include/asm-generic/rwsem.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef _ASM_GENERIC_RWSEM_H
   3 #define _ASM_GENERIC_RWSEM_H
   4
   5 #ifndef _LINUX_RWSEM_H
   6 #error "Please don't include <asm/rwsem.h> directly, use <linux/rwsem.h> instead."
   7 #endif
   8
   9 #ifdef __KERNEL__
  10
  11 /*
  12  * R/W semaphores originally for PPC using the stuff in lib/rwsem.c.
  13  * Adapted largely from include/asm-i386/rwsem.h
  14  * by Paul Mackerras <paulus@samba.org>.
  15  */
  16
  17 /*
  18  * the semaphore definition
  19  */
  20 #ifdef CONFIG_64BIT
  21 # define RWSEM_ACTIVE_MASK              0xffffffffL
  22 #else
  23 # define RWSEM_ACTIVE_MASK              0x0000ffffL
  24 #endif
  25
  26 #define RWSEM_UNLOCKED_VALUE            0x00000000L
  27 #define RWSEM_ACTIVE_BIAS               0x00000001L
  28 #define RWSEM_WAITING_BIAS              (-RWSEM_ACTIVE_MASK-1)
  29 #define RWSEM_ACTIVE_READ_BIAS          RWSEM_ACTIVE_BIAS
  30 #define RWSEM_ACTIVE_WRITE_BIAS         (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
  31
  32 /*
  33  * lock for reading
  34  */
  35 static inline void __down_read(struct rw_semaphore *sem)
  36 {
  37         if (unlikely(atomic_long_inc_return_acquire(&sem->count) <= 0))
  38                 rwsem_down_read_failed(sem);
  39 }
  40
  41 static inline int __down_read_trylock(struct rw_semaphore *sem)
  42 {
  43         long tmp;
  44
  45         while ((tmp = atomic_long_read(&sem->count)) >= 0) {
  46                 if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
  47                                    tmp + RWSEM_ACTIVE_READ_BIAS)) {
  48                         return 1;
  49                 }
  50         }
  51         return 0;
  52 }
  53
  54 /*
  55  * lock for writing
  56  */
  57 static inline void __down_write(struct rw_semaphore *sem)
  58 {
  59         long tmp;
  60
  61         tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
  62                                              &sem->count);
  63         if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
  64                 rwsem_down_write_failed(sem);
  65 }
  66
  67 static inline int __down_write_killable(struct rw_semaphore *sem)
  68 {
  69         long tmp;
  70
  71         tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
  72                                              &sem->count);
  73         if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
  74                 if (IS_ERR(rwsem_down_write_failed_killable(sem)))
  75                         return -EINTR;
  76         return 0;
  77 }
  78
  79 static inline int __down_write_trylock(struct rw_semaphore *sem)
  80 {
  81         long tmp;
  82
  83         tmp = atomic_long_cmpxchg_acquire(&sem->count, RWSEM_UNLOCKED_VALUE,
  84                       RWSEM_ACTIVE_WRITE_BIAS);
  85         return tmp == RWSEM_UNLOCKED_VALUE;
  86 }
  87
  88 /*
  89  * unlock after reading
  90  */
  91 static inline void __up_read(struct rw_semaphore *sem)
  92 {
  93         long tmp;
  94
  95         tmp = atomic_long_dec_return_release(&sem->count);
  96         if (unlikely(tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) == 0))
  97                 rwsem_wake(sem);
  98 }
  99
 100 /*
 101  * unlock after writing
 102  */
 103 static inline void __up_write(struct rw_semaphore *sem)
 104 {
 105         if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
 106                                                     &sem->count) < 0))
 107                 rwsem_wake(sem);
 108 }
 109
 110 /*
 111  * downgrade write lock to read lock
 112  */
 113 static inline void __downgrade_write(struct rw_semaphore *sem)
 114 {
 115         long tmp;
 116
 117         /*
 118          * When downgrading from exclusive to shared ownership,
 119          * anything inside the write-locked region cannot leak
 120          * into the read side. In contrast, anything in the
 121          * read-locked region is ok to be re-ordered into the
 122          * write side. As such, rely on RELEASE semantics.
 123          */
 124         tmp = atomic_long_add_return_release(-RWSEM_WAITING_BIAS, &sem->count);
 125         if (tmp < 0)
 126                 rwsem_downgrade_wake(sem);
 127 }
 128
 129 #endif  /* __KERNEL__ */
 130 #endif  /* _ASM_GENERIC_RWSEM_H */