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 #include <asm/barrier.h>
  30
  31 /*
  32  * Yes, this permits 64-bit accesses on 32-bit architectures. These will
  33  * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
  34  * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
  35  * (e.g. a virtual address) and a strong prevailing wind.
  36  */
  37 #define compiletime_assert_rwonce_type(t)                                       \
  38         compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long),  \
  39                 "Unsupported access size for {READ,WRITE}_ONCE().")
  40
  41 /*
  42  * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
  43  * atomicity or dependency ordering guarantees. Note that this may result
  44  * in tears!
  45  */
  46 #define __READ_ONCE(x)  (*(const volatile __unqual_scalar_typeof(x) *)&(x))
  47
  48 #define __READ_ONCE_SCALAR(x)                                           \
  49 ({                                                                      \
  50         __unqual_scalar_typeof(x) __x = __READ_ONCE(x);                 \
  51         smp_read_barrier_depends();                                     \
  52         (typeof(x))__x;                                                 \
  53 })
  54
  55 #define READ_ONCE(x)                                                    \
  56 ({                                                                      \
  57         compiletime_assert_rwonce_type(x);                              \
  58         __READ_ONCE_SCALAR(x);                                          \
  59 })
  60
  61 #define __WRITE_ONCE(x, val)                                            \
  62 do {                                                                    \
  63         *(volatile typeof(x) *)&(x) = (val);                            \
  64 } while (0)
  65
  66 #define WRITE_ONCE(x, val)                                              \
  67 do {                                                                    \
  68         compiletime_assert_rwonce_type(x);                              \
  69         __WRITE_ONCE(x, val);                                           \
  70 } while (0)
  71
  72 static __no_sanitize_or_inline
  73 unsigned long __read_once_word_nocheck(const void *addr)
  74 {
  75         return __READ_ONCE(*(unsigned long *)addr);
  76 }
  77
  78 /*
  79  * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need to load a
  80  * word from memory atomically but without telling KASAN/KCSAN. This is
  81  * usually used by unwinding code when walking the stack of a running process.
  82  */
  83 #define READ_ONCE_NOCHECK(x)                                            \
  84 ({                                                                      \
  85         unsigned long __x;                                              \
  86         compiletime_assert(sizeof(x) == sizeof(__x),                    \
  87                 "Unsupported access size for READ_ONCE_NOCHECK().");    \
  88         __x = __read_once_word_nocheck(&(x));                           \
  89         smp_read_barrier_depends();                                     \
  90         (typeof(x))__x;                                                 \
  91 })
  92
  93 static __no_kasan_or_inline
  94 unsigned long read_word_at_a_time(const void *addr)
  95 {
  96         kasan_check_read(addr, 1);
  97         return *(unsigned long *)addr;
  98 }
  99
 100 #endif /* __ASSEMBLY__ */
 101 #endif  /* __ASM_GENERIC_RWONCE_H */