From: Christian Borntraeger Date: Tue, 25 Nov 2014 09:16:39 +0000 (+0100) Subject: kernel: tighten rules for ACCESS ONCE X-Git-Tag: v4.0-rc1~88^2~2 X-Git-Url: http://git.cascardo.eti.br/?a=commitdiff_plain;h=927609d622a3773995f84bc03b4564f873cf0e22;hp=38c5ce936a0862a6ce2c8d1c72689a3aba301425;p=cascardo%2Flinux.git kernel: tighten rules for ACCESS ONCE Now that all non-scalar users of ACCESS_ONCE have been converted to READ_ONCE or ASSIGN once, lets tighten ACCESS_ONCE to only work on scalar types. This variant was proposed by Alexei Starovoitov. Signed-off-by: Christian Borntraeger Reviewed-by: Paul E. McKenney --- diff --git a/include/linux/compiler.h b/include/linux/compiler.h index a1c81f80978e..5e186bf6f6c1 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -447,12 +447,23 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int * to make the compiler aware of ordering is to put the two invocations of * ACCESS_ONCE() in different C statements. * - * This macro does absolutely -nothing- to prevent the CPU from reordering, - * merging, or refetching absolutely anything at any time. Its main intended - * use is to mediate communication between process-level code and irq/NMI - * handlers, all running on the same CPU. + * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE + * on a union member will work as long as the size of the member matches the + * size of the union and the size is smaller than word size. + * + * The major use cases of ACCESS_ONCE used to be (1) Mediating communication + * between process-level code and irq/NMI handlers, all running on the same CPU, + * and (2) Ensuring that the compiler does not fold, spindle, or otherwise + * mutilate accesses that either do not require ordering or that interact + * with an explicit memory barrier or atomic instruction that provides the + * required ordering. + * + * If possible use READ_ONCE/ASSIGN_ONCE instead. */ -#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) +#define __ACCESS_ONCE(x) ({ \ + __maybe_unused typeof(x) __var = 0; \ + (volatile typeof(x) *)&(x); }) +#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x)) /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */ #ifdef CONFIG_KPROBES