diff options
author | Peter Zijlstra <peterz@infradead.org> | 2020-08-18 15:57:48 +0200 |
---|---|---|
committer | Ingo Molnar <mingo@kernel.org> | 2020-09-01 09:58:05 +0200 |
commit | 452cddbff74b6a15b9354505671011700fe03710 (patch) | |
tree | 75bd4702221b4ca983ef387cfd33f49262e050f0 /include/linux/static_call.h | |
parent | c43a43e439e00ad2a4d98716895d961ade6bbbfc (diff) |
static_call: Add static_call_cond()
Extend the static_call infrastructure to optimize the following common
pattern:
if (func_ptr)
func_ptr(args...)
For the trampoline (which is in effect a tail-call), we patch the
JMP.d32 into a RET, which then directly consumes the trampoline call.
For the in-line sites we replace the CALL with a NOP5.
NOTE: this is 'obviously' limited to functions with a 'void' return type.
NOTE: DEFINE_STATIC_COND_CALL() only requires a typename, as opposed
to a full function.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20200818135805.042977182@infradead.org
Diffstat (limited to 'include/linux/static_call.h')
-rw-r--r-- | include/linux/static_call.h | 86 |
1 files changed, 86 insertions, 0 deletions
diff --git a/include/linux/static_call.h b/include/linux/static_call.h index 6f62ceda7dd9..0f74581e0e2f 100644 --- a/include/linux/static_call.h +++ b/include/linux/static_call.h @@ -16,7 +16,9 @@ * * DECLARE_STATIC_CALL(name, func); * DEFINE_STATIC_CALL(name, func); + * DEFINE_STATIC_CALL_NULL(name, typename); * static_call(name)(args...); + * static_call_cond(name)(args...); * static_call_update(name, func); * * Usage example: @@ -52,6 +54,43 @@ * rather than calling through the trampoline. This requires objtool or a * compiler plugin to detect all the static_call() sites and annotate them * in the .static_call_sites section. + * + * + * Notes on NULL function pointers: + * + * Static_call()s support NULL functions, with many of the caveats that + * regular function pointers have. + * + * Clearly calling a NULL function pointer is 'BAD', so too for + * static_call()s (although when HAVE_STATIC_CALL it might not be immediately + * fatal). A NULL static_call can be the result of: + * + * DECLARE_STATIC_CALL_NULL(my_static_call, void (*)(int)); + * + * which is equivalent to declaring a NULL function pointer with just a + * typename: + * + * void (*my_func_ptr)(int arg1) = NULL; + * + * or using static_call_update() with a NULL function. In both cases the + * HAVE_STATIC_CALL implementation will patch the trampoline with a RET + * instruction, instead of an immediate tail-call JMP. HAVE_STATIC_CALL_INLINE + * architectures can patch the trampoline call to a NOP. + * + * In all cases, any argument evaluation is unconditional. Unlike a regular + * conditional function pointer call: + * + * if (my_func_ptr) + * my_func_ptr(arg1) + * + * where the argument evaludation also depends on the pointer value. + * + * When calling a static_call that can be NULL, use: + * + * static_call_cond(name)(arg1); + * + * which will include the required value tests to avoid NULL-pointer + * dereferences. */ #include <linux/types.h> @@ -120,7 +159,16 @@ extern int static_call_text_reserved(void *start, void *end); }; \ ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func) +#define DEFINE_STATIC_CALL_NULL(name, _func) \ + DECLARE_STATIC_CALL(name, _func); \ + struct static_call_key STATIC_CALL_KEY(name) = { \ + .func = NULL, \ + .type = 1, \ + }; \ + ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) + #define static_call(name) __static_call(name) +#define static_call_cond(name) (void)__static_call(name) #define EXPORT_STATIC_CALL(name) \ EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \ @@ -143,7 +191,15 @@ struct static_call_key { }; \ ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func) +#define DEFINE_STATIC_CALL_NULL(name, _func) \ + DECLARE_STATIC_CALL(name, _func); \ + struct static_call_key STATIC_CALL_KEY(name) = { \ + .func = NULL, \ + }; \ + ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) + #define static_call(name) __static_call(name) +#define static_call_cond(name) (void)__static_call(name) static inline void __static_call_update(struct static_call_key *key, void *tramp, void *func) @@ -179,9 +235,39 @@ struct static_call_key { .func = _func, \ } +#define DEFINE_STATIC_CALL_NULL(name, _func) \ + DECLARE_STATIC_CALL(name, _func); \ + struct static_call_key STATIC_CALL_KEY(name) = { \ + .func = NULL, \ + } + #define static_call(name) \ ((typeof(STATIC_CALL_TRAMP(name))*)(STATIC_CALL_KEY(name).func)) +static inline void __static_call_nop(void) { } + +/* + * This horrific hack takes care of two things: + * + * - it ensures the compiler will only load the function pointer ONCE, + * which avoids a reload race. + * + * - it ensures the argument evaluation is unconditional, similar + * to the HAVE_STATIC_CALL variant. + * + * Sadly current GCC/Clang (10 for both) do not optimize this properly + * and will emit an indirect call for the NULL case :-( + */ +#define __static_call_cond(name) \ +({ \ + void *func = READ_ONCE(STATIC_CALL_KEY(name).func); \ + if (!func) \ + func = &__static_call_nop; \ + (typeof(STATIC_CALL_TRAMP(name))*)func; \ +}) + +#define static_call_cond(name) (void)__static_call_cond(name) + static inline void __static_call_update(struct static_call_key *key, void *tramp, void *func) { |