[Bug nptl/23137] s390: pthread_join sometimes block indefinitely (on 31bit and libc build with -Os)

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https://sourceware.org/bugzilla/show_bug.cgi?id=23137

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=1660901840dfc9fde6c5720a32f901af6f08f00a

commit 1660901840dfc9fde6c5720a32f901af6f08f00a
Author: Stefan Liebler <stli@linux.vnet.ibm.com>
Date:   Fri May 4 10:00:59 2018 +0200

    Fix blocking pthread_join. [BZ #23137]

    On s390 (31bit) if glibc is build with -Os, pthread_join sometimes
    blocks indefinitely. This is e.g. observable with
    testcase intl/tst-gettext6.

    pthread_join is calling lll_wait_tid(tid), which performs the futex-wait
    syscall in a loop as long as tid != 0 (thread is alive).

    On s390 (and build with -Os), tid is loaded from memory before
    comparing against zero and then the tid is loaded a second time
    in order to pass it to the futex-wait-syscall.
    If the thread exits in between, then the futex-wait-syscall is
    called with the value zero and it waits until a futex-wake occurs.
    As the thread is already exited, there won't be a futex-wake.

    In lll_wait_tid, the tid is stored to the local variable __tid,
    which is then used as argument for the futex-wait-syscall.
    But unfortunately the compiler is allowed to reload the value
    from memory.

    With this patch, the tid is loaded with atomic_load_acquire.
    Then the compiler is not allowed to reload the value for __tid from memory.

    ChangeLog:

        [BZ #23137]
        * sysdeps/nptl/lowlevellock.h (lll_wait_tid):
        Use atomic_load_acquire to load __tid.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                   |    6 ++++++
 sysdeps/nptl/lowlevellock.h |   13 ++++++++-----
 2 files changed, 14 insertions(+), 5 deletions(-)

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