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>

3 files changed, 28 insertions, 0 deletions

diff --git a/support/Makefile b/support/Makefile
index 432cf2fe6c..c15b93647c 100644
--- a/support/Makefile
+++ b/support/Makefile
@@ -129,6 +129,7 @@ libsupport-routines = \
   xpthread_mutexattr_settype \
   xpthread_once \
   xpthread_rwlock_init \
+  xpthread_rwlock_destroy \
   xpthread_rwlock_rdlock \
   xpthread_rwlock_unlock \
   xpthread_rwlock_wrlock \
diff --git a/support/xpthread_rwlock_destroy.c b/support/xpthread_rwlock_destroy.c
new file mode 100644
index 0000000000..6d6e953569
--- /dev/null
+++ b/support/xpthread_rwlock_destroy.c
@@ -0,0 +1,26 @@
+/* pthread_rwlock_destroy with error checking.
+   Copyright (C) 2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xthread.h>
+
+void
+xpthread_rwlock_destroy (pthread_rwlock_t *rwlock)
+{
+  xpthread_check_return ("pthread_rwlock_destroy",
+                         pthread_rwlock_destroy (rwlock));
+}
diff --git a/support/xthread.h b/support/xthread.h
index 47c23235f3..9fe1f68b3b 100644
--- a/support/xthread.h
+++ b/support/xthread.h
@@ -84,6 +84,7 @@ void xpthread_rwlockattr_setkind_np (pthread_rwlockattr_t *attr, int pref);
 void xpthread_rwlock_wrlock (pthread_rwlock_t *rwlock);
 void xpthread_rwlock_rdlock (pthread_rwlock_t *rwlock);
 void xpthread_rwlock_unlock (pthread_rwlock_t *rwlock);
+void xpthread_rwlock_destroy (pthread_rwlock_t *rwlock);
 
 __END_DECLS