380 lines
11 KiB
C
380 lines
11 KiB
C
#ifndef _LINUX_JUMP_LABEL_H
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#define _LINUX_JUMP_LABEL_H
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/*
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* Jump label support
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*
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* Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
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* Copyright (C) 2011-2012 Red Hat, Inc., Peter Zijlstra
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*
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* DEPRECATED API:
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*
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* The use of 'struct static_key' directly, is now DEPRECATED. In addition
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* static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
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*
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* struct static_key false = STATIC_KEY_INIT_FALSE;
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* struct static_key true = STATIC_KEY_INIT_TRUE;
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* static_key_true()
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* static_key_false()
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*
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* The updated API replacements are:
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*
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* DEFINE_STATIC_KEY_TRUE(key);
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* DEFINE_STATIC_KEY_FALSE(key);
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* static_branch_likely()
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* static_branch_unlikely()
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*
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* Jump labels provide an interface to generate dynamic branches using
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* self-modifying code. Assuming toolchain and architecture support, if we
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* define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)",
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* an "if (static_branch_unlikely(&key))" statement is an unconditional branch
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* (which defaults to false - and the true block is placed out of line).
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* Similarly, we can define an initially true key via
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* "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same
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* "if (static_branch_unlikely(&key))", in which case we will generate an
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* unconditional branch to the out-of-line true branch. Keys that are
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* initially true or false can be using in both static_branch_unlikely()
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* and static_branch_likely() statements.
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*
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* At runtime we can change the branch target by setting the key
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* to true via a call to static_branch_enable(), or false using
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* static_branch_disable(). If the direction of the branch is switched by
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* these calls then we run-time modify the branch target via a
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* no-op -> jump or jump -> no-op conversion. For example, for an
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* initially false key that is used in an "if (static_branch_unlikely(&key))"
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* statement, setting the key to true requires us to patch in a jump
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* to the out-of-line of true branch.
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*
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* In addition to static_branch_{enable,disable}, we can also reference count
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* the key or branch direction via static_branch_{inc,dec}. Thus,
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* static_branch_inc() can be thought of as a 'make more true' and
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* static_branch_dec() as a 'make more false'.
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*
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* Since this relies on modifying code, the branch modifying functions
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* must be considered absolute slow paths (machine wide synchronization etc.).
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* OTOH, since the affected branches are unconditional, their runtime overhead
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* will be absolutely minimal, esp. in the default (off) case where the total
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* effect is a single NOP of appropriate size. The on case will patch in a jump
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* to the out-of-line block.
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*
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* When the control is directly exposed to userspace, it is prudent to delay the
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* decrement to avoid high frequency code modifications which can (and do)
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* cause significant performance degradation. Struct static_key_deferred and
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* static_key_slow_dec_deferred() provide for this.
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*
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* Lacking toolchain and or architecture support, static keys fall back to a
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* simple conditional branch.
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*
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* Additional babbling in: Documentation/static-keys.txt
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*/
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#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
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# define HAVE_JUMP_LABEL
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#endif
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#ifndef __ASSEMBLY__
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#include <linux/types.h>
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#include <linux/compiler.h>
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#include <linux/bug.h>
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extern bool static_key_initialized;
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#define STATIC_KEY_CHECK_USE() WARN(!static_key_initialized, \
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"%s used before call to jump_label_init", \
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__func__)
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#ifdef HAVE_JUMP_LABEL
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struct static_key {
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atomic_t enabled;
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/* Set lsb bit to 1 if branch is default true, 0 ot */
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struct jump_entry *entries;
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#ifdef CONFIG_MODULES
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struct static_key_mod *next;
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#endif
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};
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#else
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struct static_key {
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atomic_t enabled;
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};
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#endif /* HAVE_JUMP_LABEL */
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#endif /* __ASSEMBLY__ */
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#ifdef HAVE_JUMP_LABEL
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#include <asm/jump_label.h>
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#endif
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#ifndef __ASSEMBLY__
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enum jump_label_type {
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JUMP_LABEL_NOP = 0,
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JUMP_LABEL_JMP,
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};
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struct module;
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#include <linux/atomic.h>
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static inline int static_key_count(struct static_key *key)
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{
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return atomic_read(&key->enabled);
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}
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#ifdef HAVE_JUMP_LABEL
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#define JUMP_TYPE_FALSE 0UL
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#define JUMP_TYPE_TRUE 1UL
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#define JUMP_TYPE_MASK 1UL
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static __always_inline bool static_key_false(struct static_key *key)
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{
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return arch_static_branch(key, false);
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}
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static __always_inline bool static_key_true(struct static_key *key)
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{
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return !arch_static_branch(key, true);
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}
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extern struct jump_entry __start___jump_table[];
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extern struct jump_entry __stop___jump_table[];
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extern void jump_label_init(void);
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extern void jump_label_lock(void);
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extern void jump_label_unlock(void);
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extern void arch_jump_label_transform(struct jump_entry *entry,
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enum jump_label_type type);
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extern void arch_jump_label_transform_static(struct jump_entry *entry,
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enum jump_label_type type);
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extern int jump_label_text_reserved(void *start, void *end);
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extern void static_key_slow_inc(struct static_key *key);
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extern void static_key_slow_dec(struct static_key *key);
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extern void jump_label_apply_nops(struct module *mod);
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#define STATIC_KEY_INIT_TRUE \
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{ .enabled = ATOMIC_INIT(1), \
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.entries = (void *)JUMP_TYPE_TRUE }
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#define STATIC_KEY_INIT_FALSE \
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{ .enabled = ATOMIC_INIT(0), \
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.entries = (void *)JUMP_TYPE_FALSE }
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#else /* !HAVE_JUMP_LABEL */
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static __always_inline void jump_label_init(void)
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{
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static_key_initialized = true;
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}
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static __always_inline bool static_key_false(struct static_key *key)
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{
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if (unlikely(static_key_count(key) > 0))
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return true;
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return false;
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}
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static __always_inline bool static_key_true(struct static_key *key)
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{
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if (likely(static_key_count(key) > 0))
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return true;
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return false;
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}
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static inline void static_key_slow_inc(struct static_key *key)
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{
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STATIC_KEY_CHECK_USE();
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atomic_inc(&key->enabled);
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}
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static inline void static_key_slow_dec(struct static_key *key)
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{
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STATIC_KEY_CHECK_USE();
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atomic_dec(&key->enabled);
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}
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static inline int jump_label_text_reserved(void *start, void *end)
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{
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return 0;
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}
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static inline void jump_label_lock(void) {}
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static inline void jump_label_unlock(void) {}
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static inline int jump_label_apply_nops(struct module *mod)
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{
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return 0;
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}
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#define STATIC_KEY_INIT_TRUE { .enabled = ATOMIC_INIT(1) }
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#define STATIC_KEY_INIT_FALSE { .enabled = ATOMIC_INIT(0) }
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#endif /* HAVE_JUMP_LABEL */
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#define STATIC_KEY_INIT STATIC_KEY_INIT_FALSE
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#define jump_label_enabled static_key_enabled
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static inline void static_key_enable(struct static_key *key)
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{
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int count = static_key_count(key);
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WARN_ON_ONCE(count < 0 || count > 1);
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if (!count)
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static_key_slow_inc(key);
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}
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static inline void static_key_disable(struct static_key *key)
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{
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int count = static_key_count(key);
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WARN_ON_ONCE(count < 0 || count > 1);
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if (count)
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static_key_slow_dec(key);
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}
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/* -------------------------------------------------------------------------- */
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/*
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* Two type wrappers around static_key, such that we can use compile time
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* type differentiation to emit the right code.
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*
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* All the below code is macros in order to play type games.
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*/
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struct static_key_true {
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struct static_key key;
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};
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struct static_key_false {
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struct static_key key;
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};
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#define STATIC_KEY_TRUE_INIT (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE, }
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#define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
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#define DEFINE_STATIC_KEY_TRUE(name) \
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struct static_key_true name = STATIC_KEY_TRUE_INIT
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#define DEFINE_STATIC_KEY_FALSE(name) \
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struct static_key_false name = STATIC_KEY_FALSE_INIT
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extern bool ____wrong_branch_error(void);
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#define static_key_enabled(x) \
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({ \
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if (!__builtin_types_compatible_p(typeof(*x), struct static_key) && \
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!__builtin_types_compatible_p(typeof(*x), struct static_key_true) &&\
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!__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
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____wrong_branch_error(); \
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static_key_count((struct static_key *)x) > 0; \
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})
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#ifdef HAVE_JUMP_LABEL
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/*
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* Combine the right initial value (type) with the right branch order
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* to generate the desired result.
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*
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*
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* type\branch| likely (1) | unlikely (0)
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* -----------+-----------------------+------------------
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* | |
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* true (1) | ... | ...
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* | NOP | JMP L
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* | <br-stmts> | 1: ...
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* | L: ... |
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* | |
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* | | L: <br-stmts>
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* | | jmp 1b
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* | |
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* -----------+-----------------------+------------------
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* | |
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* false (0) | ... | ...
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* | JMP L | NOP
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* | <br-stmts> | 1: ...
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* | L: ... |
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* | |
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* | | L: <br-stmts>
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* | | jmp 1b
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* | |
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* -----------+-----------------------+------------------
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*
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* The initial value is encoded in the LSB of static_key::entries,
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* type: 0 = false, 1 = true.
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*
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* The branch type is encoded in the LSB of jump_entry::key,
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* branch: 0 = unlikely, 1 = likely.
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*
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* This gives the following logic table:
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*
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* enabled type branch instuction
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* -----------------------------+-----------
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* 0 0 0 | NOP
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* 0 0 1 | JMP
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* 0 1 0 | NOP
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* 0 1 1 | JMP
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*
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* 1 0 0 | JMP
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* 1 0 1 | NOP
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* 1 1 0 | JMP
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* 1 1 1 | NOP
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*
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* Which gives the following functions:
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*
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* dynamic: instruction = enabled ^ branch
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* static: instruction = type ^ branch
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*
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* See jump_label_type() / jump_label_init_type().
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*/
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#define static_branch_likely(x) \
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({ \
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bool branch; \
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if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \
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branch = !arch_static_branch(&(x)->key, true); \
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else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
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branch = !arch_static_branch_jump(&(x)->key, true); \
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else \
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branch = ____wrong_branch_error(); \
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branch; \
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})
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#define static_branch_unlikely(x) \
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({ \
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bool branch; \
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if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \
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branch = arch_static_branch_jump(&(x)->key, false); \
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else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
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branch = arch_static_branch(&(x)->key, false); \
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else \
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branch = ____wrong_branch_error(); \
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branch; \
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})
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#else /* !HAVE_JUMP_LABEL */
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#define static_branch_likely(x) likely(static_key_enabled(&(x)->key))
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#define static_branch_unlikely(x) unlikely(static_key_enabled(&(x)->key))
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#endif /* HAVE_JUMP_LABEL */
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/*
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* Advanced usage; refcount, branch is enabled when: count != 0
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*/
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#define static_branch_inc(x) static_key_slow_inc(&(x)->key)
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#define static_branch_dec(x) static_key_slow_dec(&(x)->key)
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/*
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* Normal usage; boolean enable/disable.
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*/
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#define static_branch_enable(x) static_key_enable(&(x)->key)
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#define static_branch_disable(x) static_key_disable(&(x)->key)
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#endif /* _LINUX_JUMP_LABEL_H */
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#endif /* __ASSEMBLY__ */
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