static keys: Introduce 'struct static_key', static_key_true()/false() and static_key_slow_[inc|dec]()

So here's a boot tested patch on top of Jason's series that does
all the cleanups I talked about and turns jump labels into a
more intuitive to use facility. It should also address the
various misconceptions and confusions that surround jump labels.

Typical usage scenarios:

        #include <linux/static_key.h>

        struct static_key key = STATIC_KEY_INIT_TRUE;

        if (static_key_false(&key))
                do unlikely code
        else
                do likely code

Or:

        if (static_key_true(&key))
                do likely code
        else
                do unlikely code

The static key is modified via:

        static_key_slow_inc(&key);
        ...
        static_key_slow_dec(&key);

The 'slow' prefix makes it abundantly clear that this is an
expensive operation.

I've updated all in-kernel code to use this everywhere. Note
that I (intentionally) have not pushed through the rename
blindly through to the lowest levels: the actual jump-label
patching arch facility should be named like that, so we want to
decouple jump labels from the static-key facility a bit.

On non-jump-label enabled architectures static keys default to
likely()/unlikely() branches.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Jason Baron <jbaron@redhat.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: a.p.zijlstra@chello.nl
Cc: mathieu.desnoyers@efficios.com
Cc: davem@davemloft.net
Cc: ddaney.cavm@gmail.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20120222085809.GA26397@elte.hu
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 100 insertions, 39 deletions

diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index f7c69580fea7..2172da2d9bb4 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -9,15 +9,15 @@
  *
  * Jump labels provide an interface to generate dynamic branches using
  * self-modifying code. Assuming toolchain and architecture support the result
- * of a "if (static_branch(&key))" statement is a unconditional branch (which
+ * of a "if (static_key_false(&key))" statement is a unconditional branch (which
  * defaults to false - and the true block is placed out of line).
  *
- * However at runtime we can change the 'static' branch target using
- * jump_label_{inc,dec}(). These function as a 'reference' count on the key
+ * However at runtime we can change the branch target using
+ * static_key_slow_{inc,dec}(). These function as a 'reference' count on the key
  * object and for as long as there are references all branches referring to
  * that particular key will point to the (out of line) true block.
  *
- * Since this relies on modifying code the jump_label_{inc,dec}() functions
+ * Since this relies on modifying code the static_key_slow_{inc,dec}() functions
  * must be considered absolute slow paths (machine wide synchronization etc.).
  * OTOH, since the affected branches are unconditional their runtime overhead
  * will be absolutely minimal, esp. in the default (off) case where the total
@@ -26,12 +26,26 @@
  *
  * When the control is directly exposed to userspace it is prudent to delay the
  * decrement to avoid high frequency code modifications which can (and do)
- * cause significant performance degradation. Struct jump_label_key_deferred and
- * jump_label_dec_deferred() provide for this.
+ * cause significant performance degradation. Struct static_key_deferred and
+ * static_key_slow_dec_deferred() provide for this.
  *
  * Lacking toolchain and or architecture support, it falls back to a simple
  * conditional branch.
- */
+ *
+ * struct static_key my_key = STATIC_KEY_INIT_TRUE;
+ *
+ *   if (static_key_true(&my_key)) {
+ *   }
+ *
+ * will result in the true case being in-line and starts the key with a single
+ * reference. Mixing static_key_true() and static_key_false() on the same key is not
+ * allowed.
+ *
+ * Not initializing the key (static data is initialized to 0s anyway) is the
+ * same as using STATIC_KEY_INIT_FALSE and static_key_false() is
+ * equivalent with static_branch().
+ *
+*/
 
 #include <linux/types.h>
 #include <linux/compiler.h>
@@ -39,16 +53,17 @@
 
 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
 
-struct jump_label_key {
+struct static_key {
 	atomic_t enabled;
+/* Set lsb bit to 1 if branch is default true, 0 ot */
 	struct jump_entry *entries;
 #ifdef CONFIG_MODULES
-	struct jump_label_mod *next;
+	struct static_key_mod *next;
 #endif
 };
 
-struct jump_label_key_deferred {
-	struct jump_label_key key;
+struct static_key_deferred {
+	struct static_key key;
 	unsigned long timeout;
 	struct delayed_work work;
 };
@@ -66,13 +81,34 @@ struct module;
 
 #ifdef HAVE_JUMP_LABEL
 
-#ifdef CONFIG_MODULES
-#define JUMP_LABEL_INIT {ATOMIC_INIT(0), NULL, NULL}
-#else
-#define JUMP_LABEL_INIT {ATOMIC_INIT(0), NULL}
-#endif
+#define JUMP_LABEL_TRUE_BRANCH 1UL
+
+static
+inline struct jump_entry *jump_label_get_entries(struct static_key *key)
+{
+	return (struct jump_entry *)((unsigned long)key->entries
+						& ~JUMP_LABEL_TRUE_BRANCH);
+}
+
+static inline bool jump_label_get_branch_default(struct static_key *key)
+{
+	if ((unsigned long)key->entries & JUMP_LABEL_TRUE_BRANCH)
+		return true;
+	return false;
+}
+
+static __always_inline bool static_key_false(struct static_key *key)
+{
+	return arch_static_branch(key);
+}
 
-static __always_inline bool static_branch(struct jump_label_key *key)
+static __always_inline bool static_key_true(struct static_key *key)
+{
+	return !static_key_false(key);
+}
+
+/* Deprecated. Please use 'static_key_false() instead. */
+static __always_inline bool static_branch(struct static_key *key)
 {
 	return arch_static_branch(key);
 }
@@ -88,21 +124,24 @@ extern void arch_jump_label_transform(struct jump_entry *entry,
 extern void arch_jump_label_transform_static(struct jump_entry *entry,
 					     enum jump_label_type type);
 extern int jump_label_text_reserved(void *start, void *end);
-extern void jump_label_inc(struct jump_label_key *key);
-extern void jump_label_dec(struct jump_label_key *key);
-extern void jump_label_dec_deferred(struct jump_label_key_deferred *key);
-extern bool jump_label_enabled(struct jump_label_key *key);
+extern void static_key_slow_inc(struct static_key *key);
+extern void static_key_slow_dec(struct static_key *key);
+extern void static_key_slow_dec_deferred(struct static_key_deferred *key);
+extern bool static_key_enabled(struct static_key *key);
 extern void jump_label_apply_nops(struct module *mod);
-extern void jump_label_rate_limit(struct jump_label_key_deferred *key,
-		unsigned long rl);
+extern void
+jump_label_rate_limit(struct static_key_deferred *key, unsigned long rl);
+
+#define STATIC_KEY_INIT_TRUE ((struct static_key) \
+	{ .enabled = ATOMIC_INIT(1), .entries = (void *)1 })
+#define STATIC_KEY_INIT_FALSE ((struct static_key) \
+	{ .enabled = ATOMIC_INIT(0), .entries = (void *)0 })
 
 #else  /* !HAVE_JUMP_LABEL */
 
 #include <linux/atomic.h>
 
-#define JUMP_LABEL_INIT {ATOMIC_INIT(0)}
-
-struct jump_label_key {
+struct static_key {
 	atomic_t enabled;
 };
 
@@ -110,30 +149,45 @@ static __always_inline void jump_label_init(void)
 {
 }
 
-struct jump_label_key_deferred {
-	struct jump_label_key  key;
+struct static_key_deferred {
+	struct static_key  key;
 };
 
-static __always_inline bool static_branch(struct jump_label_key *key)
+static __always_inline bool static_key_false(struct static_key *key)
+{
+	if (unlikely(atomic_read(&key->enabled)) > 0)
+		return true;
+	return false;
+}
+
+static __always_inline bool static_key_true(struct static_key *key)
 {
-	if (unlikely(atomic_read(&key->enabled)))
+	if (likely(atomic_read(&key->enabled)) > 0)
 		return true;
 	return false;
 }
 
-static inline void jump_label_inc(struct jump_label_key *key)
+/* Deprecated. Please use 'static_key_false() instead. */
+static __always_inline bool static_branch(struct static_key *key)
+{
+	if (unlikely(atomic_read(&key->enabled)) > 0)
+		return true;
+	return false;
+}
+
+static inline void static_key_slow_inc(struct static_key *key)
 {
 	atomic_inc(&key->enabled);
 }
 
-static inline void jump_label_dec(struct jump_label_key *key)
+static inline void static_key_slow_dec(struct static_key *key)
 {
 	atomic_dec(&key->enabled);
 }
 
-static inline void jump_label_dec_deferred(struct jump_label_key_deferred *key)
+static inline void static_key_slow_dec_deferred(struct static_key_deferred *key)
 {
-	jump_label_dec(&key->key);
+	static_key_slow_dec(&key->key);
 }
 
 static inline int jump_label_text_reserved(void *start, void *end)
@@ -144,9 +198,9 @@ static inline int jump_label_text_reserved(void *start, void *end)
 static inline void jump_label_lock(void) {}
 static inline void jump_label_unlock(void) {}
 
-static inline bool jump_label_enabled(struct jump_label_key *key)
+static inline bool static_key_enabled(struct static_key *key)
 {
-	return !!atomic_read(&key->enabled);
+	return (atomic_read(&key->enabled) > 0);
 }
 
 static inline int jump_label_apply_nops(struct module *mod)
@@ -154,13 +208,20 @@ static inline int jump_label_apply_nops(struct module *mod)
 	return 0;
 }
 
-static inline void jump_label_rate_limit(struct jump_label_key_deferred *key,
+static inline void
+jump_label_rate_limit(struct static_key_deferred *key,
 		unsigned long rl)
 {
 }
+
+#define STATIC_KEY_INIT_TRUE ((struct static_key) \
+		{ .enabled = ATOMIC_INIT(1) })
+#define STATIC_KEY_INIT_FALSE ((struct static_key) \
+		{ .enabled = ATOMIC_INIT(0) })
+
 #endif	/* HAVE_JUMP_LABEL */
 
-#define jump_label_key_enabled	((struct jump_label_key){ .enabled = ATOMIC_INIT(1), })
-#define jump_label_key_disabled	((struct jump_label_key){ .enabled = ATOMIC_INIT(0), })
+#define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
+#define jump_label_enabled static_key_enabled
 
 #endif	/* _LINUX_JUMP_LABEL_H */