[lttng-dev] making liburcu and lttng coexist in a LGPL'ed program

[Mathieu Desnoyers]

----- On Dec 3, 2018, at 12:58 PM, Jeff Layton jlayton at redhat.com wrote:

> On Wed, 2018-11-28 at 17:06 -0500, Mathieu Desnoyers wrote:
>> ----- On Nov 27, 2018, at 1:17 PM, Jeff Layton jlayton at redhat.com wrote:
>> 
>> > The nfs-ganesha project has used lttng for quite some time to handle
>> > tracing. Recently though, we decided to start building liburcu in as a
>> > mandatory component, with an eye toward using it in certain areas.
>> > 
>> > Before this change, the code linked in liburcu-bp directly, but now we
>> > just use liburcu. Unfortunately, when we enable tracepoints in the build
>> > now, we get errors like this at link time:
>> > 
>> > --------------------8<----------------
>> > [ 96%] Linking C executable ganesha.nfsd
>> > /usr/bin/ld: libMainServices.a(nfs_worker_thread.c.o): undefined reference to
>> > symbol 'rcu_gp_bp'
>> > /usr/bin/ld: //usr/lib64/liburcu-bp.so.6: error adding symbols: DSO missing from
>> > command line
>> > collect2: error: ld returned 1 exit status
>> > make[2]: *** [MainNFSD/CMakeFiles/ganesha.nfsd.dir/build.make:308:
>> > MainNFSD/ganesha.nfsd] Error 1
>> > make[1]: *** [CMakeFiles/Makefile2:2740:
>> > MainNFSD/CMakeFiles/ganesha.nfsd.dir/all] Error 2
>> > make: *** [Makefile:152: all] Error 2
>> > --------------------8<----------------
>> > 
>> > nfs-ganesha defines _LGPL_SOURCE, and that makes lttng use the
>> > redefinitions in tracepoint-rcu.h. If I disable _LGPL_SOURCE, it all
>> > builds as expected.
>> > 
>> > I found a similar bug here:
>> > 
>> >    https://bugs.lttng.org/issues/1156
>> > 
>> > Any thoughts on the right fix for this? We'd like to eat our cake and
>> > have it too, so that we can have _LGPL_SOURCE defined, lttng enabled,
>> > and the urcu flavor be determined at runtime.
>> 
>> Hi!
>> 
>> It's great to hear that feedback. Indeed you are not the first one
>> to hit this unfortunate limitation of liburcu API.
>> 
>> The use-case involves using many liburcu flavors within the same
>> compile unit _with_ _LGPL_SOURCE defined. All the tricks done
>> in urcu/map/*.h to map all flavors to a single API conflict
>> with that use-case. And the fact that the "mb", memb", and
>> signal flavors all share the same header prevents this
>> even further. This becomes a real issue when trying to use
>> the lttng-ust tracer instrumentation (which includes urcu-bp.h)
>> along with other liburcu flavors in a compile unit.
>> 
>> So, I took the day to hack something out: beware, it's a complete
>> overhaul of the liburcu tree. You can try it out in this private
>> development branch:
>> 
>> https://github.com/compudj/userspace-rcu-dev/tree/multiflavor
>> 
>> Note: don't even try to follow the git commit history at the
>> moment, this will need editing. ;)
>> 
>> A new unit test shows how it works:
>> 
>> https://github.com/compudj/userspace-rcu-dev/blob/multiflavor/tests/unit/test_urcu_multiflavor_single_unit.c
>> 
>> And while I was there, I did a lot of namespacing cleanups for the
>> APIs, so we'll _definitely_ need to bump the major soname for this.
>> However, I'm keeping the main use-cases of the API backward compatible
>> (those which relied on the API mapping). Here is a dev branch of
>> lttng-ust adapting to those API changes:
>> 
>> https://github.com/compudj/lttng-ust-dev/tree/urcu-multiflavor
>> 
>> Those who want to be able to use many liburcu flavors in a single
>> compile unit would need to define URCU_NO_API_MAP, and use each flavor
>> namespace directly, as done in the new unit test. I'm not particularly
>> fond of the URCU_NO_API_MAP name. We could perhaps name it "URCU_MULTI_FLAVOR"
>> or something else...

By the way, with the updated liburcu tree, there is no more need to
define URCU_NO_API_MAP.

>> 
>> Please try it out and let me know how it goes.
>> 
>> Thanks,
>> 
>> Mathieu
>> 
> 
> Thanks Mathieu,
> 
> Sorry for the delay, but this was rather difficult to test. I ended up
> building a userspace-rcu package from this tree, but was unable to
> build lttng-ust as a package from your tree, and so had to just do a
> make/make install of it on the build host. Ditto for lttng-tools.

You will need lttng-ust from this tree:

https://github.com/compudj/lttng-ust-dev/tree/urcu-multiflavor

to build against the liburcu branch. This is causing your missing symbol
error.

Thanks,

Mathieu

> 
> With all of that, I reran the ganesha build and got the same error:
> 
> [ 97%] Linking C executable ganesha.nfsd
> /usr/bin/ld: libMainServices.a(nfs_worker_thread.c.o): undefined reference to
> symbol 'urcu_bp_register'
> /usr/bin/ld: //usr/lib64/liburcu-bp.so.7: error adding symbols: DSO missing from
> command line
> collect2: error: ld returned 1 exit status
> make[2]: *** [MainNFSD/CMakeFiles/ganesha.nfsd.dir/build.make:288:
> MainNFSD/ganesha.nfsd] Error 1
> make[1]: *** [CMakeFiles/Makefile2:2396:
> MainNFSD/CMakeFiles/ganesha.nfsd.dir/all] Error 2
> make: *** [Makefile:152: all] Error 2
> 
> Should we be passing in both -lurcu and -lurcu-bp on the link here?
> --
> Jeff Layton <jlayton at redhat.com>

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com