core: add refcount routines

Adds refcount_inc() and refcount_dec()

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

core/include/kernel/refcount.h

@@ -0,0 +1,71 @@
+/*
+ * Copyright (c) 2017, Linaro Limited
+ * All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#ifndef __KERNEL_REFCOUNT_H
+#define __KERNEL_REFCOUNT_H
+
+#include <atomic.h>
+
+/*
+ * Reference counter
+ *
+ * When val is 0, refcount_inc() does not change the value and returns false.
+ * Otherwise, it increments the value and returns true.
+ *
+ * refcount_dec() decrements the value and returns true when the call
+ * caused the value to become 0, false otherwise.
+ *
+ * Since each call to refcount_dec() is supposed to match a call to
+ * refcount_inc(), refcount_dec() called for val == 0 should never happen.
+ *
+ * This behaviour makes this pattern possible:
+ * if (!refcount_inc(r)) {
+ *	mutex_lock(m);
+ *	// Some other thread may have initialized o by now so check that
+ *	// we still need to initialize o.
+ *	if (!o) {
+ *		o = initialize();
+ *		refcount_set(r, 1);
+ *	}
+ *	mutex_unlock(m);
+ * }
+ *
+ * or
+ * if (refcount_dec(r)) {
+ *	mutex_lock(m);
+ *	// Now that we have the mutex o can't be ininialized/uninitialized
+ *	// by any other thread, check that the refcount value is still 0
+ *	// to guard against the thread above already having reinitialized o
+ *	if (!refcount_val(r) && o)
+ *		uninitialize(o)
+ *	mutex_unlock(m);
+ * }
+ *
+ * where r if the reference counter, o is the object and m the mutex
+ * protecting the object.
+ */
+
+struct refcount {
+	unsigned int val;
+};
+
+/* Increases refcount by 1, return true if val > 0 else false */
+bool refcount_inc(struct refcount *r);
+/* Decreases refcount by 1, return true if val == 0 else false */
+bool refcount_dec(struct refcount *r);
+
+static inline void refcount_set(struct refcount *r, unsigned int val)
+{
+	atomic_store_uint(&r->val, val);
+}
+
+static inline unsigned int refcount_val(struct refcount *r)
+{
+	return atomic_load_uint(&r->val);
+}
+
+#endif /*!__KERNEL_REFCOUNT_H*/

core/kernel/refcount.c

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2017, Linaro Limited
+ * All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <assert.h>
+#include <atomic.h>
+#include <kernel/refcount.h>
+
+bool refcount_inc(struct refcount *r)
+{
+	unsigned int nval;
+	unsigned int oval = atomic_load_uint(&r->val);
+
+	while (true) {
+		nval = oval + 1;
+
+		/* r->val is 0, we can't do anything more. */
+		if (!oval)
+			return false;
+
+		/*
+		 * Note that atomic_cas_uint() updates oval to the current
+		 * r->val read, regardless of return value.
+		 */
+		if (atomic_cas_uint(&r->val, &oval, nval))
+			return true;
+	}
+}
+
+bool refcount_dec(struct refcount *r)
+{
+	unsigned int nval;
+	unsigned int oval = atomic_load_uint(&r->val);
+
+	while (true) {
+		assert(oval);
+		nval = oval - 1;
+
+		/*
+		 * Note that atomic_cas_uint() updates oval to the current
+		 * r->val read, regardless of return value.
+		 */
+		if (atomic_cas_uint(&r->val, &oval, nval)) {
+			/*
+			 * Value has been updated, if value was set to 0
+			 * return true to indicate that.
+			 */
+			return !nval;
+		}
+	}
+}

core/kernel/sub.mk

@@ -10,3 +10,4 @@ srcs-y += interrupt.c
 srcs-$(CFG_CORE_SANITIZE_UNDEFINED) += ubsan.c
 srcs-$(CFG_CORE_SANITIZE_KADDRESS) += asan.c
 cflags-remove-asan.c-y += $(cflags_kasan)
+srcs-y += refcount.c