[Bug nptl/23844] pthread_rwlock_trywrlock results in hang

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=23844

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=86013ef5cea322b8f4b9c22f230c22cce369e947

commit 86013ef5cea322b8f4b9c22f230c22cce369e947
Author: Carlos O'Donell <carlos@redhat.com>
Date:   Mon Jan 21 22:50:12 2019 -0500

    nptl: Fix pthread_rwlock_try*lock stalls (Bug 23844)

    For a full analysis of both the pthread_rwlock_tryrdlock() stall
    and the pthread_rwlock_trywrlock() stall see:
    https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14

    In the pthread_rwlock_trydlock() function we fail to inspect for
    PTHREAD_RWLOCK_FUTEX_USED in __wrphase_futex and wake the waiting
    readers.

    In the pthread_rwlock_trywrlock() function we write 1 to
    __wrphase_futex and loose the setting of the PTHREAD_RWLOCK_FUTEX_USED
    bit, again failing to wake waiting readers during unlock.

    The fix in the case of pthread_rwlock_trydlock() is to check for
    PTHREAD_RWLOCK_FUTEX_USED and wake the readers.

    The fix in the case of pthread_rwlock_trywrlock() is to only write
    1 to __wrphase_futex if we installed the write phase, since all other
    readers would be spinning waiting for this step.

    We add two new tests, one exercises the stall for
    pthread_rwlock_trywrlock() which is easy to exercise, and one exercises
    the stall for pthread_rwlock_trydlock() which is harder to exercise.

    The pthread_rwlock_trywrlock() test fails consistently without the fix,
    and passes after. The pthread_rwlock_tryrdlock() test fails roughly
    5-10% of the time without the fix, and passes all the time after.

    Signed-off-by: Carlos O'Donell <carlos@redhat.com>
    Signed-off-by: Torvald Riegel <triegel@redhat.com>
    Signed-off-by: Rik Prohaska <prohaska7@gmail.com>
    Co-authored-by: Torvald Riegel <triegel@redhat.com>
    Co-authored-by: Rik Prohaska <prohaska7@gmail.com>
    (cherry picked from commit 5fc9ed4c4058bfbdf51ad6e7aac7d209b580e8c4)

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

Summary of changes:
 ChangeLog                         |   17 ++
 nptl/Makefile                     |    3 +-
 nptl/pthread_rwlock_tryrdlock.c   |   25 ++-
 nptl/pthread_rwlock_trywrlock.c   |    9 +-
 nptl/tst-rwlock-tryrdlock-stall.c |  355 +++++++++++++++++++++++++++++++++++++
 nptl/tst-rwlock-trywrlock-stall.c |  108 +++++++++++
 support/Makefile                  |    1 +
 support/xpthread_rwlock_destroy.c |   26 +++
 support/xthread.h                 |    1 +
 9 files changed, 534 insertions(+), 11 deletions(-)
 create mode 100644 nptl/tst-rwlock-tryrdlock-stall.c
 create mode 100644 nptl/tst-rwlock-trywrlock-stall.c
 create mode 100644 support/xpthread_rwlock_destroy.c

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