build: use initial-exec TLS when building seastar as shared library

[tchaikov]

quote from
https://patchwork.ozlabs.org/project/glibc/patch/[email protected]/

On the glibc side, we should recommend that intercepting mallocs and its
dependencies use initial-exec TLS because that kind of TLS does not use
malloc. If intercepting mallocs using dynamic TLS work at all, that's
totally by accident, and was in the past helped by glibc bug 19924.

so instead of allocating TLS variables using malloc, let's allocate them using initial-exec TLS model. another approach is to single out the static TLS variables in the code path of malloc/free and apply __attribute__ ((tls_model("initial-exec"))) to them, and optionally only do this when we are building shared library.

but this could be overkill as

1. we build static library in the release build
2. the total size of the static TLS variables is presumably small, so the application linking against the seastar shared library should be able to afford this.

see also
https://patchwork.ozlabs.org/project/glibc/patch/[email protected]/ and
https://sourceware.org/bugzilla/show_bug.cgi?id=19924

Fixes #2247
Signed-off-by: Kefu Chai [email protected]

[avikivity]

What's the downside of initial-exec? I guess it can't be used with dlopen(), which seems minor here.

[avikivity]

Seems reasonable to me. But please verify it works with scylladb.

[tchaikov]

@avikivity yes, as you put. an application would not be able to dlopen libseastar.so for accessing its TLS symbols. but i also learned that glibc's ld.so does reserve some space in static TLS block, and allows dlopen on such a shared object if its TLS size is small. but this does not apply to musl's dynamic loader. anyway, i concur with you this issue is minor.

tested with following steps

$ cd seastar
$ git cherry-pick 6c44eed
$ git show HEAD --name-only --oneline 
2ee99a7b2 (HEAD) build: use initial-exec TLS when building seastar as shared library
CMakeLists.txt
$ cd ..
$ ./configure.py --mode dev --compiler clang++ --c-compiler clang
$ ninja build/dev/scylla
$ grep CMAKE_BUILD_TYPE build/dev/seastar/CMakeCache.txt 
CMAKE_BUILD_TYPE:STRING=Dev
$ cd ~/dev/scylla-dtest
$ JAVA_HOME=/usr/lib/jvm/jre-11 pytest -s --log-cli-level=debug -c $PWD/pytest.ini --cassandra-dir $HOME/dev/scylladb/build/dev  repair_additional_test.py::TestRepairAdditional::test_repair_one_node_alter_rf
...
==================================================== 1 passed, 2 warnings in 59.91s ====================================================