1 #ifndef _LINUX_JUMP_LABEL_H
2 #define _LINUX_JUMP_LABEL_H
7 * Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
8 * Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com>
12 * The use of 'struct static_key' directly, is now DEPRECATED. In addition
13 * static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
15 * struct static_key false = STATIC_KEY_INIT_FALSE;
16 * struct static_key true = STATIC_KEY_INIT_TRUE;
20 * The updated API replacements are:
22 * DEFINE_STATIC_KEY_TRUE(key);
23 * DEFINE_STATIC_KEY_FALSE(key);
25 * statick_key_unlikely()
27 * Jump labels provide an interface to generate dynamic branches using
28 * self-modifying code. Assuming toolchain and architecture support, if we
29 * define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)",
30 * an "if (static_branch_unlikely(&key))" statement is an unconditional branch
31 * (which defaults to false - and the true block is placed out of line).
32 * Similarly, we can define an initially true key via
33 * "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same
34 * "if (static_branch_unlikely(&key))", in which case we will generate an
35 * unconditional branch to the out-of-line true branch. Keys that are
36 * initially true or false can be using in both static_branch_unlikely()
37 * and static_branch_likely() statements.
39 * At runtime we can change the branch target by setting the key
40 * to true via a call to static_branch_enable(), or false using
41 * static_branch_disable(). If the direction of the branch is switched by
42 * these calls then we run-time modify the branch target via a
43 * no-op -> jump or jump -> no-op conversion. For example, for an
44 * initially false key that is used in an "if (static_branch_unlikely(&key))"
45 * statement, setting the key to true requires us to patch in a jump
46 * to the out-of-line of true branch.
48 * In addtion to static_branch_{enable,disable}, we can also reference count
49 * the key or branch direction via static_branch_{inc,dec}. Thus,
50 * static_branch_inc() can be thought of as a 'make more true' and
51 * static_branch_dec() as a 'make more false'. The inc()/dec()
52 * interface is meant to be used exclusively from the inc()/dec() for a given
55 * Since this relies on modifying code, the branch modifying functions
56 * must be considered absolute slow paths (machine wide synchronization etc.).
57 * OTOH, since the affected branches are unconditional, their runtime overhead
58 * will be absolutely minimal, esp. in the default (off) case where the total
59 * effect is a single NOP of appropriate size. The on case will patch in a jump
60 * to the out-of-line block.
62 * When the control is directly exposed to userspace, it is prudent to delay the
63 * decrement to avoid high frequency code modifications which can (and do)
64 * cause significant performance degradation. Struct static_key_deferred and
65 * static_key_slow_dec_deferred() provide for this.
67 * Lacking toolchain and or architecture support, static keys fall back to a
68 * simple conditional branch.
70 * Additional babbling in: Documentation/static-keys.txt
73 #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
74 # define HAVE_JUMP_LABEL
79 #include <linux/types.h>
80 #include <linux/compiler.h>
81 #include <linux/bug.h>
83 extern bool static_key_initialized;
85 #define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized, \
86 "%s used before call to jump_label_init", \
89 #ifdef HAVE_JUMP_LABEL
93 /* Set lsb bit to 1 if branch is default true, 0 ot */
94 struct jump_entry *entries;
96 struct static_key_mod *next;
104 #endif /* HAVE_JUMP_LABEL */
105 #endif /* __ASSEMBLY__ */
107 #ifdef HAVE_JUMP_LABEL
108 #include <asm/jump_label.h>
113 enum jump_label_type {
120 #include <linux/atomic.h>
122 static inline int static_key_count(struct static_key *key)
124 return atomic_read(&key->enabled);
127 #ifdef HAVE_JUMP_LABEL
129 #define JUMP_TYPE_FALSE 0UL
130 #define JUMP_TYPE_TRUE 1UL
131 #define JUMP_TYPE_MASK 1UL
133 static __always_inline bool static_key_false(struct static_key *key)
135 return arch_static_branch(key, false);
138 static __always_inline bool static_key_true(struct static_key *key)
140 return !arch_static_branch(key, true);
143 extern struct jump_entry __start___jump_table[];
144 extern struct jump_entry __stop___jump_table[];
146 extern void jump_label_init(void);
147 extern void jump_label_lock(void);
148 extern void jump_label_unlock(void);
149 extern void arch_jump_label_transform(struct jump_entry *entry,
150 enum jump_label_type type);
151 extern void arch_jump_label_transform_static(struct jump_entry *entry,
152 enum jump_label_type type);
153 extern int jump_label_text_reserved(void *start, void *end);
154 extern void static_key_slow_inc(struct static_key *key);
155 extern void static_key_slow_dec(struct static_key *key);
156 extern void jump_label_apply_nops(struct module *mod);
158 #define STATIC_KEY_INIT_TRUE \
159 { .enabled = ATOMIC_INIT(1), \
160 .entries = (void *)JUMP_TYPE_TRUE }
161 #define STATIC_KEY_INIT_FALSE \
162 { .enabled = ATOMIC_INIT(0), \
163 .entries = (void *)JUMP_TYPE_FALSE }
165 #else /* !HAVE_JUMP_LABEL */
167 static __always_inline void jump_label_init(void)
169 static_key_initialized = true;
172 static __always_inline bool static_key_false(struct static_key *key)
174 if (unlikely(static_key_count(key) > 0))
179 static __always_inline bool static_key_true(struct static_key *key)
181 if (likely(static_key_count(key) > 0))
186 static inline void static_key_slow_inc(struct static_key *key)
188 STATIC_KEY_CHECK_USE();
189 atomic_inc(&key->enabled);
192 static inline void static_key_slow_dec(struct static_key *key)
194 STATIC_KEY_CHECK_USE();
195 atomic_dec(&key->enabled);
198 static inline int jump_label_text_reserved(void *start, void *end)
203 static inline void jump_label_lock(void) {}
204 static inline void jump_label_unlock(void) {}
206 static inline int jump_label_apply_nops(struct module *mod)
211 #define STATIC_KEY_INIT_TRUE { .enabled = ATOMIC_INIT(1) }
212 #define STATIC_KEY_INIT_FALSE { .enabled = ATOMIC_INIT(0) }
214 #endif /* HAVE_JUMP_LABEL */
216 #define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
217 #define jump_label_enabled static_key_enabled
219 static inline bool static_key_enabled(struct static_key *key)
221 return static_key_count(key) > 0;
224 static inline void static_key_enable(struct static_key *key)
226 int count = static_key_count(key);
228 WARN_ON_ONCE(count < 0 || count > 1);
231 static_key_slow_inc(key);
234 static inline void static_key_disable(struct static_key *key)
236 int count = static_key_count(key);
238 WARN_ON_ONCE(count < 0 || count > 1);
241 static_key_slow_dec(key);
244 /* -------------------------------------------------------------------------- */
247 * Two type wrappers around static_key, such that we can use compile time
248 * type differentiation to emit the right code.
250 * All the below code is macros in order to play type games.
253 struct static_key_true {
254 struct static_key key;
257 struct static_key_false {
258 struct static_key key;
261 #define STATIC_KEY_TRUE_INIT (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE, }
262 #define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
264 #define DEFINE_STATIC_KEY_TRUE(name) \
265 struct static_key_true name = STATIC_KEY_TRUE_INIT
267 #define DEFINE_STATIC_KEY_FALSE(name) \
268 struct static_key_false name = STATIC_KEY_FALSE_INIT
270 #ifdef HAVE_JUMP_LABEL
273 * Combine the right initial value (type) with the right branch order
274 * to generate the desired result.
277 * type\branch| likely (1) | unlikely (0)
278 * -----------+-----------------------+------------------
280 * true (1) | ... | ...
282 * | <br-stmts> | 1: ...
288 * -----------+-----------------------+------------------
290 * false (0) | ... | ...
292 * | <br-stmts> | 1: ...
298 * -----------+-----------------------+------------------
300 * The initial value is encoded in the LSB of static_key::entries,
301 * type: 0 = false, 1 = true.
303 * The branch type is encoded in the LSB of jump_entry::key,
304 * branch: 0 = unlikely, 1 = likely.
306 * This gives the following logic table:
308 * enabled type branch instuction
309 * -----------------------------+-----------
320 * Which gives the following functions:
322 * dynamic: instruction = enabled ^ branch
323 * static: instruction = type ^ branch
325 * See jump_label_type() / jump_label_init_type().
328 extern bool ____wrong_branch_error(void);
330 #define static_branch_likely(x) \
333 if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \
334 branch = !arch_static_branch(&(x)->key, true); \
335 else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
336 branch = !arch_static_branch_jump(&(x)->key, true); \
338 branch = ____wrong_branch_error(); \
342 #define static_branch_unlikely(x) \
345 if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \
346 branch = arch_static_branch_jump(&(x)->key, false); \
347 else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
348 branch = arch_static_branch(&(x)->key, false); \
350 branch = ____wrong_branch_error(); \
354 #else /* !HAVE_JUMP_LABEL */
356 #define static_branch_likely(x) likely(static_key_enabled(&(x)->key))
357 #define static_branch_unlikely(x) unlikely(static_key_enabled(&(x)->key))
359 #endif /* HAVE_JUMP_LABEL */
362 * Advanced usage; refcount, branch is enabled when: count != 0
365 #define static_branch_inc(x) static_key_slow_inc(&(x)->key)
366 #define static_branch_dec(x) static_key_slow_dec(&(x)->key)
369 * Normal usage; boolean enable/disable.
372 #define static_branch_enable(x) static_key_enable(&(x)->key)
373 #define static_branch_disable(x) static_key_disable(&(x)->key)
375 #endif /* _LINUX_JUMP_LABEL_H */
377 #endif /* __ASSEMBLY__ */