1 files changed, 86 insertions, 0 deletions
diff --git a/include/linux/once.h b/include/linux/once.h
new file mode 100644
index 000000000..bc714d414
--- /dev/null
+++ b/include/linux/once.h
@@ -0,0 +1,86 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_ONCE_H
+#define _LINUX_ONCE_H
+
+#include <linux/types.h>
+#include <linux/jump_label.h>
+
+/* Helpers used from arbitrary contexts.
+ * Hard irqs are blocked, be cautious.
+ */
+bool __do_once_start(bool *done, unsigned long *flags);
+void __do_once_done(bool *done, struct static_key_true *once_key,
+		    unsigned long *flags, struct module *mod);
+
+/* Variant for process contexts only. */
+bool __do_once_sleepable_start(bool *done);
+void __do_once_sleepable_done(bool *done, struct static_key_true *once_key,
+			      struct module *mod);
+
+/* Call a function exactly once. The idea of DO_ONCE() is to perform
+ * a function call such as initialization of random seeds, etc, only
+ * once, where DO_ONCE() can live in the fast-path. After @func has
+ * been called with the passed arguments, the static key will patch
+ * out the condition into a nop. DO_ONCE() guarantees type safety of
+ * arguments!
+ *
+ * Note that the following is not equivalent ...
+ *
+ *   DO_ONCE(func, arg);
+ *   DO_ONCE(func, arg);
+ *
+ * ... to this version:
+ *
+ *   void foo(void)
+ *   {
+ *     DO_ONCE(func, arg);
+ *   }
+ *
+ *   foo();
+ *   foo();
+ *
+ * In case the one-time invocation could be triggered from multiple
+ * places, then a common helper function must be defined, so that only
+ * a single static key will be placed there!
+ */
+#define DO_ONCE(func, ...)						     \
+	({								     \
+		bool ___ret = false;					     \
+		static bool __section(".data.once") ___done = false;	     \
+		static DEFINE_STATIC_KEY_TRUE(___once_key);		     \
+		if (static_branch_unlikely(&___once_key)) {		     \
+			unsigned long ___flags;				     \
+			___ret = __do_once_start(&___done, &___flags);	     \
+			if (unlikely(___ret)) {				     \
+				func(__VA_ARGS__);			     \
+				__do_once_done(&___done, &___once_key,	     \
+					       &___flags, THIS_MODULE);	     \
+			}						     \
+		}							     \
+		___ret;							     \
+	})
+
+/* Variant of DO_ONCE() for process/sleepable contexts. */
+#define DO_ONCE_SLEEPABLE(func, ...)						\
+	({									\
+		bool ___ret = false;						\
+		static bool __section(".data.once") ___done = false;		\
+		static DEFINE_STATIC_KEY_TRUE(___once_key);			\
+		if (static_branch_unlikely(&___once_key)) {			\
+			___ret = __do_once_sleepable_start(&___done);		\
+			if (unlikely(___ret)) {					\
+				func(__VA_ARGS__);				\
+				__do_once_sleepable_done(&___done, &___once_key,\
+						    THIS_MODULE);		\
+			}							\
+		}								\
+		___ret;								\
+	})
+
+#define get_random_once(buf, nbytes)					     \
+	DO_ONCE(get_random_bytes, (buf), (nbytes))
+
+#define get_random_sleepable_once(buf, nbytes)				     \
+	DO_ONCE_SLEEPABLE(get_random_bytes, (buf), (nbytes))
+
+#endif /* _LINUX_ONCE_H */