New pthread rwlock that is more scalable.

This replaces the pthread rwlock with a new implementation that uses a
more scalable algorithm (primarily through not using a critical section
anymore to make state changes).  The fast path for rdlock acquisition and
release is now basically a single atomic read-modify write or CAS and a few
branches.  See nptl/pthread_rwlock_common.c for details.

	* nptl/DESIGN-rwlock.txt: Remove.
	* nptl/lowlevelrwlock.sym: Remove.
	* nptl/Makefile: Add new tests.
	* nptl/pthread_rwlock_common.c: New file.  Contains the new rwlock.
	* nptl/pthreadP.h (PTHREAD_RWLOCK_PREFER_READER_P): Remove.
	(PTHREAD_RWLOCK_WRPHASE, PTHREAD_RWLOCK_WRLOCKED,
	PTHREAD_RWLOCK_RWAITING, PTHREAD_RWLOCK_READER_SHIFT,
	PTHREAD_RWLOCK_READER_OVERFLOW, PTHREAD_RWLOCK_WRHANDOVER,
	PTHREAD_RWLOCK_FUTEX_USED): New.
	* nptl/pthread_rwlock_init.c (__pthread_rwlock_init): Adapt to new
	implementation.
	* nptl/pthread_rwlock_rdlock.c (__pthread_rwlock_rdlock_slow): Remove.
	(__pthread_rwlock_rdlock): Adapt.
	* nptl/pthread_rwlock_timedrdlock.c
	(pthread_rwlock_timedrdlock): Adapt.
	* nptl/pthread_rwlock_timedwrlock.c
	(pthread_rwlock_timedwrlock): Adapt.
	* nptl/pthread_rwlock_trywrlock.c (pthread_rwlock_trywrlock): Adapt.
	* nptl/pthread_rwlock_tryrdlock.c (pthread_rwlock_tryrdlock): Adapt.
	* nptl/pthread_rwlock_unlock.c (pthread_rwlock_unlock): Adapt.
	* nptl/pthread_rwlock_wrlock.c (__pthread_rwlock_wrlock_slow): Remove.
	(__pthread_rwlock_wrlock): Adapt.
	* nptl/tst-rwlock10.c: Adapt.
	* nptl/tst-rwlock11.c: Adapt.
	* nptl/tst-rwlock17.c: New file.
	* nptl/tst-rwlock18.c: New file.
	* nptl/tst-rwlock19.c: New file.
	* nptl/tst-rwlock2b.c: New file.
	* nptl/tst-rwlock8.c: Adapt.
	* nptl/tst-rwlock9.c: Adapt.
	* sysdeps/aarch64/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/arm/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/hppa/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/ia64/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/m68k/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/microblaze/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/mips/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/nios2/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/s390/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/sh/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/sparc/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/tile/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/unix/sysv/linux/alpha/bits/pthreadtypes.h
	(pthread_rwlock_t): Adapt.
	* sysdeps/unix/sysv/linux/powerpc/bits/pthreadtypes.h
	(pthread_rwlock_t): Adapt.
	* sysdeps/x86/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* nptl/nptl-printers.py (): Adapt.
	* nptl/nptl_lock_constants.pysym: Adapt.
	* nptl/test-rwlock-printers.py: Adapt.
	* nptl/test-rwlockattr-printers.c: Adapt.
	* nptl/test-rwlockattr-printers.py: Adapt.

1 files changed, 924 insertions, 0 deletions

diff --git a/nptl/pthread_rwlock_common.c b/nptl/pthread_rwlock_common.c
new file mode 100644
index 0000000000..256508ca2a
--- /dev/null
+++ b/nptl/pthread_rwlock_common.c
@@ -0,0 +1,924 @@
+/* POSIX reader--writer lock: core parts.
+   Copyright (C) 2016-2017 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 <errno.h>
+#include <sysdep.h>
+#include <pthread.h>
+#include <pthreadP.h>
+#include <sys/time.h>
+#include <stap-probe.h>
+#include <atomic.h>
+#include <futex-internal.h>
+
+
+/* A reader--writer lock that fulfills the POSIX requirements (but operations
+   on this lock are not necessarily full barriers, as one may interpret the
+   POSIX requirement about "synchronizing memory").  All critical sections are
+   in a total order, writers synchronize with prior writers and readers, and
+   readers synchronize with prior writers.
+
+   A thread is allowed to acquire a read lock recursively (i.e., have rdlock
+   critical sections that overlap in sequenced-before) unless the kind of the
+   rwlock is set to PTHREAD_RWLOCK_PREFER_WRITERS_NONRECURSIVE_NP.
+
+   This lock is built so that workloads of mostly readers can be executed with
+   low runtime overheads.  This matches that the default kind of the lock is
+   PTHREAD_RWLOCK_PREFER_READER_NP.  Acquiring a read lock requires a single
+   atomic addition if the lock is or was previously acquired by other
+   readers; releasing the lock is a single CAS if there are no concurrent
+   writers.
+   Workloads consisting of mostly writers are of secondary importance.
+   An uncontended write lock acquisition is as fast as for a normal
+   exclusive mutex but writer contention is somewhat more costly due to
+   keeping track of the exact number of writers.  If the rwlock kind requests
+   writers to be preferred (i.e., PTHREAD_RWLOCK_PREFER_WRITERS_NP or the
+   no-recursive-readers variant of it), then writer--to--writer lock ownership
+   hand-over is fairly fast and bypasses lock acquisition attempts by readers.
+   The costs of lock ownership transfer between readers and writers vary.  If
+   the program asserts that there are no recursive readers and writers are
+   preferred, then write lock acquisition attempts will block subsequent read
+   lock acquisition attempts, so that new incoming readers do not prolong a
+   phase in which readers have acquired the lock.
+
+
+   The main components of the rwlock are a writer-only lock that allows only
+   one of the concurrent writers to be the primary writer, and a
+   single-writer-multiple-readers lock that decides between read phases, in
+   which readers have acquired the rwlock, and write phases in which a primary
+   writer or a sequence of different primary writers have acquired the rwlock.
+
+   The single-writer-multiple-readers lock is the central piece of state
+   describing the rwlock and is encoded in the __readers field (see below for
+   a detailed explanation):
+
+   State WP  WL  R   RW  Notes
+   ---------------------------
+   #1    0   0   0   0   Lock is idle (and in a read phase).
+   #2    0   0   >0  0   Readers have acquired the lock.
+   #3    0   1   0   0   Lock is not acquired; a writer is waiting for a write
+			 phase to start or will try to start one.
+   #4    0   1   >0  0   Readers have acquired the lock; a writer is waiting
+			 and explicit hand-over to the writer is required.
+   #4a   0   1   >0  1   Same as #4 except that there are further readers
+			 waiting because the writer is to be preferred.
+   #5    1   0   0   0   Lock is idle (and in a write phase).
+   #6    1   0   >0  0   Write phase; readers are waiting for a read phase to
+			 start or will try to start one.
+   #7    1   1   0   0   Lock is acquired by a writer.
+   #8    1   1   >0  0   Lock acquired by a writer and readers are waiting;
+			 explicit hand-over to the readers is required.
+
+   WP (PTHREAD_RWLOCK_WRPHASE) is true if the lock is in a write phase, so
+   potentially acquired by a primary writer.
+   WL (PTHREAD_RWLOCK_WRLOCKED) is true if there is a primary writer (i.e.,
+   the thread that was able to set this bit from false to true).
+   R (all bits in __readers except the number of least-significant bits
+   denoted in PTHREAD_RWLOCK_READER_SHIFT) is the number of readers that have
+   or are trying to acquired the lock.  There may be more readers waiting if
+   writers are preferred and there will be no recursive readers, in which
+   case RW (PTHREAD_RWLOCK_RWAITING) is true in state #4a.
+
+   We want to block using futexes but using __readers as a futex word directly
+   is not a good solution.  First, we want to wait on different conditions
+   such as waiting for a phase change vs. waiting for the primary writer to
+   release the writer-only lock.  Second, the number of readers could change
+   frequently, which would make it likely that a writer's futex_wait fails
+   frequently too because the expected value does not match the value of
+   __readers anymore.
+   Therefore, we split out the futex words into the __wrphase_futex and
+   __writers_futex fields.  The former tracks the value of the WP bit and is
+   changed after changing WP by the thread that changes WP.  However, because
+   of the POSIX requirements regarding mutex/rwlock destruction (i.e., that
+   destroying a rwlock is allowed as soon as no thread has acquired or will
+   acquire the lock), we have to be careful and hand over lock ownership (via
+   a phase change) carefully to those threads waiting.  Specifically, we must
+   prevent a situation in which we are not quite sure whether we still have
+   to unblock another thread through a change to memory (executing a
+   futex_wake on a former futex word that is now used for something else is
+   fine).
+   The scheme we use for __wrphase_futex is that waiting threads that may
+   use the futex word to block now all have to use the futex word to block; it
+   is not allowed to take the short-cut and spin-wait on __readers because
+   then the waking thread cannot just make one final change to memory to
+   unblock all potentially waiting threads.  If, for example, a reader
+   increments R in states #7 or #8, it has to then block until __wrphase_futex
+   is 0 and it can confirm that the value of 0 was stored by the primary
+   writer; in turn, the primary writer has to change to a read phase too when
+   releasing WL (i.e., to state #2), and it must change __wrphase_futex to 0
+   as the next step.  This ensures that the waiting reader will not be able to
+   acquire, release, and then destroy the lock concurrently with the pending
+   futex unblock operations by the former primary writer.  This scheme is
+   called explicit hand-over in what follows.
+   Note that waiting threads can cancel waiting only if explicit hand-over has
+   not yet started (e.g., if __readers is still in states #7 or #8 in the
+   example above).
+
+   Writers determine the primary writer through WL.  Blocking using futexes
+   is performed using __writers_futex as a futex word; primary writers will
+   enable waiting on this futex by setting it to 1 after they acquired the WL
+   bit and will disable waiting by setting it to 0 before they release WL.
+   This leaves small windows where blocking using futexes is not possible
+   although a primary writer exists, but in turn decreases complexity of the
+   writer--writer synchronization and does not affect correctness.
+   If writers are preferred, writers can hand over WL directly to other
+   waiting writers that registered by incrementing __writers:  If the primary
+   writer can CAS __writers from a non-zero value to the same value with the
+   PTHREAD_RWLOCK_WRHANDOVER bit set, it effectively transfers WL ownership
+   to one of the registered waiting writers and does not reset WL; in turn,
+   a registered writer that can clear PTHREAD_RWLOCK_WRHANDOVER using a CAS
+   then takes over WL.  Note that registered waiting writers can cancel
+   waiting by decrementing __writers, but the last writer to unregister must
+   become the primary writer if PTHREAD_RWLOCK_WRHANDOVER is set.
+   Also note that adding another state/bit to signal potential writer--writer
+   contention (e.g., as done in the normal mutex algorithm) would not be
+   helpful because we would have to conservatively assume that there is in
+   fact no other writer, and wake up readers too.
+
+   To avoid having to call futex_wake when no thread uses __wrphase_futex or
+   __writers_futex, threads will set the PTHREAD_RWLOCK_FUTEX_USED bit in the
+   respective futex words before waiting on it (using a CAS so it will only be
+   set if in a state in which waiting would be possible).  In the case of
+   __writers_futex, we wake only one thread but several threads may share
+   PTHREAD_RWLOCK_FUTEX_USED, so we must assume that there are still others.
+   This is similar to what we do in pthread_mutex_lock.  We do not need to
+   do this for __wrphase_futex because there, we always wake all waiting
+   threads.
+
+   Blocking in the state #4a simply uses __readers as futex word.  This
+   simplifies the algorithm but suffers from some of the drawbacks discussed
+   before, though not to the same extent because R can only decrease in this
+   state, so the number of potentially failing futex_wait attempts will be
+   bounded.  All threads moving from state #4a to another state must wake
+   up threads blocked on the __readers futex.
+
+   The ordering invariants that we have to take care of in the implementation
+   are primarily those necessary for a reader--writer lock; this is rather
+   straightforward and happens during write/read phase switching (potentially
+   through explicit hand-over), and between writers through synchronization
+   involving the PTHREAD_RWLOCK_WRLOCKED or PTHREAD_RWLOCK_WRHANDOVER bits.
+   Additionally, we need to take care that modifications of __writers_futex
+   and __wrphase_futex (e.g., by otherwise unordered readers) take place in
+   the writer critical sections or read/write phases, respectively, and that
+   explicit hand-over observes stores from the previous phase.  How this is
+   done is explained in more detail in comments in the code.
+
+   Many of the accesses to the futex words just need relaxed MO.  This is
+   possible because we essentially drive both the core rwlock synchronization
+   and the futex synchronization in parallel.  For example, an unlock will
+   unlock the rwlock and take part in the futex synchronization (using
+   PTHREAD_RWLOCK_FUTEX_USED, see above); even if they are not tightly
+   ordered in some way, the futex synchronization ensures that there are no
+   lost wake-ups, and woken threads will then eventually see the most recent
+   state of the rwlock.  IOW, waiting threads will always be woken up, while
+   not being able to wait using futexes (which can happen) is harmless; in
+   turn, this means that waiting threads don't need special ordering wrt.
+   waking threads.
+
+   The futex synchronization consists of the three-state futex word:
+   (1) cannot block on it, (2) can block on it, and (3) there might be a
+   thread blocked on it (i.e., with PTHREAD_RWLOCK_FUTEX_USED set).
+   Relaxed-MO atomic read-modify-write operations are sufficient to maintain
+   this (e.g., using a CAS to go from (2) to (3) but not from (1) to (3)),
+   but we need ordering of the futex word modifications by the waking threads
+   so that they collectively make correct state changes between (1)-(3).
+   The futex-internal synchronization (i.e., the conceptual critical sections
+   around futex operations in the kernel) then ensures that even an
+   unconstrained load (i.e., relaxed MO) inside of futex_wait will not lead to
+   lost wake-ups because either the waiting thread will see the change from
+   (3) to (1) when a futex_wake came first, or this futex_wake will wake this
+   waiting thread because the waiting thread came first.
+
+
+   POSIX allows but does not require rwlock acquisitions to be a cancellation
+   point.  We do not support cancellation.
+
+   TODO We do not try to elide any read or write lock acquisitions currently.
+   While this would be possible, it is unclear whether HTM performance is
+   currently predictable enough and our runtime tuning is good enough at
+   deciding when to use elision so that enabling it would lead to consistently
+   better performance.  */
+
+
+static int
+__pthread_rwlock_get_private (pthread_rwlock_t *rwlock)
+{
+  return rwlock->__data.__shared != 0 ? FUTEX_SHARED : FUTEX_PRIVATE;
+}
+
+static __always_inline void
+__pthread_rwlock_rdunlock (pthread_rwlock_t *rwlock)
+{
+  int private = __pthread_rwlock_get_private (rwlock);
+  /* We decrease the number of readers, and if we are the last reader and
+     there is a primary writer, we start a write phase.  We use a CAS to
+     make this atomic so that it is clear whether we must hand over ownership
+     explicitly.  */
+  unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
+  unsigned int rnew;
+  for (;;)
+    {
+      rnew = r - (1 << PTHREAD_RWLOCK_READER_SHIFT);
+      /* If we are the last reader, we also need to unblock any readers
+	 that are waiting for a writer to go first (PTHREAD_RWLOCK_RWAITING)
+	 so that they can register while the writer is active.  */
+      if ((rnew >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
+	{
+	  if ((rnew & PTHREAD_RWLOCK_WRLOCKED) != 0)
+	    rnew |= PTHREAD_RWLOCK_WRPHASE;
+	  rnew &= ~(unsigned int) PTHREAD_RWLOCK_RWAITING;
+	}
+      /* We need release MO here for three reasons.  First, so that we
+	 synchronize with subsequent writers.  Second, we might have been the
+	 first reader and set __wrphase_futex to 0, so we need to synchronize
+	 with the last reader that will set it to 1 (note that we will always
+	 change __readers before the last reader, or we are the last reader).
+	 Third, a writer that takes part in explicit hand-over needs to see
+	 the first reader's store to __wrphase_futex (or a later value) if
+	 the writer observes that a write phase has been started.  */
+      if (atomic_compare_exchange_weak_release (&rwlock->__data.__readers,
+	  &r, rnew))
+	break;
+      /* TODO Back-off.  */
+    }
+  if ((rnew & PTHREAD_RWLOCK_WRPHASE) != 0)
+    {
+      /* We need to do explicit hand-over.  We need the acquire MO fence so
+	 that our modification of _wrphase_futex happens after a store by
+	 another reader that started a read phase.  Relaxed MO is sufficient
+	 for the modification of __wrphase_futex because it is just used
+	 to delay acquisition by a writer until all threads are unblocked
+	 irrespective of whether they are looking at __readers or
+	 __wrphase_futex; any other synchronizes-with relations that are
+	 necessary are established through __readers.  */
+      atomic_thread_fence_acquire ();
+      if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 1)
+	   & PTHREAD_RWLOCK_FUTEX_USED) != 0)
+	futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
+    }
+  /* Also wake up waiting readers if we did reset the RWAITING flag.  */
+  if ((r & PTHREAD_RWLOCK_RWAITING) != (rnew & PTHREAD_RWLOCK_RWAITING))
+    futex_wake (&rwlock->__data.__readers, INT_MAX, private);
+}
+
+
+static __always_inline int
+__pthread_rwlock_rdlock_full (pthread_rwlock_t *rwlock,
+    const struct timespec *abstime)
+{
+  unsigned int r;
+
+  /* Make sure we are not holding the rwlock as a writer.  This is a deadlock
+     situation we recognize and report.  */
+  if (__glibc_unlikely (atomic_load_relaxed (&rwlock->__data.__cur_writer)
+      == THREAD_GETMEM (THREAD_SELF, tid)))
+    return EDEADLK;
+
+  /* If we prefer writers, recursive rdlock is disallowed, we are in a read
+     phase, and there are other readers present, we try to wait without
+     extending the read phase.  We will be unblocked by either one of the
+     other active readers, or if the writer gives up WRLOCKED (e.g., on
+     timeout).
+     If there are no other readers, we simply race with any existing primary
+     writer; it would have been a race anyway, and changing the odds slightly
+     will likely not make a big difference.  */
+  if (rwlock->__data.__flags == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP)
+    {
+      r = atomic_load_relaxed (&rwlock->__data.__readers);
+      while (((r & PTHREAD_RWLOCK_WRPHASE) == 0)
+	      && ((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
+	      && ((r >> PTHREAD_RWLOCK_READER_SHIFT) > 0))
+	{
+	  /* TODO Spin first.  */
+	  /* Try setting the flag signaling that we are waiting without having
+	     incremented the number of readers.  Relaxed MO is fine because
+	     this is just about waiting for a state change in __readers.  */
+	  if (atomic_compare_exchange_weak_relaxed
+	      (&rwlock->__data.__readers, &r, r | PTHREAD_RWLOCK_RWAITING))
+	    {
+	      /* Wait for as long as the flag is set.  An ABA situation is
+		 harmless because the flag is just about the state of
+		 __readers, and all threads set the flag under the same
+		 conditions.  */
+	      while ((atomic_load_relaxed (&rwlock->__data.__readers)
+		  & PTHREAD_RWLOCK_RWAITING) != 0)
+		{
+		  int private = __pthread_rwlock_get_private (rwlock);
+		  int err = futex_abstimed_wait (&rwlock->__data.__readers,
+		      r, abstime, private);
+		  /* We ignore EAGAIN and EINTR.  On time-outs, we can just
+		     return because we don't need to clean up anything.  */
+		  if (err == ETIMEDOUT)
+		    return err;
+		}
+	      /* It makes sense to not break out of the outer loop here
+		 because we might be in the same situation again.  */
+	    }
+	  else
+	    {
+	      /* TODO Back-off.  */
+	    }
+	}
+    }
+  /* Register as a reader, using an add-and-fetch so that R can be used as
+     expected value for future operations.  Acquire MO so we synchronize with
+     prior writers as well as the last reader of the previous read phase (see
+     below).  */
+  r = atomic_fetch_add_acquire (&rwlock->__data.__readers,
+      (1 << PTHREAD_RWLOCK_READER_SHIFT)) + (1 << PTHREAD_RWLOCK_READER_SHIFT);
+
+  /* Check whether there is an overflow in the number of readers.  We assume
+     that the total number of threads is less than half the maximum number
+     of readers that we have bits for in __readers (i.e., with 32-bit int and
+     PTHREAD_RWLOCK_READER_SHIFT of 3, we assume there are less than
+     1 << (32-3-1) concurrent threads).
+     If there is an overflow, we use a CAS to try to decrement the number of
+     readers if there still is an overflow situation.  If so, we return
+     EAGAIN; if not, we are not a thread causing an overflow situation, and so
+     we just continue.  Using a fetch-add instead of the CAS isn't possible
+     because other readers might release the lock concurrently, which could
+     make us the last reader and thus responsible for handing ownership over
+     to writers (which requires a CAS too to make the decrement and ownership
+     transfer indivisible).  */
+  while (__glibc_unlikely (r >= PTHREAD_RWLOCK_READER_OVERFLOW))
+    {
+      /* Relaxed MO is okay because we just want to undo our registration and
+	 cannot have changed the rwlock state substantially if the CAS
+	 succeeds.  */
+      if (atomic_compare_exchange_weak_relaxed (&rwlock->__data.__readers, &r,
+	  r - (1 << PTHREAD_RWLOCK_READER_SHIFT)))
+	return EAGAIN;
+    }
+
+  /* We have registered as a reader, so if we are in a read phase, we have
+     acquired a read lock.  This is also the reader--reader fast-path.
+     Even if there is a primary writer, we just return.  If writers are to
+     be preferred and we are the only active reader, we could try to enter a
+     write phase to let the writer proceed.  This would be okay because we
+     cannot have acquired the lock previously as a reader (which could result
+     in deadlock if we would wait for the primary writer to run).  However,
+     this seems to be a corner case and handling it specially not be worth the
+     complexity.  */
+  if (__glibc_likely ((r & PTHREAD_RWLOCK_WRPHASE) == 0))
+    return 0;
+
+  /* If there is no primary writer but we are in a write phase, we can try
+     to install a read phase ourself.  */
+  while (((r & PTHREAD_RWLOCK_WRPHASE) != 0)
+      && ((r & PTHREAD_RWLOCK_WRLOCKED) == 0))
+    {
+       /* Try to enter a read phase: If the CAS below succeeds, we have
+	 ownership; if it fails, we will simply retry and reassess the
+	 situation.
+	 Acquire MO so we synchronize with prior writers.  */
+      if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers, &r,
+	  r ^ PTHREAD_RWLOCK_WRPHASE))
+	{
+	  /* We started the read phase, so we are also responsible for
+	     updating the write-phase futex.  Relaxed MO is sufficient.
+	     Note that there can be no other reader that we have to wake
+	     because all other readers will see the read phase started by us
+	     (or they will try to start it themselves); if a writer started
+	     the read phase, we cannot have started it.  Furthermore, we
+	     cannot discard a PTHREAD_RWLOCK_FUTEX_USED flag because we will
+	     overwrite the value set by the most recent writer (or the readers
+	     before it in case of explicit hand-over) and we know that there
+	     are no waiting readers.  */
+	  atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0);
+	  return 0;
+	}
+      else
+	{
+	  /* TODO Back off before retrying.  Also see above.  */
+	}
+    }
+
+  if ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
+    {
+      /* We are in a write phase, and there must be a primary writer because
+	 of the previous loop.  Block until the primary writer gives up the
+	 write phase.  This case requires explicit hand-over using
+	 __wrphase_futex.
+	 However, __wrphase_futex might not have been set to 1 yet (either
+	 because explicit hand-over to the writer is still ongoing, or because
+	 the writer has started the write phase but does not yet have updated
+	 __wrphase_futex).  The least recent value of __wrphase_futex we can
+	 read from here is the modification of the last read phase (because
+	 we synchronize with the last reader in this read phase through
+	 __readers; see the use of acquire MO on the fetch_add above).
+	 Therefore, if we observe a value of 0 for __wrphase_futex, we need
+	 to subsequently check that __readers now indicates a read phase; we
+	 need to use acquire MO for this so that if we observe a read phase,
+	 we will also see the modification of __wrphase_futex by the previous
+	 writer.  We then need to load __wrphase_futex again and continue to
+	 wait if it is not 0, so that we do not skip explicit hand-over.
+	 Relaxed MO is sufficient for the load from __wrphase_futex because
+	 we just use it as an indicator for when we can proceed; we use
+	 __readers and the acquire MO accesses to it to eventually read from
+	 the proper stores to __wrphase_futex.  */
+      unsigned int wpf;
+      bool ready = false;
+      for (;;)
+	{
+	  while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
+	      | PTHREAD_RWLOCK_FUTEX_USED) == (1 | PTHREAD_RWLOCK_FUTEX_USED))
+	    {
+	      int private = __pthread_rwlock_get_private (rwlock);
+	      if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0)
+		  && !atomic_compare_exchange_weak_relaxed
+		      (&rwlock->__data.__wrphase_futex,
+		       &wpf, wpf | PTHREAD_RWLOCK_FUTEX_USED))
+		continue;
+	      int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
+		  1 | PTHREAD_RWLOCK_FUTEX_USED, abstime, private);
+	      if (err == ETIMEDOUT)
+		{
+		  /* If we timed out, we need to unregister.  If no read phase
+		     has been installed while we waited, we can just decrement
+		     the number of readers.  Otherwise, we just acquire the
+		     lock, which is allowed because we give no precise timing
+		     guarantees, and because the timeout is only required to
+		     be in effect if we would have had to wait for other
+		     threads (e.g., if futex_wait would time-out immediately
+		     because the given absolute time is in the past).  */
+		  r = atomic_load_relaxed (&rwlock->__data.__readers);
+		  while ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
+		    {
+		      /* We don't need to make anything else visible to
+			 others besides unregistering, so relaxed MO is
+			 sufficient.  */
+		      if (atomic_compare_exchange_weak_relaxed
+			  (&rwlock->__data.__readers, &r,
+			   r - (1 << PTHREAD_RWLOCK_READER_SHIFT)))
+			return ETIMEDOUT;
+		      /* TODO Back-off.  */
+		    }
+		  /* Use the acquire MO fence to mirror the steps taken in the
+		     non-timeout case.  Note that the read can happen both
+		     in the atomic_load above as well as in the failure case
+		     of the CAS operation.  */
+		  atomic_thread_fence_acquire ();
+		  /* We still need to wait for explicit hand-over, but we must
+		     not use futex_wait anymore because we would just time out
+		     in this case and thus make the spin-waiting we need
+		     unnecessarily expensive.  */
+		  while ((atomic_load_relaxed (&rwlock->__data.__wrphase_futex)
+		      | PTHREAD_RWLOCK_FUTEX_USED)
+		      == (1 | PTHREAD_RWLOCK_FUTEX_USED))
+		    {
+		      /* TODO Back-off?  */
+		    }
+		  ready = true;
+		  break;
+		}
+	      /* If we got interrupted (EINTR) or the futex word does not have the
+		 expected value (EAGAIN), retry.  */
+	    }
+	  if (ready)
+	    /* See below.  */
+	    break;
+	  /* We need acquire MO here so that we synchronize with the lock
+	     release of the writer, and so that we observe a recent value of
+	     __wrphase_futex (see below).  */
+	  if ((atomic_load_acquire (&rwlock->__data.__readers)
+	      & PTHREAD_RWLOCK_WRPHASE) == 0)
+	    /* We are in a read phase now, so the least recent modification of
+	       __wrphase_futex we can read from is the store by the writer
+	       with value 1.  Thus, only now we can assume that if we observe
+	       a value of 0, explicit hand-over is finished. Retry the loop
+	       above one more time.  */
+	    ready = true;
+	}
+    }
+
+  return 0;
+}
+
+
+static __always_inline void
+__pthread_rwlock_wrunlock (pthread_rwlock_t *rwlock)
+{
+  int private = __pthread_rwlock_get_private (rwlock);
+
+  atomic_store_relaxed (&rwlock->__data.__cur_writer, 0);
+  /* Disable waiting by writers.  We will wake up after we decided how to
+     proceed.  */
+  bool wake_writers = ((atomic_exchange_relaxed
+      (&rwlock->__data.__writers_futex, 0) & PTHREAD_RWLOCK_FUTEX_USED) != 0);
+
+  if (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP)
+    {
+      /* First, try to hand over to another writer.  */
+      unsigned int w = atomic_load_relaxed (&rwlock->__data.__writers);
+      while (w != 0)
+	{
+	  /* Release MO so that another writer that gets WRLOCKED from us will
+	     synchronize with us and thus can take over our view of
+	     __readers (including, for example, whether we are in a write
+	     phase or not).  */
+	  if (atomic_compare_exchange_weak_release (&rwlock->__data.__writers,
+	      &w, w | PTHREAD_RWLOCK_WRHANDOVER))
+	    /* Another writer will take over.  */
+	    goto done;
+	  /* TODO Back-off.  */
+	}
+    }
+
+  /* We have done everything we needed to do to prefer writers, so now we
+     either hand over explicitly to readers if there are any, or we simply
+     stay in a write phase.  See pthread_rwlock_rdunlock for more details.  */
+  unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
+  /* Release MO so that subsequent readers or writers synchronize with us.  */
+  while (!atomic_compare_exchange_weak_release
+      (&rwlock->__data.__readers, &r, (r ^ PTHREAD_RWLOCK_WRLOCKED)
+	  ^ ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0 ? 0
+	      : PTHREAD_RWLOCK_WRPHASE)))
+    {
+      /* TODO Back-off.  */
+    }
+  if ((r >> PTHREAD_RWLOCK_READER_SHIFT) != 0)
+    {
+      /* We must hand over explicitly through __wrphase_futex.  Relaxed MO is
+	 sufficient because it is just used to delay acquisition by a writer;
+	 any other synchronizes-with relations that are necessary are
+	 established through __readers.  */
+      if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0)
+	   & PTHREAD_RWLOCK_FUTEX_USED) != 0)
+	futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
+    }
+
+ done:
+  /* We released WRLOCKED in some way, so wake a writer.  */
+  if (wake_writers)
+    futex_wake (&rwlock->__data.__writers_futex, 1, private);
+}
+
+
+static __always_inline int
+__pthread_rwlock_wrlock_full (pthread_rwlock_t *rwlock,
+    const struct timespec *abstime)
+{
+  /* Make sure we are not holding the rwlock as a writer.  This is a deadlock
+     situation we recognize and report.  */
+  if (__glibc_unlikely (atomic_load_relaxed (&rwlock->__data.__cur_writer)
+      == THREAD_GETMEM (THREAD_SELF, tid)))
+    return EDEADLK;
+
+  /* First we try to acquire the role of primary writer by setting WRLOCKED;
+     if it was set before, there already is a primary writer.  Acquire MO so
+     that we synchronize with previous primary writers.
+
+     We do not try to change to a write phase right away using a fetch_or
+     because we would have to reset it again and wake readers if there are
+     readers present (some readers could try to acquire the lock more than
+     once, so setting a write phase in the middle of this could cause
+     deadlock).  Changing to a write phase eagerly would only speed up the
+     transition from a read phase to a write phase in the uncontended case,
+     but it would slow down the contended case if readers are preferred (which
+     is the default).
+     We could try to CAS from a state with no readers to a write phase, but
+     this could be less scalable if readers arrive and leave frequently.  */
+  bool may_share_futex_used_flag = false;
+  unsigned int r = atomic_fetch_or_acquire (&rwlock->__data.__readers,
+      PTHREAD_RWLOCK_WRLOCKED);
+  if (__glibc_unlikely ((r & PTHREAD_RWLOCK_WRLOCKED) != 0))
+    {
+      /* There is another primary writer.  */
+      bool prefer_writer =
+	  (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP);
+      if (prefer_writer)
+	{
+	  /* We register as a waiting writer, so that we can make use of
+	     writer--writer hand-over.  Relaxed MO is fine because we just
+	     want to register.  We assume that the maximum number of threads
+	     is less than the capacity in __writers.  */
+	  atomic_fetch_add_relaxed (&rwlock->__data.__writers, 1);
+	}
+      for (;;)
+	{
+	  /* TODO Spin until WRLOCKED is 0 before trying the CAS below.
+	     But pay attention to not delay trying writer--writer hand-over
+	     for too long (which we must try eventually anyway).  */
+	  if ((r & PTHREAD_RWLOCK_WRLOCKED) == 0)
+	    {
+	      /* Try to become the primary writer or retry.  Acquire MO as in
+		 the fetch_or above.  */
+	      if (atomic_compare_exchange_weak_acquire
+		  (&rwlock->__data.__readers, &r,
+		      r | PTHREAD_RWLOCK_WRLOCKED))
+		{
+		  if (prefer_writer)
+		    {
+		      /* Unregister as a waiting writer.  Note that because we
+			 acquired WRLOCKED, WRHANDOVER will not be set.
+			 Acquire MO on the CAS above ensures that
+			 unregistering happens after the previous writer;
+			 this sorts the accesses to __writers by all
+			 primary writers in a useful way (e.g., any other
+			 primary writer acquiring after us or getting it from
+			 us through WRHANDOVER will see both our changes to
+			 __writers).
+			 ??? Perhaps this is not strictly necessary for
+			 reasons we do not yet know of.  */
+		      atomic_fetch_add_relaxed (&rwlock->__data.__writers,
+			  -1);
+		    }
+		  break;
+		}
+	      /* Retry if the CAS fails (r will have been updated).  */
+	      continue;
+	    }
+	  /* If writer--writer hand-over is available, try to become the
+	     primary writer this way by grabbing the WRHANDOVER token.  If we
+	     succeed, we own WRLOCKED.  */
+	  if (prefer_writer)
+	    {
+	      unsigned int w = atomic_load_relaxed
+		  (&rwlock->__data.__writers);
+	      if ((w & PTHREAD_RWLOCK_WRHANDOVER) != 0)
+		{
+		  /* Acquire MO is required here so that we synchronize with
+		     the writer that handed over WRLOCKED.  We also need this
+		     for the reload of __readers below because our view of
+		     __readers must be at least as recent as the view of the
+		     writer that handed over WRLOCKED; we must avoid an ABA
+		     through WRHANDOVER, which could, for example, lead to us
+		     assuming we are still in a write phase when in fact we
+		     are not.  */
+		  if (atomic_compare_exchange_weak_acquire
+		      (&rwlock->__data.__writers,
+		       &w, (w - PTHREAD_RWLOCK_WRHANDOVER - 1)))
+		    {
+		      /* Reload so our view is consistent with the view of
+			 the previous owner of WRLOCKED.  See above.  */
+		      r = atomic_load_relaxed (&rwlock->__data.__readers);
+		      break;
+		    }
+		  /* We do not need to reload __readers here.  We should try
+		     to perform writer--writer hand-over if possible; if it
+		     is not possible anymore, we will reload __readers
+		     elsewhere in this loop.  */
+		  continue;
+		}
+	    }
+	  /* We did not acquire WRLOCKED nor were able to use writer--writer
+	     hand-over, so we block on __writers_futex.  */
+	  int private = __pthread_rwlock_get_private (rwlock);
+	  unsigned int wf = atomic_load_relaxed
+	      (&rwlock->__data.__writers_futex);
+	  if (((wf & ~(unsigned int) PTHREAD_RWLOCK_FUTEX_USED) != 1)
+	      || ((wf != (1 | PTHREAD_RWLOCK_FUTEX_USED))
+		  && !atomic_compare_exchange_weak_relaxed
+		      (&rwlock->__data.__writers_futex, &wf,
+		       1 | PTHREAD_RWLOCK_FUTEX_USED)))
+	    {
+	      /* If we cannot block on __writers_futex because there is no
+		 primary writer, or we cannot set PTHREAD_RWLOCK_FUTEX_USED,
+		 we retry.  We must reload __readers here in case we cannot
+		 block on __writers_futex so that we can become the primary
+		 writer and are not stuck in a loop that just continuously
+		 fails to block on __writers_futex.  */
+	      r = atomic_load_relaxed (&rwlock->__data.__readers);
+	      continue;
+	    }
+	  /* We set the flag that signals that the futex is used, or we could
+	     have set it if we had been faster than other waiters.  As a
+	     result, we may share the flag with an unknown number of other
+	     writers.  Therefore, we must keep this flag set when we acquire
+	     the lock.  We do not need to do this when we do not reach this
+	     point here because then we are not part of the group that may
+	     share the flag, and another writer will wake one of the writers
+	     in this group.  */
+	  may_share_futex_used_flag = true;
+	  int err = futex_abstimed_wait (&rwlock->__data.__writers_futex,
+	      1 | PTHREAD_RWLOCK_FUTEX_USED, abstime, private);
+	  if (err == ETIMEDOUT)
+	    {
+	      if (prefer_writer)
+		{
+		  /* We need to unregister as a waiting writer.  If we are the
+		     last writer and writer--writer hand-over is available,
+		     we must make use of it because nobody else will reset
+		     WRLOCKED otherwise.  (If we use it, we simply pretend
+		     that this happened before the timeout; see
+		     pthread_rwlock_rdlock_full for the full reasoning.)
+		     Also see the similar code above.  */
+		  unsigned int w = atomic_load_relaxed
+		      (&rwlock->__data.__writers);
+		  while (!atomic_compare_exchange_weak_acquire
+		      (&rwlock->__data.__writers, &w,
+			  (w == PTHREAD_RWLOCK_WRHANDOVER + 1 ? 0 : w - 1)))
+		    {
+		      /* TODO Back-off.  */
+		    }
+		  if (w == PTHREAD_RWLOCK_WRHANDOVER + 1)
+		    {
+		      /* We must continue as primary writer.  See above.  */
+		      r = atomic_load_relaxed (&rwlock->__data.__readers);
+		      break;
+		    }
+		}
+	      /* We cleaned up and cannot have stolen another waiting writer's
+		 futex wake-up, so just return.  */
+	      return ETIMEDOUT;
+	    }
+	  /* If we got interrupted (EINTR) or the futex word does not have the
+	     expected value (EAGAIN), retry after reloading __readers.  */
+	  r = atomic_load_relaxed (&rwlock->__data.__readers);
+	}
+      /* Our snapshot of __readers is up-to-date at this point because we
+	 either set WRLOCKED using a CAS or were handed over WRLOCKED from
+	 another writer whose snapshot of __readers we inherit.  */
+    }
+
+  /* If we are in a read phase and there are no readers, try to start a write
+     phase.  */
+  while (((r & PTHREAD_RWLOCK_WRPHASE) == 0)
+      && ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0))
+    {
+      /* Acquire MO so that we synchronize with prior writers and do
+	 not interfere with their updates to __writers_futex, as well
+	 as regarding prior readers and their updates to __wrphase_futex,
+	 respectively.  */
+      if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers,
+	  &r, r | PTHREAD_RWLOCK_WRPHASE))
+	{
+	  /* We have started a write phase, so need to enable readers to wait.
+	     See the similar case in__pthread_rwlock_rdlock_full.  */
+	  atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
+	  /* Make sure we fall through to the end of the function.  */
+	  r |= PTHREAD_RWLOCK_WRPHASE;
+	  break;
+	}
+      /* TODO Back-off.  */
+    }
+
+  /* We are the primary writer; enable blocking on __writers_futex.  Relaxed
+     MO is sufficient for futex words; acquire MO on the previous
+     modifications of __readers ensures that this store happens after the
+     store of value 0 by the previous primary writer.  */
+  atomic_store_relaxed (&rwlock->__data.__writers_futex,
+      1 | (may_share_futex_used_flag ? PTHREAD_RWLOCK_FUTEX_USED : 0));
+
+  if (__glibc_unlikely ((r & PTHREAD_RWLOCK_WRPHASE) == 0))
+    {
+      /* We are not in a read phase and there are readers (because of the
+	 previous loop).  Thus, we have to wait for explicit hand-over from
+	 one of these readers.
+	 We basically do the same steps as for the similar case in
+	 __pthread_rwlock_rdlock_full, except that we additionally might try
+	 to directly hand over to another writer and need to wake up
+	 other writers or waiting readers (i.e., PTHREAD_RWLOCK_RWAITING).  */
+      unsigned int wpf;
+      bool ready = false;
+      for (;;)
+	{
+	  while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
+	      | PTHREAD_RWLOCK_FUTEX_USED) == PTHREAD_RWLOCK_FUTEX_USED)
+	    {
+	      int private = __pthread_rwlock_get_private (rwlock);
+	      if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0)
+		  && !atomic_compare_exchange_weak_relaxed
+		      (&rwlock->__data.__wrphase_futex, &wpf,
+		       PTHREAD_RWLOCK_FUTEX_USED))
+		continue;
+	      int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
+		  PTHREAD_RWLOCK_FUTEX_USED, abstime, private);
+	      if (err == ETIMEDOUT)
+		{
+		  if (rwlock->__data.__flags
+		      != PTHREAD_RWLOCK_PREFER_READER_NP)
+		    {
+		      /* We try writer--writer hand-over.  */
+		      unsigned int w = atomic_load_relaxed
+			  (&rwlock->__data.__writers);
+		      if (w != 0)
+			{
+			  /* We are about to hand over WRLOCKED, so we must
+			     release __writers_futex too; otherwise, we'd have
+			     a pending store, which could at least prevent
+			     other threads from waiting using the futex
+			     because it could interleave with the stores
+			     by subsequent writers.  In turn, this means that
+			     we have to clean up when we do not hand over
+			     WRLOCKED.
+			     Release MO so that another writer that gets
+			     WRLOCKED from us can take over our view of
+			     __readers.  */
+			  unsigned int wf = atomic_exchange_relaxed
+			      (&rwlock->__data.__writers_futex, 0);
+			  while (w != 0)
+			    {
+			      if (atomic_compare_exchange_weak_release
+				  (&rwlock->__data.__writers, &w,
+				      w | PTHREAD_RWLOCK_WRHANDOVER))
+				{
+				  /* Wake other writers.  */
+				  if ((wf & PTHREAD_RWLOCK_FUTEX_USED) != 0)
+				    futex_wake
+					(&rwlock->__data.__writers_futex, 1,
+					 private);
+				  return ETIMEDOUT;
+				}
+			      /* TODO Back-off.  */
+			    }
+			  /* We still own WRLOCKED and someone else might set
+			     a write phase concurrently, so enable waiting
+			     again.  Make sure we don't loose the flag that
+			     signals whether there are threads waiting on
+			     this futex.  */
+			  atomic_store_relaxed
+			      (&rwlock->__data.__writers_futex, wf);
+			}
+		    }
+		  /* If we timed out and we are not in a write phase, we can
+		     just stop being a primary writer.  Otherwise, we just
+		     acquire the lock.  */
+		  r = atomic_load_relaxed (&rwlock->__data.__readers);
+		  if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
+		    {
+		      /* We are about to release WRLOCKED, so we must release
+			 __writers_futex too; see the handling of
+			 writer--writer hand-over above.  */
+		      unsigned int wf = atomic_exchange_relaxed
+			  (&rwlock->__data.__writers_futex, 0);
+		      while ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
+			{
+			  /* While we don't need to make anything from a
+			     caller's critical section visible to other
+			     threads, we need to ensure that our changes to
+			     __writers_futex are properly ordered.
+			     Therefore, use release MO to synchronize with
+			     subsequent primary writers.  Also wake up any
+			     waiting readers as they are waiting because of
+			     us.  */
+			  if (atomic_compare_exchange_weak_release
+			      (&rwlock->__data.__readers, &r,
+			       (r ^ PTHREAD_RWLOCK_WRLOCKED)
+			       & ~(unsigned int) PTHREAD_RWLOCK_RWAITING))
+			    {
+			      /* Wake other writers.  */
+			      if ((wf & PTHREAD_RWLOCK_FUTEX_USED) != 0)
+				futex_wake (&rwlock->__data.__writers_futex,
+				    1, private);
+			      /* Wake waiting readers.  */
+			      if ((r & PTHREAD_RWLOCK_RWAITING) != 0)
+				futex_wake (&rwlock->__data.__readers,
+				    INT_MAX, private);
+			      return ETIMEDOUT;
+			    }
+			}
+		      /* We still own WRLOCKED and someone else might set a
+			 write phase concurrently, so enable waiting again.
+			 Make sure we don't loose the flag that signals
+			 whether there are threads waiting on this futex.  */
+		      atomic_store_relaxed (&rwlock->__data.__writers_futex,
+			  wf);
+		    }
+		  /* Use the acquire MO fence to mirror the steps taken in the
+		     non-timeout case.  Note that the read can happen both
+		     in the atomic_load above as well as in the failure case
+		     of the CAS operation.  */
+		  atomic_thread_fence_acquire ();
+		  /* We still need to wait for explicit hand-over, but we must
+		     not use futex_wait anymore.  */
+		  while ((atomic_load_relaxed
+		      (&rwlock->__data.__wrphase_futex)
+		       | PTHREAD_RWLOCK_FUTEX_USED)
+		      == PTHREAD_RWLOCK_FUTEX_USED)
+		    {
+		      /* TODO Back-off.  */
+		    }
+		  ready = true;
+		  break;
+		}
+	      /* If we got interrupted (EINTR) or the futex word does not have
+		 the expected value (EAGAIN), retry.  */
+	    }
+	  /* See pthread_rwlock_rdlock_full.  */
+	  if (ready)
+	    break;
+	  if ((atomic_load_acquire (&rwlock->__data.__readers)
+	      & PTHREAD_RWLOCK_WRPHASE) != 0)
+	    ready = true;
+	}
+    }
+
+  atomic_store_relaxed (&rwlock->__data.__cur_writer,
+      THREAD_GETMEM (THREAD_SELF, tid));
+  return 0;
+}