[RFC PATCH glibc 0/3] Rewrite THREAD_GSCOPE

[Sergey Bugaev]

Alright, here's my second attempt.

On Sat, May 8, 2021 at 10:54 PM Samuel Thibault <samuel.thibault@gnu.org> wrote:
> While at it, rather try to reproduce the other archs' behavior: use
> gscope_flag from the tcb, and switch THREAD_GSCOPE_IN_TCB to 1. That
> will make the implementation much simpler and scalable.

Well, this has been much less traightforward than the patch I sent
yesterday (but unlike that patch, it appears to build and work). I am
much less familiar with glibc internals than I am with Hurd, and it
shows. Please don't throw stones at me :)

For one thing, I don't think I understand how the buildsystem works.
(For instance, how do I even add a new source file to a library? Or
where does it decide which system-specific parts to build, such as
whether to build NPTL or HTL? How do I debug the buildsystem when
something doesn't work as expected?) It must be that I'm used to
either straightforward Makefiles, or higher-level buildsystems (like
CMake or Meson) that autogenerate the low-level boilerplate.

Next, I was somewhat confused by what the GL macro is. My current
understanding is: some data needs to be shared between the dynamic
loader and glibc proper, if glibc is being built as a shared library.
To support that case, such data is placed into a special struct
instance that itself resides in ld.so. Then the GL macros is used
to actually access those symbols. It switches, mostly transparently,
between accessing said global struct instance in ld.so, and simply
declaring global variables for the case of static build. Naming
such variable with the dl_ prefix is just a convention.

I likely got something wrong there. If I got the general idea wrong,
the second patch must be completely bogus.

Initially, I tried to get rid of THREAD_GSCOPE_IN_TCB completely,
since THREAD_GSCOPE is now in TCB in all ports. But it turned out
that the way it was actualy being used was not to check if the
thread scope flag is in TCB or not, but simply to distingusish NPTL
from HTL. Namely, files like elf/dl-tls.c would directly use NPTL
specifics like struct pthread if THREAD_GSCOPE_IN_TCB is 1.

The actual implementation of THREAD_GSCOPE_* is similar to the one
in NPTL. Each thread uses a flag in its TCB. THREAD_GSCOPE_WAIT()
has to iterate over all the threads and wait for each one to leave
the global scope. This has to be available from inside ld.so, which
is why I had to convert __pthread_threads (and friends) to
GL (dl_pthread_threads). NPTL does a similar thing with its global
thread lists, too.

Now, on to the good parts. It builds, and even appears to work
somewhat. And all the gsync_wake () calls are gone!

~/glibc/build$ ./testrun.sh /bin/uname
GNU
~/glibc/build$ ./testrun.sh --tool=rpctrace /bin/uname |& grep gsync
~/glibc/build$

Sergey Bugaev (3):
  XXX: Rename THREAD_GSCOPE_IN_TCB -> THREAD_GSCOPE_LINK_MAP
  XXX: Move __pthread_threads to ld.so
  XXX: Reimplement gscope

 elf/dl-reloc.c                           |  4 +--
 elf/dl-support.c                         |  6 ++--
 elf/dl-tls.c                             |  6 ++--
 elf/rtld.c                               |  2 +-
 htl/Versions                             |  2 +-
 htl/pt-alloc.c                           | 46 ++++++++++++------------
 htl/pt-create.c                          | 11 ++----
 htl/pt-internal.h                        | 28 +++++----------
 sysdeps/aarch64/nptl/tls.h               |  2 +-
 sysdeps/alpha/nptl/tls.h                 |  2 +-
 sysdeps/arc/nptl/tls.h                   |  2 +-
 sysdeps/arm/nptl/tls.h                   |  2 +-
 sysdeps/csky/nptl/tls.h                  |  2 +-
 sysdeps/generic/ldsodefs.h               | 18 ++++++----
 sysdeps/generic/tls.h                    |  3 +-
 sysdeps/hppa/nptl/tls.h                  |  2 +-
 sysdeps/htl/dl-thread_gscope_wait.c      | 36 +++++++++++++++++++
 sysdeps/htl/pt-key-delete.c              |  8 ++---
 sysdeps/htl/pthreadP.h                   |  2 +-
 sysdeps/htl/raise.c                      |  9 +++--
 sysdeps/htl/thrd_current.c               |  8 +++--
 sysdeps/i386/nptl/tls.h                  |  2 +-
 sysdeps/ia64/nptl/tls.h                  |  2 +-
 sysdeps/m68k/nptl/tls.h                  |  2 +-
 sysdeps/mach/hurd/htl/pt-sigstate-init.c |  2 +-
 sysdeps/mach/hurd/htl/pt-sysdep.c        |  2 +-
 sysdeps/mach/hurd/htl/pt-sysdep.h        |  2 +-
 sysdeps/mach/hurd/i386/tls.h             | 22 ++++++++++++
 sysdeps/mach/hurd/tls.h                  | 20 -----------
 sysdeps/microblaze/nptl/tls.h            |  2 +-
 sysdeps/mips/nptl/tls.h                  |  2 +-
 sysdeps/nios2/nptl/tls.h                 |  2 +-
 sysdeps/powerpc/nptl/tls.h               |  2 +-
 sysdeps/riscv/nptl/tls.h                 |  2 +-
 sysdeps/s390/nptl/tls.h                  |  2 +-
 sysdeps/sh/nptl/tls.h                    |  2 +-
 sysdeps/sparc/nptl/tls.h                 |  2 +-
 sysdeps/x86_64/nptl/tls.h                |  2 +-
 38 files changed, 156 insertions(+), 117 deletions(-)
 create mode 100644 sysdeps/htl/dl-thread_gscope_wait.c

-- 
2.31.1