From a5460b5e5fb82656807840d40d3deaecad094044 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Mon, 16 Dec 2019 16:51:45 +0000 Subject: [PATCH] READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE() The implementations of {READ,WRITE}_ONCE() suffer from a significant amount of indirection and complexity due to a historic GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 which was originally worked around by 230fa253df63 ("kernel: Provide READ_ONCE and ASSIGN_ONCE"). Since GCC 4.8 is fairly vintage at this point and we emit a warning if we detect it during the build, return {READ,WRITE}_ONCE() to their former glory with an implementation that is easier to understand and, crucially, more amenable to optimisation. A side effect of this simplification is that WRITE_ONCE() no longer returns a value, but nobody seems to be relying on that and the new behaviour is aligned with smp_store_release(). Suggested-by: Linus Torvalds Cc: Peter Zijlstra Cc: Michael Ellerman Cc: Arnd Bergmann Cc: Christian Borntraeger Signed-off-by: Will Deacon --- include/linux/compiler.h | 118 +++++++++++++-------------------------- 1 file changed, 39 insertions(+), 79 deletions(-) diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 034b0a644efc..338111a448d0 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -177,60 +177,6 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, # define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__) #endif -#include - -#define __READ_ONCE_SIZE \ -({ \ - switch (size) { \ - case 1: *(__u8 *)res = *(volatile __u8 *)p; break; \ - case 2: *(__u16 *)res = *(volatile __u16 *)p; break; \ - case 4: *(__u32 *)res = *(volatile __u32 *)p; break; \ - case 8: *(__u64 *)res = *(volatile __u64 *)p; break; \ - default: \ - barrier(); \ - __builtin_memcpy((void *)res, (const void *)p, size); \ - barrier(); \ - } \ -}) - -static __always_inline -void __read_once_size(const volatile void *p, void *res, int size) -{ - __READ_ONCE_SIZE; -} - -#ifdef CONFIG_KASAN -/* - * We can't declare function 'inline' because __no_sanitize_address confilcts - * with inlining. Attempt to inline it may cause a build failure. - * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368 - * '__maybe_unused' allows us to avoid defined-but-not-used warnings. - */ -# define __no_kasan_or_inline __no_sanitize_address notrace __maybe_unused -#else -# define __no_kasan_or_inline __always_inline -#endif - -static __no_kasan_or_inline -void __read_once_size_nocheck(const volatile void *p, void *res, int size) -{ - __READ_ONCE_SIZE; -} - -static __always_inline void __write_once_size(volatile void *p, void *res, int size) -{ - switch (size) { - case 1: *(volatile __u8 *)p = *(__u8 *)res; break; - case 2: *(volatile __u16 *)p = *(__u16 *)res; break; - case 4: *(volatile __u32 *)p = *(__u32 *)res; break; - case 8: *(volatile __u64 *)p = *(__u64 *)res; break; - default: - barrier(); - __builtin_memcpy((void *)p, (const void *)res, size); - barrier(); - } -} - /* * Prevent the compiler from merging or refetching reads or writes. The * compiler is also forbidden from reordering successive instances of @@ -240,11 +186,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s * statements. * * These two macros will also work on aggregate data types like structs or - * unions. If the size of the accessed data type exceeds the word size of - * the machine (e.g., 32 bits or 64 bits) READ_ONCE() and WRITE_ONCE() will - * fall back to memcpy(). There's at least two memcpy()s: one for the - * __builtin_memcpy() and then one for the macro doing the copy of variable - * - '__u' allocated on the stack. + * unions. * * Their two major use cases are: (1) Mediating communication between * process-level code and irq/NMI handlers, all running on the same CPU, @@ -256,23 +198,49 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s #include #include -#define __READ_ONCE(x, check) \ +#define __READ_ONCE(x) (*(volatile typeof(x) *)&(x)) + +#define READ_ONCE(x) \ ({ \ - union { typeof(x) __val; char __c[1]; } __u; \ - if (check) \ - __read_once_size(&(x), __u.__c, sizeof(x)); \ - else \ - __read_once_size_nocheck(&(x), __u.__c, sizeof(x)); \ - smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \ - __u.__val; \ + typeof(x) __x = __READ_ONCE(x); \ + smp_read_barrier_depends(); \ + __x; \ }) -#define READ_ONCE(x) __READ_ONCE(x, 1) + +#define WRITE_ONCE(x, val) \ +do { \ + *(volatile typeof(x) *)&(x) = (val); \ +} while (0) + +#ifdef CONFIG_KASAN +/* + * We can't declare function 'inline' because __no_sanitize_address conflicts + * with inlining. Attempt to inline it may cause a build failure. + * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368 + * '__maybe_unused' allows us to avoid defined-but-not-used warnings. + */ +# define __no_kasan_or_inline __no_sanitize_address notrace __maybe_unused +#else +# define __no_kasan_or_inline __always_inline +#endif + +static __no_kasan_or_inline +unsigned long __read_once_word_nocheck(const void *addr) +{ + return __READ_ONCE(*(unsigned long *)addr); +} /* - * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need - * to hide memory access from KASAN. + * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need to load a + * word from memory atomically but without telling KASAN. This is usually + * used by unwinding code when walking the stack of a running process. */ -#define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0) +#define READ_ONCE_NOCHECK(x) \ +({ \ + unsigned long __x = __read_once_word_nocheck(&(x)); \ + smp_read_barrier_depends(); \ + __x; \ +}) static __no_kasan_or_inline unsigned long read_word_at_a_time(const void *addr) @@ -281,14 +249,6 @@ unsigned long read_word_at_a_time(const void *addr) return *(unsigned long *)addr; } -#define WRITE_ONCE(x, val) \ -({ \ - union { typeof(x) __val; char __c[1]; } __u = \ - { .__val = (__force typeof(x)) (val) }; \ - __write_once_size(&(x), __u.__c, sizeof(x)); \ - __u.__val; \ -}) - #endif /* __KERNEL__ */ /* -- 2.39.5