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+/*
+ * Copyright (c) 2018 Richard Braun.
+ *
+ * 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 <http://www.gnu.org/licenses/>.
+ *
+ *
+ * Read-Copy Update.
+ *
+ * This module provides a synchronization framework for read-only lockless
+ * operations, best used on read-mostly data structures.
+ *
+ * To learn more about RCU, see "Is Parallel Programming Hard, And, If So,
+ * What Can You Do About It ?" by Paul E. McKenney.
+ *
+ * A thorough discussion of RCU's requirements can be found at [1].
+ * However, the memory model and terminology used in that document being
+ * different from C11, the memory barrier guarantees are hereby translated
+ * into memory ordering guarantees :
+ *
+ *  1. Release-acquire ordering is enforced between the end of all
+ *     read-side critical sections starting before a grace period starts
+ *     and the start of all works deferred before the same grace period
+ *     starts.
+ *  2. Release-acquire ordering is enforced between the start of all
+ *     read-side critical sections ending after a grace period ends
+ *     and all the work deferrals done before the same grace period
+ *     starts.
+ *  3. Release-acquire ordering is enforced between all the work deferrals
+ *     done before a grace period starts and the start of all works deferred
+ *     before the same grace period starts.
+ *
+ * [1] https://www.kernel.org/doc/Documentation/RCU/Design/Requirements/Requirements.html.
+ */
+
+#ifndef _KERN_RCU_H
+#define _KERN_RCU_H
+
+#include <kern/atomic.h>
+#include <kern/init.h>
+#include <kern/rcu_i.h>
+#include <kern/rcu_types.h>
+#include <kern/thread.h>
+#include <kern/work.h>
+
+/*
+ * Thread-local RCU data.
+ */
+struct rcu_reader;
+
+/*
+ * Safely store a pointer.
+ *
+ * This macro enforces release ordering on the given pointer.
+ */
+#define rcu_store_ptr(ptr, value) atomic_store(&(ptr), value, ATOMIC_RELEASE)
+
+/*
+ * Safely load a pointer.
+ *
+ * This macro enforces consume ordering on the given pointer.
+ */
+#define rcu_load_ptr(ptr) atomic_load(&(ptr), ATOMIC_CONSUME)
+
+/*
+ * Read-side critical section functions.
+ *
+ * Critical sections are preemptible, may safely nest, and may safely
+ * be used in interrupt context. It is not allowed to sleep inside a
+ * read-side critical section. However, it is allowed to acquire locks
+ * that don't sleep, such as spin locks.
+ *
+ * Entering or leaving a critical section doesn't provide any memory
+ * ordering guarantee.
+ */
+
+static inline void
+rcu_read_enter(void)
+{
+    rcu_reader_inc(thread_rcu_reader(thread_self()));
+    barrier();
+}
+
+static inline void
+rcu_read_leave(void)
+{
+    barrier();
+    rcu_reader_dec(thread_rcu_reader(thread_self()));
+}
+
+/*
+ * Initialize a RCU reader.
+ */
+void rcu_reader_init(struct rcu_reader *reader);
+
+/*
+ * Report a context switch on the current processor.
+ *
+ * The argument is the RCU reader of the preempted thread.
+ *
+ * Interrupts and preemption must be disabled when calling this function.
+ */
+void rcu_report_context_switch(struct rcu_reader *reader);
+
+/*
+ * Report a periodic event on the current processor.
+ *
+ * Interrupts and preemption must be disabled when calling this function.
+ */
+void rcu_report_periodic_event(void);
+
+/*
+ * Defer a work until all existing read-side references are dropped,
+ * without blocking.
+ *
+ * The given work is scheduled when all existing read-side references
+ * are dropped.
+ */
+void rcu_defer(struct work *work);
+
+/*
+ * Wait for all existing read-side references to be dropped.
+ *
+ * This function sleeps, and may do so for a moderately long duration,
+ * at leat a few system timer ticks, sometimes a lot more.
+ */
+void rcu_wait(void);
+
+/*
+ * This init operation provides :
+ *  - read-side critical sections may be used
+ *  - module fully initialized
+ */
+INIT_OP_DECLARE(rcu_setup);
+
+#endif /* _KERN_RCU_H */