include/asm-generic/rwonce.h

   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 */