[NFS-Ganesha-Devel] Re: [lttng-dev] making liburcu and lttng coexist in a LGPL'ed program

----- On Dec 3, 2018, at 12:58 PM, Jeff Layton jlayton(a)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(a)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/... > > > > 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(a)redhat.com&gt;