1 /* SPDX-License-Identifier: GPL-2.0 */ 2 /* 3 * Prevent the compiler from merging or refetching reads or writes. The 4 * compiler is also forbidden from reordering successive instances of 5 * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some 6 * particular ordering. One way to make the compiler aware of ordering is to 7 * put the two invocations of READ_ONCE or WRITE_ONCE in different C 8 * statements. 9 * 10 * These two macros will also work on aggregate data types like structs or 11 * unions. 12 * 13 * Their two major use cases are: (1) Mediating communication between 14 * process-level code and irq/NMI handlers, all running on the same CPU, 15 * and (2) Ensuring that the compiler does not fold, spindle, or otherwise 16 * mutilate accesses that either do not require ordering or that interact 17 * with an explicit memory barrier or atomic instruction that provides the 18 * required ordering. 19 */ 20 #ifndef __ASM_GENERIC_RWONCE_H 21 #define __ASM_GENERIC_RWONCE_H 22 23 #ifndef __ASSEMBLY__ 24 25 #include <linux/compiler_types.h> 26 #include <linux/kasan-checks.h> 27 #include <linux/kcsan-checks.h> 28 29 /* 30 * Yes, this permits 64-bit accesses on 32-bit architectures. These will 31 * actually be atomic in some cases (namely Armv7 + LPAE), but for others we 32 * rely on the access being split into 2x32-bit accesses for a 32-bit quantity 33 * (e.g. a virtual address) and a strong prevailing wind. 34 */ 35 #define compiletime_assert_rwonce_type(t) \ 36 compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long), \ 37 "Unsupported access size for {READ,WRITE}_ONCE().") 38 39 /* 40 * Use __READ_ONCE() instead of READ_ONCE() if you do not require any 41 * atomicity. Note that this may result in tears! 42 */ 43 #ifndef __READ_ONCE 44 #define __READ_ONCE(x) (*(const volatile __unqual_scalar_typeof(x) *)&(x)) 45 #endif 46 47 #define READ_ONCE(x) \ 48 ({ \ 49 compiletime_assert_rwonce_type(x); \ 50 __READ_ONCE(x); \ 51 }) 52 53 #define __WRITE_ONCE(x, val) \ 54 do { \ 55 *(volatile typeof(x) *)&(x) = (val); \ 56 } while (0) 57 58 #define WRITE_ONCE(x, val) \ 59 do { \ 60 compiletime_assert_rwonce_type(x); \ 61 __WRITE_ONCE(x, val); \ 62 } while (0) 63 64 static __no_sanitize_or_inline 65 unsigned long __read_once_word_nocheck(const void *addr) 66 { 67 return __READ_ONCE(*(unsigned long *)addr); 68 } 69 70 /* 71 * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need to load a 72 * word from memory atomically but without telling KASAN/KCSAN. This is 73 * usually used by unwinding code when walking the stack of a running process. 74 */ 75 #define READ_ONCE_NOCHECK(x) \ 76 ({ \ 77 compiletime_assert(sizeof(x) == sizeof(unsigned long), \ 78 "Unsupported access size for READ_ONCE_NOCHECK()."); \ 79 (typeof(x))__read_once_word_nocheck(&(x)); \ 80 }) 81 82 static __no_kasan_or_inline 83 unsigned long read_word_at_a_time(const void *addr) 84 { 85 kasan_check_read(addr, 1); 86 return *(unsigned long *)addr; 87 } 88 89 #endif /* __ASSEMBLY__ */ 90 #endif /* __ASM_GENERIC_RWONCE_H */ 91