1 files changed, 57 insertions, 0 deletions

diff --git a/include/linux/once.h b/include/linux/once.h
new file mode 100644
index 000000000000..285f12cb40e6
--- /dev/null
+++ b/include/linux/once.h
@@ -0,0 +1,57 @@
+#ifndef _LINUX_ONCE_H
+#define _LINUX_ONCE_H
+
+#include <linux/types.h>
+#include <linux/jump_label.h>
+
+bool __do_once_start(bool *done, unsigned long *flags);
+void __do_once_done(bool *done, struct static_key *once_key,
+                    unsigned long *flags);
+
+/* 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!
+ *
+ * Not 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 ___done = false;                                 \
+                static struct static_key ___once_key = STATIC_KEY_INIT_TRUE; \
+                if (static_key_true(&___once_key)) {                         \
+                        unsigned long ___flags;                              \
+                        ___ret = __do_once_start(&___done, &___flags);       \
+                        if (unlikely(___ret)) {                              \
+                                func(__VA_ARGS__);                           \
+                                __do_once_done(&___done, &___once_key,       \
+                                               &___flags);                   \
+                        }                                                    \
+                }                                                            \
+                ___ret;                                                      \
+        })
+
+#define get_random_once(buf, nbytes)                                         \
+        DO_ONCE(get_random_bytes, (buf), (nbytes))
+
+#endif /* _LINUX_ONCE_H */