[Bug nptl/18743] PowerPC: findutils testcase fails with --enable-lock-elision

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https://sourceware.org/bugzilla/show_bug.cgi?id=18743

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=5b319ce2949cf6fb97862ff81558944f76c704f1

commit 5b319ce2949cf6fb97862ff81558944f76c704f1
Author: Tulio Magno Quites Machado Filho <tuliom@linux.vnet.ibm.com>
Date:   Wed Jul 22 09:26:02 2015 -0300

    PowerPC: Fix a race condition when eliding a lock

    The previous code used to evaluate the preprocessor token is_lock_free to
    a variable before starting a transaction.  This behavior can cause an
    error if another thread got the lock (without using a transaction)
    between the evaluation of the token and the beginning of the transaction.

    This bug can be triggered with the following order of events:
    1. The lock accessed by is_lock_free is free.
    2. Thread T1 evaluates is_lock_free and stores into register R1 that the
       lock is free.
    3. Thread T2 acquires the same lock used in is_lock_free.
    4. T1 begins the transaction, creating a memory barrier where is_lock_free
       is false, but R1 is true.
    5. T1 reads R1 and doesn't abort the transaction.
    6. T1 calls ELIDE_UNLOCK, which reads false from is_lock_free and decides
       to unlock a lock acquired by T2, leading to undefined behavior.

    This patch delays the evaluation of is_lock_free to inside a transaction
    by moving this part of the code to the macro ELIDE_LOCK.

        [BZ #18743]
        * sysdeps/powerpc/nptl/elide.h (__elide_lock): Move most of this
        code to...
        (ELIDE_LOCK): ...here.
        (__get_new_count): New function with part of the code from
        __elide_lock that updates the value of adapt_count after a
        transaction abort.
        (__elided_trylock): Moved this code to...
        (ELIDE_TRYLOCK): ...here.

    (cherry picked from commit 6ec52bf634b7650b57ff67b5f5053bce8992d549)

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

Summary of changes:
 ChangeLog                    |   12 ++++
 NEWS                         |    4 +-
 sysdeps/powerpc/nptl/elide.h |  115 +++++++++++++++++++++++-------------------
 3 files changed, 77 insertions(+), 54 deletions(-)

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