• Bruno Haible's avatar
    lock: Provide guarantee to avoid writer starvation for rwlocks. · be95b17a
    Bruno Haible authored
    The rationale is: 1) Read-preferring read-write locks are prone to
    writer starvation if the number of reader threads multiplied by the
    percentage of time they have the lock held is too high. 2) Write-
    preferring read-write locks are the only reliable way to avoid this.
    3) There have been reports of 'test-lock' hanging on glibc systems
    http://lists.gnu.org/archive/html/bug-gnulib/2017-01/msg00009.html,
    and glibc indeed implements read-preferring rwlocks by default, see
    http://man7.org/linux/man-pages/man3/pthread_rwlockattr_setkind_np.3.html
    and https://sourceware.org/bugzilla/show_bug.cgi?id=13701 .
    
    * m4/pthread_rwlock_rdlock.m4: New file.
    * m4/lock.m4 (gl_LOCK): Invoke gl_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER.
    * lib/glthread/lock.h [USE_POSIX_THREADS]: Test
    HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER. Use a different implementation
    of rwlock initialization on glibc systems without
    HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER. Use a different implementation
    of rwlocks altogether on non-glibc systems without
    HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER.
    [USE_PTH_THREADS]: Use a different implementation of rwlocks altogether.
    * lib/glthread/lock.c [USE_POSIX_THREADS]
    (glthread_rwlock_init_for_glibc): New function.
    [USE_POSIX_THREADS] (glthread_rwlock_rdlock_multithreaded): Update
    comment.
    [USE_PTH_THREADS]: New implementation of rwlocks.
    [USE_WINDOWS_THREADS] (glthread_rwlock_rdlock_func): Prefer writers over
    readers.
    * modules/lock (Files): Add m4/pthread_rwlock_rdlock.m4.
    (Depends-on): Add 'extensions'.
    * tests/test-rwlock1.c: New file.
    * lock-tests (Files): Add it.
    (Depends-on): Add usleep.
    (Makefile.am): Add test-rwlock1 to the tests.
    be95b17a