1 files changed, 103 insertions, 0 deletions
diff --git a/lib/isc/include/isc/rwlock.h b/lib/isc/include/isc/rwlock.h
new file mode 100644
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+++ b/lib/isc/include/isc/rwlock.h
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+/*
+ * 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.
+ */
+
+#pragma once
+
+#include <inttypes.h>
+
+/*! \file isc/rwlock.h */
+
+#include <isc/atomic.h>
+#include <isc/condition.h>
+#include <isc/lang.h>
+#include <isc/types.h>
+
+ISC_LANG_BEGINDECLS
+
+typedef enum {
+	isc_rwlocktype_none = 0,
+	isc_rwlocktype_read,
+	isc_rwlocktype_write
+} isc_rwlocktype_t;
+
+#if USE_PTHREAD_RWLOCK
+#include <pthread.h>
+
+struct isc_rwlock {
+	pthread_rwlock_t rwlock;
+	atomic_bool	 downgrade;
+};
+
+#else /* USE_PTHREAD_RWLOCK */
+
+struct isc_rwlock {
+	/* Unlocked. */
+	unsigned int	    magic;
+	isc_mutex_t	    lock;
+	atomic_int_fast32_t spins;
+
+	/*
+	 * When some atomic instructions with hardware assistance are
+	 * available, rwlock will use those so that concurrent readers do not
+	 * interfere with each other through mutex as long as no writers
+	 * appear, massively reducing the lock overhead in the typical case.
+	 *
+	 * The basic algorithm of this approach is the "simple
+	 * writer-preference lock" shown in the following URL:
+	 * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/rw.html
+	 * but our implementation does not rely on the spin lock unlike the
+	 * original algorithm to be more portable as a user space application.
+	 */
+
+	/* Read or modified atomically. */
+	atomic_int_fast32_t write_requests;
+	atomic_int_fast32_t write_completions;
+	atomic_int_fast32_t cnt_and_flag;
+
+	/* Locked by lock. */
+	isc_condition_t readable;
+	isc_condition_t writeable;
+	unsigned int	readers_waiting;
+
+	/* Locked by rwlock itself. */
+	atomic_uint_fast32_t write_granted;
+
+	/* Unlocked. */
+	unsigned int write_quota;
+};
+
+#endif /* USE_PTHREAD_RWLOCK */
+
+void
+isc_rwlock_init(isc_rwlock_t *rwl, unsigned int read_quota,
+		unsigned int write_quota);
+
+isc_result_t
+isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type);
+
+isc_result_t
+isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type);
+
+isc_result_t
+isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type);
+
+isc_result_t
+isc_rwlock_tryupgrade(isc_rwlock_t *rwl);
+
+void
+isc_rwlock_downgrade(isc_rwlock_t *rwl);
+
+void
+isc_rwlock_destroy(isc_rwlock_t *rwl);
+
+ISC_LANG_ENDDECLS