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-rw-r--r--tools/include/linux/compiler.h21
1 files changed, 9 insertions, 12 deletions
diff --git a/tools/include/linux/compiler.h b/tools/include/linux/compiler.h
index 07fd03c74a77..04e32f965ad7 100644
--- a/tools/include/linux/compiler.h
+++ b/tools/include/linux/compiler.h
@@ -84,8 +84,6 @@
84 84
85#define uninitialized_var(x) x = *(&(x)) 85#define uninitialized_var(x) x = *(&(x))
86 86
87#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
88
89#include <linux/types.h> 87#include <linux/types.h>
90 88
91/* 89/*
@@ -135,20 +133,19 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
135/* 133/*
136 * Prevent the compiler from merging or refetching reads or writes. The 134 * Prevent the compiler from merging or refetching reads or writes. The
137 * compiler is also forbidden from reordering successive instances of 135 * compiler is also forbidden from reordering successive instances of
138 * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the 136 * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
139 * compiler is aware of some particular ordering. One way to make the 137 * particular ordering. One way to make the compiler aware of ordering is to
140 * compiler aware of ordering is to put the two invocations of READ_ONCE, 138 * put the two invocations of READ_ONCE or WRITE_ONCE in different C
141 * WRITE_ONCE or ACCESS_ONCE() in different C statements. 139 * statements.
142 * 140 *
143 * In contrast to ACCESS_ONCE these two macros will also work on aggregate 141 * These two macros will also work on aggregate data types like structs or
144 * data types like structs or unions. If the size of the accessed data 142 * unions. If the size of the accessed data type exceeds the word size of
145 * type exceeds the word size of the machine (e.g., 32 bits or 64 bits) 143 * the machine (e.g., 32 bits or 64 bits) READ_ONCE() and WRITE_ONCE() will
146 * READ_ONCE() and WRITE_ONCE() will fall back to memcpy and print a 144 * fall back to memcpy and print a compile-time warning.
147 * compile-time warning.
148 * 145 *
149 * Their two major use cases are: (1) Mediating communication between 146 * Their two major use cases are: (1) Mediating communication between
150 * process-level code and irq/NMI handlers, all running on the same CPU, 147 * process-level code and irq/NMI handlers, all running on the same CPU,
151 * and (2) Ensuring that the compiler does not fold, spindle, or otherwise 148 * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
152 * mutilate accesses that either do not require ordering or that interact 149 * mutilate accesses that either do not require ordering or that interact
153 * with an explicit memory barrier or atomic instruction that provides the 150 * with an explicit memory barrier or atomic instruction that provides the
154 * required ordering. 151 * required ordering.