Line data    Source code

       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 24540014962 :         kasan_check_read(addr, 1);
      86 24540014962 :         return *(unsigned long *)addr;
      87             : }
      88             : 
      89             : #endif /* __ASSEMBLY__ */
      90             : #endif  /* __ASM_GENERIC_RWONCE_H */