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>

kernel/jump_label.c

@@ -12,7 +12,7 @@
 #include <linux/slab.h>
 #include <linux/sort.h>
 #include <linux/err.h>
-#include <linux/jump_label.h>
+#include <linux/static_key.h>
 
 #ifdef HAVE_JUMP_LABEL
 
@@ -29,10 +29,11 @@ void jump_label_unlock(void)
 	mutex_unlock(&jump_label_mutex);
 }
 
-bool jump_label_enabled(struct jump_label_key *key)
+bool static_key_enabled(struct static_key *key)
 {
-	return !!atomic_read(&key->enabled);
+	return (atomic_read(&key->enabled) > 0);
 }
+EXPORT_SYMBOL_GPL(static_key_enabled);
 
 static int jump_label_cmp(const void *a, const void *b)
 {
@@ -58,22 +59,26 @@ jump_label_sort_entries(struct jump_entry *start, struct jump_entry *stop)
 	sort(start, size, sizeof(struct jump_entry), jump_label_cmp, NULL);
 }
 
-static void jump_label_update(struct jump_label_key *key, int enable);
+static void jump_label_update(struct static_key *key, int enable);
 
-void jump_label_inc(struct jump_label_key *key)
+void static_key_slow_inc(struct static_key *key)
 {
 	if (atomic_inc_not_zero(&key->enabled))
 		return;
 
 	jump_label_lock();
-	if (atomic_read(&key->enabled) == 0)
-		jump_label_update(key, JUMP_LABEL_ENABLE);
+	if (atomic_read(&key->enabled) == 0) {
+		if (!jump_label_get_branch_default(key))
+			jump_label_update(key, JUMP_LABEL_ENABLE);
+		else
+			jump_label_update(key, JUMP_LABEL_DISABLE);
+	}
 	atomic_inc(&key->enabled);
 	jump_label_unlock();
 }
-EXPORT_SYMBOL_GPL(jump_label_inc);
+EXPORT_SYMBOL_GPL(static_key_slow_inc);
 
-static void __jump_label_dec(struct jump_label_key *key,
+static void __static_key_slow_dec(struct static_key *key,
 		unsigned long rate_limit, struct delayed_work *work)
 {
 	if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex)) {
@@ -85,32 +90,35 @@ static void __jump_label_dec(struct jump_label_key *key,
 	if (rate_limit) {
 		atomic_inc(&key->enabled);
 		schedule_delayed_work(work, rate_limit);
-	} else
-		jump_label_update(key, JUMP_LABEL_DISABLE);
-
+	} else {
+		if (!jump_label_get_branch_default(key))
+			jump_label_update(key, JUMP_LABEL_DISABLE);
+		else
+			jump_label_update(key, JUMP_LABEL_ENABLE);
+	}
 	jump_label_unlock();
 }
-EXPORT_SYMBOL_GPL(jump_label_dec);
 
 static void jump_label_update_timeout(struct work_struct *work)
 {
-	struct jump_label_key_deferred *key =
-		container_of(work, struct jump_label_key_deferred, work.work);
-	__jump_label_dec(&key->key, 0, NULL);
+	struct static_key_deferred *key =
+		container_of(work, struct static_key_deferred, work.work);
+	__static_key_slow_dec(&key->key, 0, NULL);
 }
 
-void jump_label_dec(struct jump_label_key *key)
+void static_key_slow_dec(struct static_key *key)
 {
-	__jump_label_dec(key, 0, NULL);
+	__static_key_slow_dec(key, 0, NULL);
 }
+EXPORT_SYMBOL_GPL(static_key_slow_dec);
 
-void jump_label_dec_deferred(struct jump_label_key_deferred *key)
+void static_key_slow_dec_deferred(struct static_key_deferred *key)
 {
-	__jump_label_dec(&key->key, key->timeout, &key->work);
+	__static_key_slow_dec(&key->key, key->timeout, &key->work);
 }
+EXPORT_SYMBOL_GPL(static_key_slow_dec_deferred);
 
-
-void jump_label_rate_limit(struct jump_label_key_deferred *key,
+void jump_label_rate_limit(struct static_key_deferred *key,
 		unsigned long rl)
 {
 	key->timeout = rl;
@@ -153,7 +161,7 @@ void __weak __init_or_module arch_jump_label_transform_static(struct jump_entry
 	arch_jump_label_transform(entry, type);	
 }
 
-static void __jump_label_update(struct jump_label_key *key,
+static void __jump_label_update(struct static_key *key,
 				struct jump_entry *entry,
 				struct jump_entry *stop, int enable)
 {
@@ -170,27 +178,40 @@ static void __jump_label_update(struct jump_label_key *key,
 	}
 }
 
+static enum jump_label_type jump_label_type(struct static_key *key)
+{
+	bool true_branch = jump_label_get_branch_default(key);
+	bool state = static_key_enabled(key);
+
+	if ((!true_branch && state) || (true_branch && !state))
+		return JUMP_LABEL_ENABLE;
+
+	return JUMP_LABEL_DISABLE;
+}
+
 void __init jump_label_init(void)
 {
 	struct jump_entry *iter_start = __start___jump_table;
 	struct jump_entry *iter_stop = __stop___jump_table;
-	struct jump_label_key *key = NULL;
+	struct static_key *key = NULL;
 	struct jump_entry *iter;
 
 	jump_label_lock();
 	jump_label_sort_entries(iter_start, iter_stop);
 
 	for (iter = iter_start; iter < iter_stop; iter++) {
-		struct jump_label_key *iterk;
+		struct static_key *iterk;
 
-		iterk = (struct jump_label_key *)(unsigned long)iter->key;
-		arch_jump_label_transform_static(iter, jump_label_enabled(iterk) ?
-						 JUMP_LABEL_ENABLE : JUMP_LABEL_DISABLE);
+		iterk = (struct static_key *)(unsigned long)iter->key;
+		arch_jump_label_transform_static(iter, jump_label_type(iterk));
 		if (iterk == key)
 			continue;
 
 		key = iterk;
-		key->entries = iter;
+		/*
+		 * Set key->entries to iter, but preserve JUMP_LABEL_TRUE_BRANCH.
+		 */
+		*((unsigned long *)&key->entries) += (unsigned long)iter;
 #ifdef CONFIG_MODULES
 		key->next = NULL;
 #endif
@@ -200,8 +221,8 @@ void __init jump_label_init(void)
 
 #ifdef CONFIG_MODULES
 
-struct jump_label_mod {
-	struct jump_label_mod *next;
+struct static_key_mod {
+	struct static_key_mod *next;
 	struct jump_entry *entries;
 	struct module *mod;
 };
@@ -221,9 +242,9 @@ static int __jump_label_mod_text_reserved(void *start, void *end)
 				start, end);
 }
 
-static void __jump_label_mod_update(struct jump_label_key *key, int enable)
+static void __jump_label_mod_update(struct static_key *key, int enable)
 {
-	struct jump_label_mod *mod = key->next;
+	struct static_key_mod *mod = key->next;
 
 	while (mod) {
 		struct module *m = mod->mod;
@@ -254,11 +275,7 @@ void jump_label_apply_nops(struct module *mod)
 		return;
 
 	for (iter = iter_start; iter < iter_stop; iter++) {
-		struct jump_label_key *iterk;
-
-		iterk = (struct jump_label_key *)(unsigned long)iter->key;
-		arch_jump_label_transform_static(iter, jump_label_enabled(iterk) ?
-				JUMP_LABEL_ENABLE : JUMP_LABEL_DISABLE);
+		arch_jump_label_transform_static(iter, JUMP_LABEL_DISABLE);
 	}
 }
 
@@ -267,8 +284,8 @@ static int jump_label_add_module(struct module *mod)
 	struct jump_entry *iter_start = mod->jump_entries;
 	struct jump_entry *iter_stop = iter_start + mod->num_jump_entries;
 	struct jump_entry *iter;
-	struct jump_label_key *key = NULL;
-	struct jump_label_mod *jlm;
+	struct static_key *key = NULL;
+	struct static_key_mod *jlm;
 
 	/* if the module doesn't have jump label entries, just return */
 	if (iter_start == iter_stop)
@@ -277,28 +294,30 @@ static int jump_label_add_module(struct module *mod)
 	jump_label_sort_entries(iter_start, iter_stop);
 
 	for (iter = iter_start; iter < iter_stop; iter++) {
-		if (iter->key == (jump_label_t)(unsigned long)key)
+		struct static_key *iterk;
+
+		iterk = (struct static_key *)(unsigned long)iter->key;
+		if (iterk == key)
 			continue;
 
-		key = (struct jump_label_key *)(unsigned long)iter->key;
-
+		key = iterk;
 		if (__module_address(iter->key) == mod) {
-			atomic_set(&key->enabled, 0);
-			key->entries = iter;
+			/*
+			 * Set key->entries to iter, but preserve JUMP_LABEL_TRUE_BRANCH.
+			 */
+			*((unsigned long *)&key->entries) += (unsigned long)iter;
 			key->next = NULL;
 			continue;
 		}
-
-		jlm = kzalloc(sizeof(struct jump_label_mod), GFP_KERNEL);
+		jlm = kzalloc(sizeof(struct static_key_mod), GFP_KERNEL);
 		if (!jlm)
 			return -ENOMEM;
-
 		jlm->mod = mod;
 		jlm->entries = iter;
 		jlm->next = key->next;
 		key->next = jlm;
 
-		if (jump_label_enabled(key))
+		if (jump_label_type(key) == JUMP_LABEL_ENABLE)
 			__jump_label_update(key, iter, iter_stop, JUMP_LABEL_ENABLE);
 	}
 
@@ -310,14 +329,14 @@ static void jump_label_del_module(struct module *mod)
 	struct jump_entry *iter_start = mod->jump_entries;
 	struct jump_entry *iter_stop = iter_start + mod->num_jump_entries;
 	struct jump_entry *iter;
-	struct jump_label_key *key = NULL;
-	struct jump_label_mod *jlm, **prev;
+	struct static_key *key = NULL;
+	struct static_key_mod *jlm, **prev;
 
 	for (iter = iter_start; iter < iter_stop; iter++) {
 		if (iter->key == (jump_label_t)(unsigned long)key)
 			continue;
 
-		key = (struct jump_label_key *)(unsigned long)iter->key;
+		key = (struct static_key *)(unsigned long)iter->key;
 
 		if (__module_address(iter->key) == mod)
 			continue;
@@ -419,9 +438,10 @@ int jump_label_text_reserved(void *start, void *end)
 	return ret;
 }
 
-static void jump_label_update(struct jump_label_key *key, int enable)
+static void jump_label_update(struct static_key *key, int enable)
 {
-	struct jump_entry *entry = key->entries, *stop = __stop___jump_table;
+	struct jump_entry *stop = __stop___jump_table;
+	struct jump_entry *entry = jump_label_get_entries(key);
 
 #ifdef CONFIG_MODULES
 	struct module *mod = __module_address((unsigned long)key);