diff options
Diffstat (limited to 'include/linux/jump_label.h')
| -rw-r--r-- | include/linux/jump_label.h | 261 |
1 files changed, 210 insertions, 51 deletions
diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h index f4de473f226b..7f653e8f6690 100644 --- a/include/linux/jump_label.h +++ b/include/linux/jump_label.h @@ -7,17 +7,52 @@ * Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com> * Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com> * + * DEPRECATED API: + * + * The use of 'struct static_key' directly, is now DEPRECATED. In addition + * static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following: + * + * struct static_key false = STATIC_KEY_INIT_FALSE; + * struct static_key true = STATIC_KEY_INIT_TRUE; + * static_key_true() + * static_key_false() + * + * The updated API replacements are: + * + * DEFINE_STATIC_KEY_TRUE(key); + * DEFINE_STATIC_KEY_FALSE(key); + * static_key_likely() + * statick_key_unlikely() + * * Jump labels provide an interface to generate dynamic branches using - * self-modifying code. Assuming toolchain and architecture support, the result - * of a "if (static_key_false(&key))" statement is an unconditional branch (which - * defaults to false - and the true block is placed out of line). + * self-modifying code. Assuming toolchain and architecture support, if we + * define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)", + * an "if (static_branch_unlikely(&key))" statement is an unconditional branch + * (which defaults to false - and the true block is placed out of line). + * Similarly, we can define an initially true key via + * "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same + * "if (static_branch_unlikely(&key))", in which case we will generate an + * unconditional branch to the out-of-line true branch. Keys that are + * initially true or false can be using in both static_branch_unlikely() + * and static_branch_likely() statements. + * + * At runtime we can change the branch target by setting the key + * to true via a call to static_branch_enable(), or false using + * static_branch_disable(). If the direction of the branch is switched by + * these calls then we run-time modify the branch target via a + * no-op -> jump or jump -> no-op conversion. For example, for an + * initially false key that is used in an "if (static_branch_unlikely(&key))" + * statement, setting the key to true requires us to patch in a jump + * to the out-of-line of true branch. * - * 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. + * In addtion to static_branch_{enable,disable}, we can also reference count + * the key or branch direction via static_branch_{inc,dec}. Thus, + * static_branch_inc() can be thought of as a 'make more true' and + * static_branch_dec() as a 'make more false'. The inc()/dec() + * interface is meant to be used exclusively from the inc()/dec() for a given + * key. * - * Since this relies on modifying code, the static_key_slow_{inc,dec}() functions + * Since this relies on modifying code, the branch modifying 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 @@ -29,20 +64,10 @@ * cause significant performance degradation. Struct static_key_deferred and * static_key_slow_dec_deferred() provide for this. * - * Lacking toolchain and or architecture support, jump labels fall back to a simple - * conditional branch. - * - * struct static_key my_key = STATIC_KEY_INIT_TRUE; - * - * if (static_key_true(&my_key)) { - * } + * Lacking toolchain and or architecture support, static keys fall back to a + * simple conditional branch. * - * 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. + * Additional babbling in: Documentation/static-keys.txt */ #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL) @@ -86,8 +111,8 @@ struct static_key { #ifndef __ASSEMBLY__ enum jump_label_type { - JUMP_LABEL_DISABLE = 0, - JUMP_LABEL_ENABLE, + JUMP_LABEL_NOP = 0, + JUMP_LABEL_JMP, }; struct module; @@ -101,33 +126,18 @@ static inline int static_key_count(struct static_key *key) #ifdef HAVE_JUMP_LABEL -#define JUMP_LABEL_TYPE_FALSE_BRANCH 0UL -#define JUMP_LABEL_TYPE_TRUE_BRANCH 1UL -#define JUMP_LABEL_TYPE_MASK 1UL - -static -inline struct jump_entry *jump_label_get_entries(struct static_key *key) -{ - return (struct jump_entry *)((unsigned long)key->entries - & ~JUMP_LABEL_TYPE_MASK); -} - -static inline bool jump_label_get_branch_default(struct static_key *key) -{ - if (((unsigned long)key->entries & JUMP_LABEL_TYPE_MASK) == - JUMP_LABEL_TYPE_TRUE_BRANCH) - return true; - return false; -} +#define JUMP_TYPE_FALSE 0UL +#define JUMP_TYPE_TRUE 1UL +#define JUMP_TYPE_MASK 1UL static __always_inline bool static_key_false(struct static_key *key) { - return arch_static_branch(key); + return arch_static_branch(key, false); } static __always_inline bool static_key_true(struct static_key *key) { - return !static_key_false(key); + return !arch_static_branch(key, true); } extern struct jump_entry __start___jump_table[]; @@ -145,12 +155,12 @@ extern void static_key_slow_inc(struct static_key *key); extern void static_key_slow_dec(struct static_key *key); extern void jump_label_apply_nops(struct module *mod); -#define STATIC_KEY_INIT_TRUE ((struct static_key) \ +#define STATIC_KEY_INIT_TRUE \ { .enabled = ATOMIC_INIT(1), \ - .entries = (void *)JUMP_LABEL_TYPE_TRUE_BRANCH }) -#define STATIC_KEY_INIT_FALSE ((struct static_key) \ + .entries = (void *)JUMP_TYPE_TRUE } +#define STATIC_KEY_INIT_FALSE \ { .enabled = ATOMIC_INIT(0), \ - .entries = (void *)JUMP_LABEL_TYPE_FALSE_BRANCH }) + .entries = (void *)JUMP_TYPE_FALSE } #else /* !HAVE_JUMP_LABEL */ @@ -198,10 +208,8 @@ static inline int jump_label_apply_nops(struct module *mod) return 0; } -#define STATIC_KEY_INIT_TRUE ((struct static_key) \ - { .enabled = ATOMIC_INIT(1) }) -#define STATIC_KEY_INIT_FALSE ((struct static_key) \ - { .enabled = ATOMIC_INIT(0) }) +#define STATIC_KEY_INIT_TRUE { .enabled = ATOMIC_INIT(1) } +#define STATIC_KEY_INIT_FALSE { .enabled = ATOMIC_INIT(0) } #endif /* HAVE_JUMP_LABEL */ @@ -213,6 +221,157 @@ static inline bool static_key_enabled(struct static_key *key) return static_key_count(key) > 0; } +static inline void static_key_enable(struct static_key *key) +{ + int count = static_key_count(key); + + WARN_ON_ONCE(count < 0 || count > 1); + + if (!count) + static_key_slow_inc(key); +} + +static inline void static_key_disable(struct static_key *key) +{ + int count = static_key_count(key); + + WARN_ON_ONCE(count < 0 || count > 1); + + if (count) + static_key_slow_dec(key); +} + +/* -------------------------------------------------------------------------- */ + +/* + * Two type wrappers around static_key, such that we can use compile time + * type differentiation to emit the right code. + * + * All the below code is macros in order to play type games. + */ + +struct static_key_true { + struct static_key key; +}; + +struct static_key_false { + struct static_key key; +}; + +#define STATIC_KEY_TRUE_INIT (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE, } +#define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, } + +#define DEFINE_STATIC_KEY_TRUE(name) \ + struct static_key_true name = STATIC_KEY_TRUE_INIT + +#define DEFINE_STATIC_KEY_FALSE(name) \ + struct static_key_false name = STATIC_KEY_FALSE_INIT + +#ifdef HAVE_JUMP_LABEL + +/* + * Combine the right initial value (type) with the right branch order + * to generate the desired result. + * + * + * type\branch| likely (1) | unlikely (0) + * -----------+-----------------------+------------------ + * | | + * true (1) | ... | ... + * | NOP | JMP L + * | <br-stmts> | 1: ... + * | L: ... | + * | | + * | | L: <br-stmts> + * | | jmp 1b + * | | + * -----------+-----------------------+------------------ + * | | + * false (0) | ... | ... + * | JMP L | NOP + * | <br-stmts> | 1: ... + * | L: ... | + * | | + * | | L: <br-stmts> + * | | jmp 1b + * | | + * -----------+-----------------------+------------------ + * + * The initial value is encoded in the LSB of static_key::entries, + * type: 0 = false, 1 = true. + * + * The branch type is encoded in the LSB of jump_entry::key, + * branch: 0 = unlikely, 1 = likely. + * + * This gives the following logic table: + * + * enabled type branch instuction + * -----------------------------+----------- + * 0 0 0 | NOP + * 0 0 1 | JMP + * 0 1 0 | NOP + * 0 1 1 | JMP + * + * 1 0 0 | JMP + * 1 0 1 | NOP + * 1 1 0 | JMP + * 1 1 1 | NOP + * + * Which gives the following functions: + * + * dynamic: instruction = enabled ^ branch + * static: instruction = type ^ branch + * + * See jump_label_type() / jump_label_init_type(). + */ + +extern bool ____wrong_branch_error(void); + +#define static_branch_likely(x) \ +({ \ + bool branch; \ + if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \ + branch = !arch_static_branch(&(x)->key, true); \ + else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \ + branch = !arch_static_branch_jump(&(x)->key, true); \ + else \ + branch = ____wrong_branch_error(); \ + branch; \ +}) + +#define static_branch_unlikely(x) \ +({ \ + bool branch; \ + if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \ + branch = arch_static_branch_jump(&(x)->key, false); \ + else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \ + branch = arch_static_branch(&(x)->key, false); \ + else \ + branch = ____wrong_branch_error(); \ + branch; \ +}) + +#else /* !HAVE_JUMP_LABEL */ + +#define static_branch_likely(x) likely(static_key_enabled(&(x)->key)) +#define static_branch_unlikely(x) unlikely(static_key_enabled(&(x)->key)) + +#endif /* HAVE_JUMP_LABEL */ + +/* + * Advanced usage; refcount, branch is enabled when: count != 0 + */ + +#define static_branch_inc(x) static_key_slow_inc(&(x)->key) +#define static_branch_dec(x) static_key_slow_dec(&(x)->key) + +/* + * Normal usage; boolean enable/disable. + */ + +#define static_branch_enable(x) static_key_enable(&(x)->key) +#define static_branch_disable(x) static_key_disable(&(x)->key) + #endif /* _LINUX_JUMP_LABEL_H */ #endif /* __ASSEMBLY__ */ |