Date:   Fri, 9 Nov 2018 13:33:37 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC PATCH 1/3] static_call: Add static call infrastructure

On Thu,  8 Nov 2018 15:15:51 -0600
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> Add a static call infrastructure.  Static calls use code patching to
> hard-code function pointers into direct branch instructions.  They give
> the flexibility of function pointers, but with improved performance.
> This is especially important for cases where retpolines would otherwise
> be used, as retpolines can significantly impact performance.
> 
> This code is heavily inspired by the jump label code (aka "static
> jumps"), as some of the concepts are very similar.
> 
> There are three implementations, depending on arch support:
> 
> 1) optimized: patched call sites (CONFIG_HAVE_STATIC_CALL_OPTIMIZED)

Thanks for doing this.

> 2) unoptimized: patched trampolines (CONFIG_HAVE_STATIC_CALL_UNOPTIMIZED)

 #2 looks like my approach.

> 3) basic function pointers
> 
> For more details, see the comments in include/linux/static_call.h.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  arch/Kconfig                      |   6 +
>  include/asm-generic/vmlinux.lds.h |  11 ++
>  include/linux/module.h            |  10 +
>  include/linux/static_call.h       | 186 +++++++++++++++++++
>  include/linux/static_call_types.h |  19 ++
>  kernel/Makefile                   |   1 +
>  kernel/module.c                   |   5 +
>  kernel/static_call.c              | 297 ++++++++++++++++++++++++++++++
>  8 files changed, 535 insertions(+)
>  create mode 100644 include/linux/static_call.h
>  create mode 100644 include/linux/static_call_types.h
>  create mode 100644 kernel/static_call.c
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index e1e540ffa979..9657ad5a80d1 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -879,6 +879,12 @@ config HAVE_ARCH_PREL32_RELOCATIONS
>  	  architectures, and don't require runtime relocation on relocatable
>  	  kernels.
>  
> +config HAVE_STATIC_CALL_OPTIMIZED
> +	bool
> +
> +config HAVE_STATIC_CALL_UNOPTIMIZED
> +	bool
> +

Let's also have

config HAVE_STATIC_CALL
	def_bool Y
	depends on HAVE_STATIC_CALL_OPTIMIZED || HAVE_STATIC_CALL_UNOPTIMIZED


>  source "kernel/gcov/Kconfig"
>  
>  source "scripts/gcc-plugins/Kconfig"
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 3d7a6a9c2370..45d8ced28bd0 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -320,6 +320,7 @@
>  	__start_ro_after_init = .;					\
>  	*(.data..ro_after_init)						\
>  	JUMP_TABLE_DATA							\
> +	STATIC_CALL_SITES						\
>  	__end_ro_after_init = .;
>  #endif
>  
> @@ -725,6 +726,16 @@
>  #define BUG_TABLE
>  #endif
>  
> +#ifdef CONFIG_HAVE_STATIC_CALL_OPTIMIZED
> +#define STATIC_CALL_SITES						\
> +	. = ALIGN(8);							\
> +	__start_static_call_sites = .;					\
> +	KEEP(*(.static_call_sites))					\
> +	__stop_static_call_sites = .;
> +#else
> +#define STATIC_CALL_SITES
> +#endif
> +
>  #ifdef CONFIG_UNWINDER_ORC
>  #define ORC_UNWIND_TABLE						\
>  	. = ALIGN(4);							\
> diff --git a/include/linux/module.h b/include/linux/module.h
> index fce6b4335e36..91baf181247a 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -21,6 +21,7 @@
>  #include <linux/rbtree_latch.h>
>  #include <linux/error-injection.h>
>  #include <linux/tracepoint-defs.h>
> +#include <linux/static_call_types.h>
>  
>  #include <linux/percpu.h>
>  #include <asm/module.h>
> @@ -450,6 +451,10 @@ struct module {
>  	unsigned int num_ftrace_callsites;
>  	unsigned long *ftrace_callsites;
>  #endif
> +#ifdef CONFIG_HAVE_STATIC_CALL_OPTIMIZED
> +	int num_static_call_sites;
> +	struct static_call_site *static_call_sites;
> +#endif
>  
>  #ifdef CONFIG_LIVEPATCH
>  	bool klp; /* Is this a livepatch module? */
> @@ -682,6 +687,11 @@ static inline bool is_module_text_address(unsigned long addr)
>  	return false;
>  }
>  
> +static inline bool within_module_init(unsigned long addr, const struct module *mod)
> +{
> +	return false;
> +}
> +
>  /* Get/put a kernel symbol (calls should be symmetric) */
>  #define symbol_get(x) ({ extern typeof(x) x __attribute__((weak)); &(x); })
>  #define symbol_put(x) do { } while (0)
> diff --git a/include/linux/static_call.h b/include/linux/static_call.h
> new file mode 100644
> index 000000000000..b22987057cf1
> --- /dev/null
> +++ b/include/linux/static_call.h
> @@ -0,0 +1,186 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_STATIC_CALL_H
> +#define _LINUX_STATIC_CALL_H
> +
> +/*
> + * Static call support
> + *
> + * Static calls use code patching to hard-code function pointers into direct
> + * branch instructions.  They give the flexibility of function pointers, but
> + * with improved performance.  This is especially important for cases where
> + * retpolines would otherwise be used, as retpolines can significantly impact
> + * performance.
> + *
> + *
> + * API overview:
> + *
> + *   DECLARE_STATIC_CALL(key, func);
> + *   DEFINE_STATIC_CALL(key, func);
> + *   static_call(key, args...);
> + *   static_call_update(key, func);
> + *
> + *
> + * Usage example:
> + *
> + *   # Start with the following functions (with identical prototypes):
> + *   int func_a(int arg1, int arg2);
> + *   int func_b(int arg1, int arg2);
> + *
> + *   # Define a 'my_key' reference, associated with func_a() by default
> + *   DEFINE_STATIC_CALL(my_key, func_a);
> + *
> + *   # Call func_a()
> + *   static_call(my_key, arg1, arg2);
> + *
> + *   # Update 'my_key' to point to func_b()
> + *   static_call_update(my_key, func_b);
> + *
> + *   # Call func_b()
> + *   static_call(my_key, arg1, arg2);
> + *
> + *
> + * Implementation details:
> + *
> + * There are three different implementations:
> + *
> + * 1) Optimized static calls (patched call sites)
> + *
> + *    This requires objtool, which detects all the static_call() sites and
> + *    annotates them in the '.static_call_sites' section.  By default, the call
> + *    sites will call into a temporary per-key trampoline which has an indirect
> + *    branch to the current destination function associated with the key.
> + *    During system boot (or module init), all call sites are patched to call
> + *    their destination functions directly.  Updates to a key will patch all
> + *    call sites associated with that key.
> + *
> + * 2) Unoptimized static calls (patched trampolines)
> + *
> + *    Each static_call() site calls into a permanent trampoline associated with
> + *    the key.  The trampoline has a direct branch to the default function.
> + *    Updates to a key will modify the direct branch in the key's trampoline.
> + *
> + * 3) Generic implementation
> + *
> + *    This is the default implementation if the architecture hasn't implemented
> + *    CONFIG_HAVE_STATIC_CALL_[UN]OPTIMIZED.  In this case, a basic
> + *    function pointer is used.
> + */
> +
> +#include <linux/types.h>
> +#include <linux/cpu.h>
> +#include <linux/static_call_types.h>
> +
> +#if defined(CONFIG_HAVE_STATIC_CALL_OPTIMIZED) || \
> +    defined(CONFIG_HAVE_STATIC_CALL_UNOPTIMIZED)

Now we can have:

#ifdef CONFIG_HAVE_STATIC_CALL


> +
> +#include <asm/static_call.h>
> +
> +extern void arch_static_call_transform(unsigned long insn, void *dest);
> +
> +#endif /* CONFIG_HAVE_STATIC_CALL_[UN]OPTIMIZED */
> +
> +#ifdef CONFIG_HAVE_STATIC_CALL_OPTIMIZED
> +/* Optimized implementation */
> +
> +struct static_call_mod {
> +	struct list_head list;
> +	struct module *mod; /* NULL means vmlinux */
> +	struct static_call_site *sites;
> +};
> +
> +struct static_call_key {
> +	/*
> +	 * This field should always be first because the trampolines expect it.
> +	 * This points to the current call destination.
> +	 */
> +	void *func;
> +
> +	/*
> +	 * List of modules (including vmlinux) and the call sites which need to
> +	 * be patched for this key.
> +	 */
> +	struct list_head site_mods;
> +};
> +
> +extern void __static_call_update(struct static_call_key *key, void *func);
> +extern void arch_static_call_poison_tramp(unsigned long insn);
> +
> +#define DECLARE_STATIC_CALL(key, func)					\
> +	extern struct static_call_key key;				\
> +	extern typeof(func) STATIC_CALL_TRAMP(key);			\
> +	/* Preserve the ELF symbol so objtool can access it: */		\
> +	__ADDRESSABLE(key)

Does the __ADDRESSABLE(key) need to be in the DECLARE part?
If so, there needs to be more explanation than just the comment above
it.


> +
> +#define DEFINE_STATIC_CALL(key, _func)					\
> +	DECLARE_STATIC_CALL(key, _func);				\
> +	struct static_call_key key = {					\
> +		.func = _func,						\
> +		.site_mods = LIST_HEAD_INIT(key.site_mods),		\
> +	};								\
> +	ARCH_STATIC_CALL_TEMPORARY_TRAMP(key)
> +
> +#define static_call(key, args...) STATIC_CALL_TRAMP(key)(args)
> +
> +#define static_call_update(key, func)					\
> +({									\
> +	BUILD_BUG_ON(!__same_type(typeof(func), typeof(STATIC_CALL_TRAMP(key)))); \
> +	__static_call_update(&key, func);				\
> +})
> +
> +#define EXPORT_STATIC_CALL(key)						\
> +	EXPORT_SYMBOL(key);						\
> +	EXPORT_SYMBOL(STATIC_CALL_TRAMP(key))
> +
> +#define EXPORT_STATIC_CALL_GPL(key)					\
> +	EXPORT_SYMBOL_GPL(key);						\
> +	EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(key))
> +
> +
> +#elif defined(CONFIG_HAVE_STATIC_CALL_UNOPTIMIZED)
> +/* Unoptimized implementation */
> +
> +#define DECLARE_STATIC_CALL(key, func)					\
> +	extern typeof(func) STATIC_CALL_TRAMP(key)
> +
> +#define DEFINE_STATIC_CALL(key, func)					\
> +	DECLARE_STATIC_CALL(key, func);					\
> +	ARCH_STATIC_CALL_TRAMP(key, func)
> +
> +#define static_call(key, args...) STATIC_CALL_TRAMP(key)(args)
> +
> +#define static_call_update(key, func)					\
> +({									\
> +	BUILD_BUG_ON(!__same_type(func, STATIC_CALL_TRAMP(key)));	\
> +	cpus_read_lock();						\
> +	arch_static_call_transform((unsigned long)STATIC_CALL_TRAMP(key),\
> +				   func);				\
> +	cpus_read_unlock();						\
> +})
> +
> +#define EXPORT_STATIC_CALL(key)						\
> +	EXPORT_SYMBOL(STATIC_CALL_TRAMP(key))
> +
> +#define EXPORT_STATIC_CALL_GPL(key)					\
> +	EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(key))
> +
> +
> +#else /* Generic implementation */
> +
> +#define DECLARE_STATIC_CALL(key, func)					\
> +	extern typeof(func) *key
> +
> +#define DEFINE_STATIC_CALL(key, func)					\
> +	typeof(func) *key = func
> +
> +#define static_call(key, args...)					\
> +	key(args)
> +
> +#define static_call_update(key, func)					\
> +	WRITE_ONCE(key, func)
> +
> +#define EXPORT_STATIC_CALL(key) EXPORT_SYMBOL(key)
> +#define EXPORT_STATIC_CALL_GPL(key) EXPORT_SYMBOL_GPL(key)
> +
> +#endif /* CONFIG_HAVE_STATIC_CALL_OPTIMIZED */
> +
> +#endif /* _LINUX_STATIC_CALL_H */
> diff --git a/include/linux/static_call_types.h b/include/linux/static_call_types.h
> new file mode 100644
> index 000000000000..7dd4b3d7ec2b
> --- /dev/null
> +++ b/include/linux/static_call_types.h
> @@ -0,0 +1,19 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _STATIC_CALL_TYPES_H
> +#define _STATIC_CALL_TYPES_H
> +
> +#include <linux/stringify.h>
> +
> +#define STATIC_CALL_TRAMP_PREFIX ____static_call_tramp_
> +#define STATIC_CALL_TRAMP_PREFIX_STR __stringify(STATIC_CALL_TRAMP_PREFIX)
> +
> +#define STATIC_CALL_TRAMP(key) STATIC_CALL_TRAMP_PREFIX##key
> +#define STATIC_CALL_TRAMP_STR(key) __stringify(STATIC_CALL_TRAMP(key))
> +
> +/* The static call site table is created by objtool. */
> +struct static_call_site {
> +	s32 addr;
> +	s32 key;
> +};
> +
> +#endif /* _STATIC_CALL_TYPES_H */
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 7343b3a9bff0..888bb476ddaf 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -103,6 +103,7 @@ obj-$(CONFIG_TRACEPOINTS) += trace/
>  obj-$(CONFIG_IRQ_WORK) += irq_work.o
>  obj-$(CONFIG_CPU_PM) += cpu_pm.o
>  obj-$(CONFIG_BPF) += bpf/
> +obj-$(CONFIG_HAVE_STATIC_CALL_OPTIMIZED) += static_call.o
>  
>  obj-$(CONFIG_PERF_EVENTS) += events/
>  
> diff --git a/kernel/module.c b/kernel/module.c
> index 49a405891587..11124d991fcd 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3121,6 +3121,11 @@ static int find_module_sections(struct module *mod, struct load_info *info)
>  	mod->ei_funcs = section_objs(info, "_error_injection_whitelist",
>  					    sizeof(*mod->ei_funcs),
>  					    &mod->num_ei_funcs);
> +#endif
> +#ifdef CONFIG_HAVE_STATIC_CALL_OPTIMIZED
> +	mod->static_call_sites = section_objs(info, ".static_call_sites",
> +					      sizeof(*mod->static_call_sites),
> +					      &mod->num_static_call_sites);
>  #endif
>  	mod->extable = section_objs(info, "__ex_table",
>  				    sizeof(*mod->extable), &mod->num_exentries);
> diff --git a/kernel/static_call.c b/kernel/static_call.c
> new file mode 100644
> index 000000000000..599ebc6fc4f1
> --- /dev/null
> +++ b/kernel/static_call.c
> @@ -0,0 +1,297 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#include <linux/init.h>
> +#include <linux/static_call.h>
> +#include <linux/bug.h>
> +#include <linux/smp.h>
> +#include <linux/sort.h>
> +#include <linux/slab.h>
> +#include <linux/module.h>
> +#include <linux/cpu.h>
> +#include <linux/processor.h>
> +#include <asm/sections.h>
> +
> +extern struct static_call_site __start_static_call_sites[],
> +			       __stop_static_call_sites[];
> +
> +static bool static_call_initialized;
> +
> +/* mutex to protect key modules/sites */
> +static DEFINE_MUTEX(static_call_mutex);
> +
> +static void static_call_lock(void)
> +{
> +	mutex_lock(&static_call_mutex);
> +}
> +
> +static void static_call_unlock(void)
> +{
> +	mutex_unlock(&static_call_mutex);
> +}
> +
> +static inline unsigned long static_call_addr(struct static_call_site *site)
> +{
> +	return (long)site->addr + (long)&site->addr;
> +}
> +
> +static inline struct static_call_key *static_call_key(const struct static_call_site *site)
> +{
> +	return (struct static_call_key *)((long)site->key + (long)&site->key);
> +}
> +
> +static int static_call_site_cmp(const void *_a, const void *_b)
> +{
> +	const struct static_call_site *a = _a;
> +	const struct static_call_site *b = _b;
> +	const struct static_call_key *key_a = static_call_key(a);
> +	const struct static_call_key *key_b = static_call_key(b);
> +
> +	if (key_a < key_b)
> +		return -1;
> +
> +	if (key_a > key_b)
> +		return 1;
> +
> +	return 0;
> +}
> +
> +static void static_call_site_swap(void *_a, void *_b, int size)
> +{
> +	long delta = (unsigned long)_a - (unsigned long)_b;
> +	struct static_call_site *a = _a;
> +	struct static_call_site *b = _b;
> +	struct static_call_site tmp = *a;
> +
> +	a->addr = b->addr  - delta;
> +	a->key  = b->key   - delta;
> +
> +	b->addr = tmp.addr + delta;
> +	b->key  = tmp.key  + delta;
> +}
> +
> +static inline void static_call_sort_entries(struct static_call_site *start,
> +					    struct static_call_site *stop)
> +{
> +	sort(start, stop - start, sizeof(struct static_call_site),
> +	     static_call_site_cmp, static_call_site_swap);
> +}
> +
> +void __static_call_update(struct static_call_key *key, void *func)
> +{
> +	struct static_call_mod *mod;
> +	struct static_call_site *site, *stop;
> +
> +	cpus_read_lock();
> +	static_call_lock();
> +
> +	if (key->func == func)
> +		goto done;
> +
> +	key->func = func;
> +
> +	/*
> +	 * If called before init, leave the call sites unpatched for now.
> +	 * In the meantime they'll continue to call the temporary trampoline.
> +	 */
> +	if (!static_call_initialized)
> +		goto done;
> +
> +	list_for_each_entry(mod, &key->site_mods, list) {

Since I'm expecting a lot of sites, I'm wondering if we should just do
this as an array, like I do with the ftrace call sites.

But this can be an enhancement for later. Let's focus on getting this
working first.

> +		if (!mod->sites) {
> +			/*
> +			 * This can happen if the static call key is defined in
> +			 * a module which doesn't use it.
> +			 */
> +			continue;
> +		}
> +
> +		stop = __stop_static_call_sites;
> +
> +#ifdef CONFIG_MODULES
> +		if (mod->mod) {

BTW, "mod" is an unfortunate name.

> +			stop = mod->mod->static_call_sites +
> +			       mod->mod->num_static_call_sites;
> +		}
> +#endif
> +
> +		for (site = mod->sites;
> +		     site < stop && static_call_key(site) == key; site++) {
> +			unsigned long addr = static_call_addr(site);
> +
> +			if (!mod->mod && init_section_contains((void *)addr, 1))
> +				continue;
> +			if (mod->mod && within_module_init(addr, mod->mod))
> +				continue;
> +


So what's the reason for skipping init calls?

-- Steve


> +			arch_static_call_transform(addr, func);
> +		}
> +	}
> +
> +done:
> +	static_call_unlock();
> +	cpus_read_unlock();
> +}
> +EXPORT_SYMBOL_GPL(__static_call_update);
> +
> +#ifdef CONFIG_MODULES
> +
> +static int static_call_add_module(struct module *mod)
> +{
> +	struct static_call_site *start = mod->static_call_sites;
> +	struct static_call_site *stop = mod->static_call_sites +
> +					mod->num_static_call_sites;
> +	struct static_call_site *site;
> +	struct static_call_key *key, *prev_key = NULL;
> +	struct static_call_mod *static_call_mod;
> +
> +	if (start == stop)
> +		return 0;
> +
> +	module_disable_ro(mod);
> +	static_call_sort_entries(start, stop);
> +	module_enable_ro(mod, false);
> +
> +	for (site = start; site < stop; site++) {
> +		unsigned long addr = static_call_addr(site);
> +
> +		if (within_module_init(addr, mod))
> +			continue;
> +
> +		key = static_call_key(site);
> +		if (key != prev_key) {
> +			prev_key = key;
> +
> +			static_call_mod = kzalloc(sizeof(*static_call_mod), GFP_KERNEL);
> +			if (!static_call_mod)
> +				return -ENOMEM;
> +
> +			static_call_mod->mod = mod;
> +			static_call_mod->sites = site;
> +			list_add_tail(&static_call_mod->list, &key->site_mods);
> +
> +			if (is_module_address((unsigned long)key)) {
> +				/*
> +				 * The trampoline should no longer be used.
> +				 * Poison it it with a BUG() to catch any stray
> +				 * callers.
> +				 */
> +				arch_static_call_poison_tramp(addr);
> +			}
> +		}
> +
> +		arch_static_call_transform(addr, key->func);
> +	}
> +
> +	return 0;
> +}
> +
> +static void static_call_del_module(struct module *mod)
> +{
> +	struct static_call_site *start = mod->static_call_sites;
> +	struct static_call_site *stop = mod->static_call_sites +
> +					mod->num_static_call_sites;
> +	struct static_call_site *site;
> +	struct static_call_key *key, *prev_key = NULL;
> +	struct static_call_mod *static_call_mod;
> +
> +	for (site = start; site < stop; site++) {
> +		key = static_call_key(site);
> +		if (key == prev_key)
> +			continue;
> +		prev_key = key;
> +
> +		list_for_each_entry(static_call_mod, &key->site_mods, list) {
> +			if (static_call_mod->mod == mod) {
> +				list_del(&static_call_mod->list);
> +				kfree(static_call_mod);
> +				break;
> +			}
> +		}
> +	}
> +}
> +
> +static int static_call_module_notify(struct notifier_block *nb,
> +				     unsigned long val, void *data)
> +{
> +	struct module *mod = data;
> +	int ret = 0;
> +
> +	cpus_read_lock();
> +	static_call_lock();
> +
> +	switch (val) {
> +	case MODULE_STATE_COMING:
> +		ret = static_call_add_module(mod);
> +		if (ret) {
> +			WARN(1, "Failed to allocate memory for static calls");
> +			static_call_del_module(mod);
> +		}
> +		break;
> +	case MODULE_STATE_GOING:
> +		static_call_del_module(mod);
> +		break;
> +	}
> +
> +	static_call_unlock();
> +	cpus_read_unlock();
> +
> +	return notifier_from_errno(ret);
> +}
> +
> +static struct notifier_block static_call_module_nb = {
> +	.notifier_call = static_call_module_notify,
> +};
> +
> +#endif /* CONFIG_MODULES */
> +
> +static void __init static_call_init(void)
> +{
> +	struct static_call_site *start = __start_static_call_sites;
> +	struct static_call_site *stop  = __stop_static_call_sites;
> +	struct static_call_site *site;
> +
> +	if (start == stop) {
> +		pr_warn("WARNING: empty static call table\n");
> +		return;
> +	}
> +
> +	cpus_read_lock();
> +	static_call_lock();
> +
> +	static_call_sort_entries(start, stop);
> +
> +	for (site = start; site < stop; site++) {
> +		struct static_call_key *key = static_call_key(site);
> +		unsigned long addr = static_call_addr(site);
> +
> +		if (list_empty(&key->site_mods)) {
> +			struct static_call_mod *mod;
> +
> +			mod = kzalloc(sizeof(*mod), GFP_KERNEL);
> +			if (!mod) {
> +				WARN(1, "Failed to allocate memory for static calls");
> +				return;
> +			}
> +
> +			mod->sites = site;
> +			list_add_tail(&mod->list, &key->site_mods);
> +
> +			/*
> +			 * The trampoline should no longer be used.  Poison it
> +			 * it with a BUG() to catch any stray callers.
> +			 */
> +			arch_static_call_poison_tramp(addr);
> +		}
> +
> +		arch_static_call_transform(addr, key->func);
> +	}
> +
> +	static_call_initialized = true;
> +
> +	static_call_unlock();
> +	cpus_read_unlock();
> +
> +#ifdef CONFIG_MODULES
> +	register_module_notifier(&static_call_module_nb);
> +#endif
> +}
> +early_initcall(static_call_init);

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