Adding upstream version 1:9.16.44.upstream/1%9.16.44

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 646 insertions, 0 deletions

diff --git a/lib/isc/rwlock.c b/lib/isc/rwlock.c
new file mode 100644
index 0000000..c69b6d7
--- /dev/null
+++ b/lib/isc/rwlock.c
@@ -0,0 +1,646 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+/*! \file */
+
+#include <inttypes.h>
+#include <stdbool.h>
+#include <stddef.h>
+
+#if defined(sun) && (defined(__sparc) || defined(__sparc__))
+#include <synch.h> /* for smt_pause(3c) */
+#endif /* if defined(sun) && (defined(__sparc) || defined(__sparc__)) */
+
+#include <isc/atomic.h>
+#include <isc/magic.h>
+#include <isc/platform.h>
+#include <isc/print.h>
+#include <isc/rwlock.h>
+#include <isc/util.h>
+
+#if USE_PTHREAD_RWLOCK
+
+#include <errno.h>
+#include <pthread.h>
+
+void
+isc_rwlock_init(isc_rwlock_t *rwl, unsigned int read_quota,
+		unsigned int write_quota) {
+	UNUSED(read_quota);
+	UNUSED(write_quota);
+	REQUIRE(pthread_rwlock_init(&rwl->rwlock, NULL) == 0);
+	atomic_init(&rwl->downgrade, false);
+}
+
+isc_result_t
+isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
+	switch (type) {
+	case isc_rwlocktype_read:
+		REQUIRE(pthread_rwlock_rdlock(&rwl->rwlock) == 0);
+		break;
+	case isc_rwlocktype_write:
+		while (true) {
+			REQUIRE(pthread_rwlock_wrlock(&rwl->rwlock) == 0);
+			/* Unlock if in middle of downgrade operation */
+			if (atomic_load_acquire(&rwl->downgrade)) {
+				REQUIRE(pthread_rwlock_unlock(&rwl->rwlock) ==
+					0);
+				while (atomic_load_acquire(&rwl->downgrade)) {
+				}
+				continue;
+			}
+			break;
+		}
+		break;
+	default:
+		UNREACHABLE();
+	}
+	return (ISC_R_SUCCESS);
+}
+
+isc_result_t
+isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
+	int ret = 0;
+	switch (type) {
+	case isc_rwlocktype_read:
+		ret = pthread_rwlock_tryrdlock(&rwl->rwlock);
+		break;
+	case isc_rwlocktype_write:
+		ret = pthread_rwlock_trywrlock(&rwl->rwlock);
+		if ((ret == 0) && atomic_load_acquire(&rwl->downgrade)) {
+			isc_rwlock_unlock(rwl, type);
+			return (ISC_R_LOCKBUSY);
+		}
+		break;
+	default:
+		UNREACHABLE();
+	}
+
+	switch (ret) {
+	case 0:
+		return (ISC_R_SUCCESS);
+	case EBUSY:
+		return (ISC_R_LOCKBUSY);
+	case EAGAIN:
+		return (ISC_R_LOCKBUSY);
+	default:
+		UNREACHABLE();
+	}
+}
+
+isc_result_t
+isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
+	UNUSED(type);
+	REQUIRE(pthread_rwlock_unlock(&rwl->rwlock) == 0);
+	return (ISC_R_SUCCESS);
+}
+
+isc_result_t
+isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
+	UNUSED(rwl);
+	return (ISC_R_LOCKBUSY);
+}
+
+void
+isc_rwlock_downgrade(isc_rwlock_t *rwl) {
+	isc_result_t result;
+	atomic_store_release(&rwl->downgrade, true);
+	result = isc_rwlock_unlock(rwl, isc_rwlocktype_write);
+	RUNTIME_CHECK(result == ISC_R_SUCCESS);
+	result = isc_rwlock_lock(rwl, isc_rwlocktype_read);
+	RUNTIME_CHECK(result == ISC_R_SUCCESS);
+	atomic_store_release(&rwl->downgrade, false);
+}
+
+void
+isc_rwlock_destroy(isc_rwlock_t *rwl) {
+	pthread_rwlock_destroy(&rwl->rwlock);
+}
+
+#else /* if USE_PTHREAD_RWLOCK */
+
+#define RWLOCK_MAGIC	  ISC_MAGIC('R', 'W', 'L', 'k')
+#define VALID_RWLOCK(rwl) ISC_MAGIC_VALID(rwl, RWLOCK_MAGIC)
+
+#ifndef RWLOCK_DEFAULT_READ_QUOTA
+#define RWLOCK_DEFAULT_READ_QUOTA 4
+#endif /* ifndef RWLOCK_DEFAULT_READ_QUOTA */
+
+#ifndef RWLOCK_DEFAULT_WRITE_QUOTA
+#define RWLOCK_DEFAULT_WRITE_QUOTA 4
+#endif /* ifndef RWLOCK_DEFAULT_WRITE_QUOTA */
+
+#ifndef RWLOCK_MAX_ADAPTIVE_COUNT
+#define RWLOCK_MAX_ADAPTIVE_COUNT 100
+#endif /* ifndef RWLOCK_MAX_ADAPTIVE_COUNT */
+
+#if defined(_MSC_VER)
+#include <intrin.h>
+#define isc_rwlock_pause() YieldProcessor()
+#elif defined(__x86_64__)
+#include <immintrin.h>
+#define isc_rwlock_pause() _mm_pause()
+#elif defined(__i386__)
+#define isc_rwlock_pause() __asm__ __volatile__("rep; nop")
+#elif defined(__ia64__)
+#define isc_rwlock_pause() __asm__ __volatile__("hint @pause")
+#elif defined(__arm__) && HAVE_ARM_YIELD
+#define isc_rwlock_pause() __asm__ __volatile__("yield")
+#elif defined(sun) && (defined(__sparc) || defined(__sparc__))
+#define isc_rwlock_pause() smt_pause()
+#elif (defined(__sparc) || defined(__sparc__)) && HAVE_SPARC_PAUSE
+#define isc_rwlock_pause() __asm__ __volatile__("pause")
+#elif defined(__ppc__) || defined(_ARCH_PPC) || defined(_ARCH_PWR) || \
+	defined(_ARCH_PWR2) || defined(_POWER)
+#define isc_rwlock_pause() __asm__ volatile("or 27,27,27")
+#else /* if defined(_MSC_VER) */
+#define isc_rwlock_pause()
+#endif /* if defined(_MSC_VER) */
+
+static isc_result_t
+isc__rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type);
+
+#ifdef ISC_RWLOCK_TRACE
+#include <stdio.h> /* Required for fprintf/stderr. */
+
+#include <isc/thread.h> /* Required for isc_thread_self(). */
+
+static void
+print_lock(const char *operation, isc_rwlock_t *rwl, isc_rwlocktype_t type) {
+	fprintf(stderr,
+		"rwlock %p thread %" PRIuPTR " %s(%s): "
+		"write_requests=%u, write_completions=%u, "
+		"cnt_and_flag=0x%x, readers_waiting=%u, "
+		"write_granted=%u, write_quota=%u\n",
+		rwl, isc_thread_self(), operation,
+		(type == isc_rwlocktype_read ? "read" : "write"),
+		atomic_load_acquire(&rwl->write_requests),
+		atomic_load_acquire(&rwl->write_completions),
+		atomic_load_acquire(&rwl->cnt_and_flag), rwl->readers_waiting,
+		atomic_load_acquire(&rwl->write_granted), rwl->write_quota);
+}
+#endif			/* ISC_RWLOCK_TRACE */
+
+void
+isc_rwlock_init(isc_rwlock_t *rwl, unsigned int read_quota,
+		unsigned int write_quota) {
+	REQUIRE(rwl != NULL);
+
+	/*
+	 * In case there's trouble initializing, we zero magic now.  If all
+	 * goes well, we'll set it to RWLOCK_MAGIC.
+	 */
+	rwl->magic = 0;
+
+	atomic_init(&rwl->spins, 0);
+	atomic_init(&rwl->write_requests, 0);
+	atomic_init(&rwl->write_completions, 0);
+	atomic_init(&rwl->cnt_and_flag, 0);
+	rwl->readers_waiting = 0;
+	atomic_init(&rwl->write_granted, 0);
+	if (read_quota != 0) {
+		UNEXPECTED_ERROR(__FILE__, __LINE__,
+				 "read quota is not supported");
+	}
+	if (write_quota == 0) {
+		write_quota = RWLOCK_DEFAULT_WRITE_QUOTA;
+	}
+	rwl->write_quota = write_quota;
+
+	isc_mutex_init(&rwl->lock);
+
+	isc_condition_init(&rwl->readable);
+	isc_condition_init(&rwl->writeable);
+
+	rwl->magic = RWLOCK_MAGIC;
+}
+
+void
+isc_rwlock_destroy(isc_rwlock_t *rwl) {
+	REQUIRE(VALID_RWLOCK(rwl));
+
+	REQUIRE(atomic_load_acquire(&rwl->write_requests) ==
+			atomic_load_acquire(&rwl->write_completions) &&
+		atomic_load_acquire(&rwl->cnt_and_flag) == 0 &&
+		rwl->readers_waiting == 0);
+
+	rwl->magic = 0;
+	(void)isc_condition_destroy(&rwl->readable);
+	(void)isc_condition_destroy(&rwl->writeable);
+	isc_mutex_destroy(&rwl->lock);
+}
+
+/*
+ * When some architecture-dependent atomic operations are available,
+ * rwlock can be more efficient than the generic algorithm defined below.
+ * The basic algorithm is described in the following URL:
+ *   http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/rw.html
+ *
+ * The key is to use the following integer variables modified atomically:
+ *   write_requests, write_completions, and cnt_and_flag.
+ *
+ * write_requests and write_completions act as a waiting queue for writers
+ * in order to ensure the FIFO order.  Both variables begin with the initial
+ * value of 0.  When a new writer tries to get a write lock, it increments
+ * write_requests and gets the previous value of the variable as a "ticket".
+ * When write_completions reaches the ticket number, the new writer can start
+ * writing.  When the writer completes its work, it increments
+ * write_completions so that another new writer can start working.  If the
+ * write_requests is not equal to write_completions, it means a writer is now
+ * working or waiting.  In this case, a new readers cannot start reading, or
+ * in other words, this algorithm basically prefers writers.
+ *
+ * cnt_and_flag is a "lock" shared by all readers and writers.  This integer
+ * variable is a kind of structure with two members: writer_flag (1 bit) and
+ * reader_count (31 bits).  The writer_flag shows whether a writer is working,
+ * and the reader_count shows the number of readers currently working or almost
+ * ready for working.  A writer who has the current "ticket" tries to get the
+ * lock by exclusively setting the writer_flag to 1, provided that the whole
+ * 32-bit is 0 (meaning no readers or writers working).  On the other hand,
+ * a new reader tries to increment the "reader_count" field provided that
+ * the writer_flag is 0 (meaning there is no writer working).
+ *
+ * If some of the above operations fail, the reader or the writer sleeps
+ * until the related condition changes.  When a working reader or writer
+ * completes its work, some readers or writers are sleeping, and the condition
+ * that suspended the reader or writer has changed, it wakes up the sleeping
+ * readers or writers.
+ *
+ * As already noted, this algorithm basically prefers writers.  In order to
+ * prevent readers from starving, however, the algorithm also introduces the
+ * "writer quota" (Q).  When Q consecutive writers have completed their work,
+ * suspending readers, the last writer will wake up the readers, even if a new
+ * writer is waiting.
+ *
+ * Implementation specific note: due to the combination of atomic operations
+ * and a mutex lock, ordering between the atomic operation and locks can be
+ * very sensitive in some cases.  In particular, it is generally very important
+ * to check the atomic variable that requires a reader or writer to sleep after
+ * locking the mutex and before actually sleeping; otherwise, it could be very
+ * likely to cause a deadlock.  For example, assume "var" is a variable
+ * atomically modified, then the corresponding code would be:
+ *	if (var == need_sleep) {
+ *		LOCK(lock);
+ *		if (var == need_sleep)
+ *			WAIT(cond, lock);
+ *		UNLOCK(lock);
+ *	}
+ * The second check is important, since "var" is protected by the atomic
+ * operation, not by the mutex, and can be changed just before sleeping.
+ * (The first "if" could be omitted, but this is also important in order to
+ * make the code efficient by avoiding the use of the mutex unless it is
+ * really necessary.)
+ */
+
+#define WRITER_ACTIVE 0x1
+#define READER_INCR   0x2
+
+static isc_result_t
+isc__rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
+	int32_t cntflag;
+
+	REQUIRE(VALID_RWLOCK(rwl));
+
+#ifdef ISC_RWLOCK_TRACE
+	print_lock("prelock", rwl, type);
+#endif /* ifdef ISC_RWLOCK_TRACE */
+
+	if (type == isc_rwlocktype_read) {
+		if (atomic_load_acquire(&rwl->write_requests) !=
+		    atomic_load_acquire(&rwl->write_completions))
+		{
+			/* there is a waiting or active writer */
+			LOCK(&rwl->lock);
+			if (atomic_load_acquire(&rwl->write_requests) !=
+			    atomic_load_acquire(&rwl->write_completions))
+			{
+				rwl->readers_waiting++;
+				WAIT(&rwl->readable, &rwl->lock);
+				rwl->readers_waiting--;
+			}
+			UNLOCK(&rwl->lock);
+		}
+
+		cntflag = atomic_fetch_add_release(&rwl->cnt_and_flag,
+						   READER_INCR);
+		POST(cntflag);
+		while (1) {
+			if ((atomic_load_acquire(&rwl->cnt_and_flag) &
+			     WRITER_ACTIVE) == 0)
+			{
+				break;
+			}
+
+			/* A writer is still working */
+			LOCK(&rwl->lock);
+			rwl->readers_waiting++;
+			if ((atomic_load_acquire(&rwl->cnt_and_flag) &
+			     WRITER_ACTIVE) != 0)
+			{
+				WAIT(&rwl->readable, &rwl->lock);
+			}
+			rwl->readers_waiting--;
+			UNLOCK(&rwl->lock);
+
+			/*
+			 * Typically, the reader should be able to get a lock
+			 * at this stage:
+			 *   (1) there should have been no pending writer when
+			 *       the reader was trying to increment the
+			 *       counter; otherwise, the writer should be in
+			 *       the waiting queue, preventing the reader from
+			 *       proceeding to this point.
+			 *   (2) once the reader increments the counter, no
+			 *       more writer can get a lock.
+			 * Still, it is possible another writer can work at
+			 * this point, e.g. in the following scenario:
+			 *   A previous writer unlocks the writer lock.
+			 *   This reader proceeds to point (1).
+			 *   A new writer appears, and gets a new lock before
+			 *   the reader increments the counter.
+			 *   The reader then increments the counter.
+			 *   The previous writer notices there is a waiting
+			 *   reader who is almost ready, and wakes it up.
+			 * So, the reader needs to confirm whether it can now
+			 * read explicitly (thus we loop).  Note that this is
+			 * not an infinite process, since the reader has
+			 * incremented the counter at this point.
+			 */
+		}
+
+		/*
+		 * If we are temporarily preferred to writers due to the writer
+		 * quota, reset the condition (race among readers doesn't
+		 * matter).
+		 */
+		atomic_store_release(&rwl->write_granted, 0);
+	} else {
+		int32_t prev_writer;
+
+		/* enter the waiting queue, and wait for our turn */
+		prev_writer = atomic_fetch_add_release(&rwl->write_requests, 1);
+		while (atomic_load_acquire(&rwl->write_completions) !=
+		       prev_writer)
+		{
+			LOCK(&rwl->lock);
+			if (atomic_load_acquire(&rwl->write_completions) !=
+			    prev_writer)
+			{
+				WAIT(&rwl->writeable, &rwl->lock);
+				UNLOCK(&rwl->lock);
+				continue;
+			}
+			UNLOCK(&rwl->lock);
+			break;
+		}
+
+		while (!atomic_compare_exchange_weak_acq_rel(
+			&rwl->cnt_and_flag, &(int_fast32_t){ 0 },
+			WRITER_ACTIVE))
+		{
+			/* Another active reader or writer is working. */
+			LOCK(&rwl->lock);
+			if (atomic_load_acquire(&rwl->cnt_and_flag) != 0) {
+				WAIT(&rwl->writeable, &rwl->lock);
+			}
+			UNLOCK(&rwl->lock);
+		}
+
+		INSIST((atomic_load_acquire(&rwl->cnt_and_flag) &
+			WRITER_ACTIVE));
+		atomic_fetch_add_release(&rwl->write_granted, 1);
+	}
+
+#ifdef ISC_RWLOCK_TRACE
+	print_lock("postlock", rwl, type);
+#endif /* ifdef ISC_RWLOCK_TRACE */
+
+	return (ISC_R_SUCCESS);
+}
+
+isc_result_t
+isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
+	int32_t cnt = 0;
+	int32_t spins = atomic_load_acquire(&rwl->spins) * 2 + 10;
+	int32_t max_cnt = ISC_MAX(spins, RWLOCK_MAX_ADAPTIVE_COUNT);
+	isc_result_t result = ISC_R_SUCCESS;
+
+	do {
+		if (cnt++ >= max_cnt) {
+			result = isc__rwlock_lock(rwl, type);
+			break;
+		}
+		isc_rwlock_pause();
+	} while (isc_rwlock_trylock(rwl, type) != ISC_R_SUCCESS);
+
+	atomic_fetch_add_release(&rwl->spins, (cnt - spins) / 8);
+
+	return (result);
+}
+
+isc_result_t
+isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
+	int32_t cntflag;
+
+	REQUIRE(VALID_RWLOCK(rwl));
+
+#ifdef ISC_RWLOCK_TRACE
+	print_lock("prelock", rwl, type);
+#endif /* ifdef ISC_RWLOCK_TRACE */
+
+	if (type == isc_rwlocktype_read) {
+		/* If a writer is waiting or working, we fail. */
+		if (atomic_load_acquire(&rwl->write_requests) !=
+		    atomic_load_acquire(&rwl->write_completions))
+		{
+			return (ISC_R_LOCKBUSY);
+		}
+
+		/* Otherwise, be ready for reading. */
+		cntflag = atomic_fetch_add_release(&rwl->cnt_and_flag,
+						   READER_INCR);
+		if ((cntflag & WRITER_ACTIVE) != 0) {
+			/*
+			 * A writer is working.  We lose, and cancel the read
+			 * request.
+			 */
+			cntflag = atomic_fetch_sub_release(&rwl->cnt_and_flag,
+							   READER_INCR);
+			/*
+			 * If no other readers are waiting and we've suspended
+			 * new writers in this short period, wake them up.
+			 */
+			if (cntflag == READER_INCR &&
+			    atomic_load_acquire(&rwl->write_completions) !=
+				    atomic_load_acquire(&rwl->write_requests))
+			{
+				LOCK(&rwl->lock);
+				BROADCAST(&rwl->writeable);
+				UNLOCK(&rwl->lock);
+			}
+
+			return (ISC_R_LOCKBUSY);
+		}
+	} else {
+		/* Try locking without entering the waiting queue. */
+		int_fast32_t zero = 0;
+		if (!atomic_compare_exchange_strong_acq_rel(
+			    &rwl->cnt_and_flag, &zero, WRITER_ACTIVE))
+		{
+			return (ISC_R_LOCKBUSY);
+		}
+
+		/*
+		 * XXXJT: jump into the queue, possibly breaking the writer
+		 * order.
+		 */
+		atomic_fetch_sub_release(&rwl->write_completions, 1);
+		atomic_fetch_add_release(&rwl->write_granted, 1);
+	}
+
+#ifdef ISC_RWLOCK_TRACE
+	print_lock("postlock", rwl, type);
+#endif /* ifdef ISC_RWLOCK_TRACE */
+
+	return (ISC_R_SUCCESS);
+}
+
+isc_result_t
+isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
+	REQUIRE(VALID_RWLOCK(rwl));
+
+	int_fast32_t reader_incr = READER_INCR;
+
+	/* Try to acquire write access. */
+	atomic_compare_exchange_strong_acq_rel(&rwl->cnt_and_flag, &reader_incr,
+					       WRITER_ACTIVE);
+	/*
+	 * There must have been no writer, and there must have
+	 * been at least one reader.
+	 */
+	INSIST((reader_incr & WRITER_ACTIVE) == 0 &&
+	       (reader_incr & ~WRITER_ACTIVE) != 0);
+
+	if (reader_incr == READER_INCR) {
+		/*
+		 * We are the only reader and have been upgraded.
+		 * Now jump into the head of the writer waiting queue.
+		 */
+		atomic_fetch_sub_release(&rwl->write_completions, 1);
+	} else {
+		return (ISC_R_LOCKBUSY);
+	}
+
+	return (ISC_R_SUCCESS);
+}
+
+void
+isc_rwlock_downgrade(isc_rwlock_t *rwl) {
+	int32_t prev_readers;
+
+	REQUIRE(VALID_RWLOCK(rwl));
+
+	/* Become an active reader. */
+	prev_readers = atomic_fetch_add_release(&rwl->cnt_and_flag,
+						READER_INCR);
+	/* We must have been a writer. */
+	INSIST((prev_readers & WRITER_ACTIVE) != 0);
+
+	/* Complete write */
+	atomic_fetch_sub_release(&rwl->cnt_and_flag, WRITER_ACTIVE);
+	atomic_fetch_add_release(&rwl->write_completions, 1);
+
+	/* Resume other readers */
+	LOCK(&rwl->lock);
+	if (rwl->readers_waiting > 0) {
+		BROADCAST(&rwl->readable);
+	}
+	UNLOCK(&rwl->lock);
+}
+
+isc_result_t
+isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
+	int32_t prev_cnt;
+
+	REQUIRE(VALID_RWLOCK(rwl));
+
+#ifdef ISC_RWLOCK_TRACE
+	print_lock("preunlock", rwl, type);
+#endif /* ifdef ISC_RWLOCK_TRACE */
+
+	if (type == isc_rwlocktype_read) {
+		prev_cnt = atomic_fetch_sub_release(&rwl->cnt_and_flag,
+						    READER_INCR);
+		/*
+		 * If we're the last reader and any writers are waiting, wake
+		 * them up.  We need to wake up all of them to ensure the
+		 * FIFO order.
+		 */
+		if (prev_cnt == READER_INCR &&
+		    atomic_load_acquire(&rwl->write_completions) !=
+			    atomic_load_acquire(&rwl->write_requests))
+		{
+			LOCK(&rwl->lock);
+			BROADCAST(&rwl->writeable);
+			UNLOCK(&rwl->lock);
+		}
+	} else {
+		bool wakeup_writers = true;
+
+		/*
+		 * Reset the flag, and (implicitly) tell other writers
+		 * we are done.
+		 */
+		atomic_fetch_sub_release(&rwl->cnt_and_flag, WRITER_ACTIVE);
+		atomic_fetch_add_release(&rwl->write_completions, 1);
+
+		if ((atomic_load_acquire(&rwl->write_granted) >=
+		     rwl->write_quota) ||
+		    (atomic_load_acquire(&rwl->write_requests) ==
+		     atomic_load_acquire(&rwl->write_completions)) ||
+		    (atomic_load_acquire(&rwl->cnt_and_flag) & ~WRITER_ACTIVE))
+		{
+			/*
+			 * We have passed the write quota, no writer is
+			 * waiting, or some readers are almost ready, pending
+			 * possible writers.  Note that the last case can
+			 * happen even if write_requests != write_completions
+			 * (which means a new writer in the queue), so we need
+			 * to catch the case explicitly.
+			 */
+			LOCK(&rwl->lock);
+			if (rwl->readers_waiting > 0) {
+				wakeup_writers = false;
+				BROADCAST(&rwl->readable);
+			}
+			UNLOCK(&rwl->lock);
+		}
+
+		if ((atomic_load_acquire(&rwl->write_requests) !=
+		     atomic_load_acquire(&rwl->write_completions)) &&
+		    wakeup_writers)
+		{
+			LOCK(&rwl->lock);
+			BROADCAST(&rwl->writeable);
+			UNLOCK(&rwl->lock);
+		}
+	}
+
+#ifdef ISC_RWLOCK_TRACE
+	print_lock("postunlock", rwl, type);
+#endif /* ifdef ISC_RWLOCK_TRACE */
+
+	return (ISC_R_SUCCESS);
+}
+
+#endif /* USE_PTHREAD_RWLOCK */