is rather related to
the OpenEmbedded toolchain.
I was about to post the "opposite" of your patch, as I don't see the
need to have those symbols as weak instead. In the end, doesn't weak
only allow for a further re-definition? In this case we're only
declaring it as extern, aren't we?
Definition actually happens by magic, as far as I can tell.
But please correct me if I got it all wrong.
Thank you,
Gerlando
On 05/27/2014 04:32 PM, Woegerer, Paul wrote:
Hi Martin, Hi Gerlando,
this sounds a lot like the compiler bug I found recently in Yocto 1.6
(reproducible on ARM, x86 and PPC)
The problem in my case is that the Yocto generated GCC cross-compiler
translates:
extern struct tracepoint * const __start___tracepoints_ptrs[]
__attribute__((weak, visibility("hidden")));
extern struct tracepoint * const __stop___tracepoints_ptrs[]
__attribute__((weak, visibility("hidden")));
incorrectly to assembly. For these symbols that are declared with
__attribute__((weak, visibility("hidden")));
that are also defined to be external, in the assembly the following lines are
missing:
.hidden __stop___tracepoints_ptrs
.hidden __start___tracepoints_ptrs
This causes __stop___tracepoints_ptrs and __start___tracepoints_ptrs to be
further treated as ordinary weak symbols instead of per-shared-object weak
symbols.
That further will cause the linker to resolve any such symbols with the first
definition of those symbols that it can see (it will not constrain itself to
only consider definitions from within the same shared object). The net result
is that only one tracepoint provider gets activated (the first one the linker
sees) instead of all the tracepoint providers used in various source files.
To fix this I use the following lttng-ust workaround (for now):
diff --git a/include/lttng/tracepoint.h b/include/lttng/tracepoint.h
index 66e2abd..50cef26 100644
--- a/include/lttng/tracepoint.h
+++ b/include/lttng/tracepoint.h
@@ -313,9 +313,11 @@ __tracepoints__destroy(void)
* (or for the whole main program).
*/
extern struct tracepoint * const __start___tracepoints_ptrs[]
- __attribute__((weak, visibility("hidden")));
+ __attribute__((weak));
+asm(".hidden __start___tracepoints_ptrs");
extern struct tracepoint * const __stop___tracepoints_ptrs[]
- __attribute__((weak, visibility("hidden")));
+ __attribute__((weak));
+asm(".hidden __stop___tracepoints_ptrs");
/*
* When TRACEPOINT_PROBE_DYNAMIC_LINKAGE is defined, we do not emit a
Note that this issue is not reproducible with my GCC on host:
gcc version 4.8.1 20130909 [gcc-4_8-branch revision 202388] (SUSE Linux)
and also not with the latest Codebench 2014.05 ARM-Linux cross-toolchain.
--
Best,
Paul
On 05/27/2014 01:55 PM, Gerlando Falauto wrote:
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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