include/asm-generic/rwsem.h

   1 #ifndef _ASM_GENERIC_RWSEM_H
   2 #define _ASM_GENERIC_RWSEM_H
   3
   4 #ifndef _LINUX_RWSEM_H
   5 #error "Please don't include <asm/rwsem.h> directly, use <linux/rwsem.h> instead."
   6 #endif
   7
   8 #ifdef __KERNEL__
   9
  10 /*
  11  * R/W semaphores originally for PPC using the stuff in lib/rwsem.c.
  12  * Adapted largely from include/asm-i386/rwsem.h
  13  * by Paul Mackerras <paulus@samba.org>.
  14  */
  15
  16 /*
  17  * the semaphore definition
  18  */
  19 #ifdef CONFIG_64BIT
  20 # define RWSEM_ACTIVE_MASK              0xffffffffL
  21 #else
  22 # define RWSEM_ACTIVE_MASK              0x0000ffffL
  23 #endif
  24
  25 #define RWSEM_UNLOCKED_VALUE            0x00000000L
  26 #define RWSEM_ACTIVE_BIAS               0x00000001L
  27 #define RWSEM_WAITING_BIAS              (-RWSEM_ACTIVE_MASK-1)
  28 #define RWSEM_ACTIVE_READ_BIAS          RWSEM_ACTIVE_BIAS
  29 #define RWSEM_ACTIVE_WRITE_BIAS         (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
  30
  31 /*
  32  * lock for reading
  33  */
  34 static inline void __down_read(struct rw_semaphore *sem)
  35 {
  36         if (unlikely(atomic_long_inc_return_acquire((atomic_long_t *)&sem->count) <= 0))
  37                 rwsem_down_read_failed(sem);
  38 }
  39
  40 static inline int __down_read_trylock(struct rw_semaphore *sem)
  41 {
  42         long tmp;
  43
  44         while ((tmp = sem->count) >= 0) {
  45                 if (tmp == cmpxchg_acquire(&sem->count, tmp,
  46                                    tmp + RWSEM_ACTIVE_READ_BIAS)) {
  47                         return 1;
  48                 }
  49         }
  50         return 0;
  51 }
  52
  53 /*
  54  * lock for writing
  55  */
  56 static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
  57 {
  58         long tmp;
  59
  60         tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
  61                                      (atomic_long_t *)&sem->count);
  62         if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
  63                 rwsem_down_write_failed(sem);
  64 }
  65
  66 static inline void __down_write(struct rw_semaphore *sem)
  67 {
  68         __down_write_nested(sem, 0);
  69 }
  70
  71 static inline int __down_write_trylock(struct rw_semaphore *sem)
  72 {
  73         long tmp;
  74
  75         tmp = cmpxchg_acquire(&sem->count, RWSEM_UNLOCKED_VALUE,
  76                       RWSEM_ACTIVE_WRITE_BIAS);
  77         return tmp == RWSEM_UNLOCKED_VALUE;
  78 }
  79
  80 /*
  81  * unlock after reading
  82  */
  83 static inline void __up_read(struct rw_semaphore *sem)
  84 {
  85         long tmp;
  86
  87         tmp = atomic_long_dec_return_release((atomic_long_t *)&sem->count);
  88         if (unlikely(tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) == 0))
  89                 rwsem_wake(sem);
  90 }
  91
  92 /*
  93  * unlock after writing
  94  */
  95 static inline void __up_write(struct rw_semaphore *sem)
  96 {
  97         if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
  98                                  (atomic_long_t *)&sem->count) < 0))
  99                 rwsem_wake(sem);
 100 }
 101
 102 /*
 103  * implement atomic add functionality
 104  */
 105 static inline void rwsem_atomic_add(long delta, struct rw_semaphore *sem)
 106 {
 107         atomic_long_add(delta, (atomic_long_t *)&sem->count);
 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,
 125                                      (atomic_long_t *)&sem->count);
 126         if (tmp < 0)
 127                 rwsem_downgrade_wake(sem);
 128 }
 129
 130 /*
 131  * implement exchange and add functionality
 132  */
 133 static inline long rwsem_atomic_update(long delta, struct rw_semaphore *sem)
 134 {
 135         return atomic_long_add_return(delta, (atomic_long_t *)&sem->count);
 136 }
 137
 138 #endif  /* __KERNEL__ */
 139 #endif  /* _ASM_GENERIC_RWSEM_H */