once: make helper generic for calling functions once

Make the get_random_once() helper generic enough, so that functions
in general would only be called once, where one user of this is then
net_get_random_once().

The only implementation specific call is to get_random_bytes(), all
the rest of this *_once() facility would be duplicated among different
subsystems otherwise. The new DO_ONCE() helper will be used by prandom()
later on, but might also be useful for other scenarios/subsystems as
well where a one-time initialization in often-called, possibly fast
path code could occur.

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/once.h b/include/linux/once.h
index 2a83b538dd6ac23b87bc93e57fbfef4ff7aafab1..285f12cb40e6a3d6d21db8fbaca418c133a93d97 100644 (file)
--- a/include/linux/once.h
+++ b/include/linux/once.h
@@ -4,21 +4,54 @@
 #include <linux/types.h>
 #include <linux/jump_label.h>
 
-bool __get_random_once(void *buf, int nbytes, bool *done,
-                      struct static_key *once_key);
+bool __do_once_start(bool *done, unsigned long *flags);
+void __do_once_done(bool *done, struct static_key *once_key,
+                   unsigned long *flags);
 
-#define get_random_once(buf, nbytes)                                   \
-       ({                                                              \
-               bool ___ret = false;                                    \
-               static bool ___done = false;                            \
-               static struct static_key ___once_key =                  \
-                       STATIC_KEY_INIT_TRUE;                           \
-               if (static_key_true(&___once_key))                      \
-                       ___ret = __get_random_once((buf),               \
-                                                  (nbytes),            \
-                                                  &___done,            \
-                                                  &___once_key);       \
-               ___ret;                                                 \
+/* 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 */

diff --git a/lib/once.c b/lib/once.c
index 2d5a7de17abafbe966535be0000f6d620ba9008d..05c8604627eb0ae085913a91a17013604e83f359 100644 (file)
--- a/lib/once.c
+++ b/lib/once.c
@@ -3,52 +3,60 @@
 #include <linux/once.h>
 #include <linux/random.h>
 
-struct __random_once_work {
+struct once_work {
        struct work_struct work;
        struct static_key *key;
 };
 
-static void __random_once_deferred(struct work_struct *w)
+static void once_deferred(struct work_struct *w)
 {
-       struct __random_once_work *work;
+       struct once_work *work;
 
-       work = container_of(w, struct __random_once_work, work);
+       work = container_of(w, struct once_work, work);
        BUG_ON(!static_key_enabled(work->key));
        static_key_slow_dec(work->key);
        kfree(work);
 }
 
-static void __random_once_disable_jump(struct static_key *key)
+static void once_disable_jump(struct static_key *key)
 {
-       struct __random_once_work *w;
+       struct once_work *w;
 
        w = kmalloc(sizeof(*w), GFP_ATOMIC);
        if (!w)
                return;
 
-       INIT_WORK(&w->work, __random_once_deferred);
+       INIT_WORK(&w->work, once_deferred);
        w->key = key;
        schedule_work(&w->work);
 }
 
-bool __get_random_once(void *buf, int nbytes, bool *done,
-                      struct static_key *once_key)
-{
-       static DEFINE_SPINLOCK(lock);
-       unsigned long flags;
+static DEFINE_SPINLOCK(once_lock);
 
-       spin_lock_irqsave(&lock, flags);
+bool __do_once_start(bool *done, unsigned long *flags)
+       __acquires(once_lock)
+{
+       spin_lock_irqsave(&once_lock, *flags);
        if (*done) {
-               spin_unlock_irqrestore(&lock, flags);
+               spin_unlock_irqrestore(&once_lock, *flags);
+               /* Keep sparse happy by restoring an even lock count on
+                * this lock. In case we return here, we don't call into
+                * __do_once_done but return early in the DO_ONCE() macro.
+                */
+               __acquire(once_lock);
                return false;
        }
 
-       get_random_bytes(buf, nbytes);
-       *done = true;
-       spin_unlock_irqrestore(&lock, flags);
-
-       __random_once_disable_jump(once_key);
-
        return true;
 }
-EXPORT_SYMBOL(__get_random_once);
+EXPORT_SYMBOL(__do_once_start);
+
+void __do_once_done(bool *done, struct static_key *once_key,
+                   unsigned long *flags)
+       __releases(once_lock)
+{
+       *done = true;
+       spin_unlock_irqrestore(&once_lock, *flags);
+       once_disable_jump(once_key);
+}
+EXPORT_SYMBOL(__do_once_done);