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diff --git a/src/contrib/spinlock.h b/src/contrib/spinlock.h
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+/* Copyright (C) 2021 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+
+ This program 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 General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/*!
+ * \brief Multiplatform spinlock.
+ */
+
+#pragma once
+
+#if (__STDC_VERSION__ >= 201112L) && !defined(__STDC_NO_ATOMICS__)
+/* Not tested and activated yet. */
+/* #define HAVE_STDATOMIC */
+#endif
+
+#if defined(__APPLE__)
+# if defined(MAC_OS_X_VERSION_10_12) || \
+ MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_12
+# define APPLE_SPIN_NEW
+# else
+# define APPLE_SPIN_OLD
+# endif /* MAC_OS_X_VERSION_10_12 ... */
+#endif /* __APPLE__ */
+
+#if defined(HAVE_SYNC_ATOMIC) || defined(HAVE_ATOMIC)
+# include <stdbool.h>
+#elif defined(HAVE_STDATOMIC)
+# include <stdbool.h>
+# include <stdatomic.h>
+#elif defined(APPLE_SPIN_NEW)
+# include <os/lock.h>
+#elif defined(APPLE_SPIN_OLD)
+# include <libkern/OSAtomic.h>
+#else /* POSIX pthread spinlock. */
+# include <pthread.h>
+#endif
+
+/*! \brief Spinlock lock variable type. */
+typedef
+#if defined(HAVE_SYNC_ATOMIC) || defined(HAVE_ATOMIC)
+ bool /*!< Spinlock lock - a simple & fast atomic version. */
+#elif defined(HAVE_STDATOMIC)
+ atomic_bool /*!< Spinlock lock - a simple & fast atomic version, C11 */
+#elif defined(APPLE_SPIN_NEW)
+ os_unfair_lock /*!< Spinlock lock - a newer macOS version (macOS >= 10.12). */
+#elif defined(APPLE_SPIN_OLD)
+ OSSpinLock /*!< Spinlock lock - an older macOS version (macOS < 10.12). */
+#else /* POSIX */
+ pthread_spinlock_t /*!< Spinlock lock - a POSIX pthread version. */
+#endif
+ knot_spin_t;
+
+/*! \brief Initialize the spinlock pointed to by the parameter "lock". */
+static inline void knot_spin_init(knot_spin_t *lock)
+{
+#if defined(HAVE_SYNC_ATOMIC) || defined(HAVE_ATOMIC)
+ *lock = false;
+#elif defined(HAVE_STDATOMIC)
+ atomic_init(lock, false);
+#elif defined(APPLE_SPIN_NEW)
+ *lock = OS_UNFAIR_LOCK_INIT;
+#elif defined(APPLE_SPIN_OLD)
+ *lock = OS_SPINLOCK_INIT;
+#else /* POSIX */
+ pthread_spin_init(lock, PTHREAD_PROCESS_PRIVATE);
+#endif
+}
+
+/*! \brief Destroy the spinlock pointed to by the parameter "lock". */
+static inline void knot_spin_destroy(knot_spin_t *lock)
+{
+#if defined(HAVE_SYNC_ATOMIC) || defined(HAVE_ATOMIC) || defined(HAVE_STDATOMIC) || \
+ defined(APPLE_SPIN_NEW) || defined(APPLE_SPIN_OLD)
+ /* Nothing needed here. */
+#else /* POSIX */
+ pthread_spin_destroy(lock);
+#endif
+}
+
+/*! \brief Lock the spinlock pointed to by the parameter "lock". */
+static inline void knot_spin_lock(knot_spin_t *lock)
+{
+#if defined(HAVE_SYNC_ATOMIC)
+ while (__sync_lock_test_and_set(lock, 1)) {
+ }
+#elif defined(HAVE_ATOMIC)
+ int expected = 0;
+ while (!__atomic_compare_exchange_n(lock, &expected, 1, false, __ATOMIC_RELAXED, __ATOMIC_RELAXED)) {
+ expected = 0;
+ }
+#elif defined(HAVE_STDATOMIC)
+ int expected = 0;
+ while (!atomic_compare_exchange_strong(lock, &expected, false)) {
+ expected = 0;
+ }
+#elif defined(APPLE_SPIN_NEW)
+ os_unfair_lock_lock(lock);
+#elif defined(APPLE_SPIN_OLD)
+ OSSpinLockLock(lock);
+#else /* POSIX */
+ pthread_spin_lock(lock);
+#endif
+}
+
+/*! \brief Unlock the spinlock pointed to by the parameter "lock". */
+static inline void knot_spin_unlock(knot_spin_t *lock)
+{
+#if defined(HAVE_SYNC_ATOMIC)
+ __sync_lock_release(lock);
+#elif defined(HAVE_ATOMIC)
+ __atomic_clear(lock, __ATOMIC_RELAXED);
+#elif defined(HAVE_STDATOMIC)
+ atomic_store(lock, false);
+#elif defined(APPLE_SPIN_NEW)
+ os_unfair_lock_unlock(lock);
+#elif defined(APPLE_SPIN_OLD)
+ OSSpinLockUnlock(lock);
+#else /* POSIX */
+ pthread_spin_unlock(lock);
+#endif
+}