Hi Paul,
thanks for your explanation, but I'm more puzzled than ever.
I'm definitely lacking the appropriate background in both terminology
and internals, so I tried to figure out how the whole magic works by
empirical testing.
Now, when you say:
> The reason is that you can have the same tracepoint provider be USED in
> several compilation units that will all become part of one and the same
> shared object (or executable).
>
> Then all those __start/stop___tracepoints_ptrs references in different
> compilation units should refer to the same
> __start/stop___tracepoints_ptrs definitions for the shared object (or
> executable) they are part of. This is required because the
> initialization of the tracepoints will only happen once per shared
> object (or executable) with the static ctor mechanism also defined in
> tracepoint.h
Who's responsible for initializating the tracepoints? Isn't it the
PROVIDER, instead of the user?
Here's what I understood (or rather, speculated!), so please point out
where my understanding falls short.
Tracepoint providers (where TRACEPOINT_DEFINE is defined) are what
actually implement tracepoints. You can have multiple source files, each
defining one or more tracepoints. So in the end each object file will
contain one or more tracepoint pointers within its "__tracepoints_ptrs"
section (courtesy of the compiler). When linking (e.g. towards a shared
object), a single section __tracepoints_ptrs in the output ELF binary
will merge all the sections of the above objects, and hold all the
pointers as a contiguous array. This time, courtesy of the linker, who
also automagically defines __start___tracepoints_ptrs /
__stop___tracepoints_ptrs symbols to hold pointers to the beginning and
end parts of the section.
Each object file will contain its own __tracepoints__ptrs_init()
constructor, responsible for registering ALL the tracepoints it
provides. Actually, we want only ONE constructor per shared object to
register all the tracepoint pointers provided by the whole shared object
(contained within __start___tracepoints_ptrs/__stop___tracepoints_ptrs).
This is where, for instance, __tracepoint_ptrs_registered comes into
play. Multiple invocations of the constructor (one per object file)
should be avoided and only the first one needs to be performed. And this
is why __tracepoint_ptrs_registered needs to be weak (multiple source
files could lead to multiple definitions -- we want one and only one per
shared object) *AND* hidden (each shared object should have its own copy).
If I remove the weak attribute from __tracepoint_ptrs_registered, the
linker starts screaming as soon as I compile one of the examples.
On the other hand, __start___tracepoints_ptrs/__stop___tracepoints_ptrs
are generated by the linker (or so I want to believe!) so only one
instance is emitted.
Keeping them hidden prevents the name clash during dynamic linking, as
the symbol will not be visible from other shared objects or binaries.
But I don't see why they should also be weak.
As a matter of fact, removing the weak attribute seems to fix my problem
(as far as I could test).
What am I missing?
Thank you again for your patience,
Gerlando
On 05/27/2014 04:58 PM, Woegerer, Paul wrote:
On 05/27/2014 04:41 PM, Gerlando Falauto wrote:
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.
It's more complicated.
You absolutely need those symbol to be declared as:
.weak __start___tracepoints_ptrs
.weak __stop___tracepoints_ptrs
*and*
.hidden __start___tracepoints_ptrs
.hidden __stop___tracepoints_ptrs
The reason is that you can have the same tracepoint provider be USED in
several compilation units that will all become part of one and the same
shared object (or executable).
Then all those __start/stop___tracepoints_ptrs references in different
compilation units should refer to the same
__start/stop___tracepoints_ptrs definitions for the shared object (or
executable) they are part of. This is required because the
initialization of the tracepoints will only happen once per shared
object (or executable) with the static ctor mechanism also defined in
tracepoint.h
HTH,
Paul
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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